Allow user properties as pre-encoded UTF-8 binary buffer (#2228)

* Add support for binary user properties in MQTT 5.0

* Optimize byte array reversal in SubscriptionTopicHashTests

* Use InvariantCulture for topic string conversion in Load_Tests

* switched to `Microsoft.Testing.Platform`

* Refactor CI workflow to separate signed and unsigned package builds

* Add condition to sign job to prevent execution on external pull requests

* Remove fallback to value as string in MqttUserProperty

* Refactor MqttUserProperty handling to use byte arrays for improved performance and add deprecation warnings for legacy methods

.github/workflows/ci.yml

@@ -23,15 +23,21 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Load strong name certificate
+        if: ${{ github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository }}
         run: |
           echo "$SNC_BASE64" | base64 --decode > "${{ github.workspace }}/certificate.snk"
         shell: bash
         env:
           SNC_BASE64: ${{ secrets.SNC_BASE64 }}
 
-      - name: Build package
+      - name: Build package (signed)
+        if: ${{ github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository }}
         run: dotnet build MQTTnet.sln --configuration Release /p:FileVersion=${{ env.VERSION }} /p:AssemblyVersion=${{ env.VERSION }} /p:PackageVersion=${{ env.VERSION }}${{ env.PACKAGE_SUFFIX }} /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=${{ github.workspace }}/certificate.snk
 
+      - name: Build package (unsigned)
+        if: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name != github.repository }}
+        run: dotnet build MQTTnet.sln --configuration Release /p:FileVersion=${{ env.VERSION }} /p:AssemblyVersion=${{ env.VERSION }} /p:PackageVersion=${{ env.VERSION }}${{ env.PACKAGE_SUFFIX }}
+
       - name: Upload nuget packages
         uses: actions/upload-artifact@v4
         with:
@@ -55,12 +61,12 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Execute tests
-        run: dotnet test --framework net10.0 --configuration Release Source/MQTTnet.Tests/MQTTnet.Tests.csproj
+        run: dotnet test --framework net10.0 --configuration Release --project Source/MQTTnet.Tests/MQTTnet.Tests.csproj
 
   sign:
     needs: build
     runs-on: windows-latest # Code signing must run on a Windows agent for Authenticode signing (dll/exe)
-    if: github.repository == 'dotnet/MQTTnet'
+    if: github.repository == 'dotnet/MQTTnet' && (github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository)
     steps:
       - name: Setup .NET SDK
         uses: actions/setup-dotnet@v4

Samples/Client/Client_Publish_Samples.cs

@@ -6,6 +6,9 @@
 // ReSharper disable UnusedMember.Global
 // ReSharper disable InconsistentNaming
 
+using System;
+using System.Text;
+
 namespace MQTTnet.Samples.Client;
 
 public static class Client_Publish_Samples
@@ -84,4 +87,21 @@ public static class Client_Publish_Samples
 
         Console.WriteLine("MQTT application message is published.");
     }
+
+    public static MqttApplicationMessage Create_Message_With_Binary_User_Property()
+    {
+        /*
+         * MQTT v5 user properties are encoded as UTF-8 strings. When the UTF-8 payload is already available
+         * as a byte buffer, the builder APIs can avoid creating intermediate strings.
+         */
+
+        var encodedValue = Encoding.UTF8.GetBytes("sensor-01");
+
+        return new MqttApplicationMessageBuilder()
+            .WithTopic("samples/metadata/binary")
+            .WithUserProperty("client-id", encodedValue.AsMemory())
+            .WithUserProperty("checksum", new ArraySegment<byte>(encodedValue))
+            .WithPayload("metadata")
+            .Build();
+    }
 }

Samples/Client/Client_Subscribe_Samples.cs

@@ -7,6 +7,7 @@
 // ReSharper disable InconsistentNaming
 // ReSharper disable UnusedMember.Local
 
+using System.Text;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 using MQTTnet.Samples.Helpers;
@@ -67,7 +68,7 @@ public static class Client_Subscribe_Samples
             args.ResponseReasonString = "That did not work!";
 
             // User properties require MQTT v5!
-            args.ResponseUserProperties.Add(new MqttUserProperty("My", "Data"));
+            args.ResponseUserProperties.Add(new MqttUserProperty("My", Encoding.UTF8.GetBytes("Data")));
 
             // Now the broker will resend the message again.
             return Task.CompletedTask;

Source/MQTTnet.Server/Disconnecting/MqttServerClientDisconnectOptionsBuilder.cs

@@ -40,6 +40,7 @@ public sealed class MqttServerClientDisconnectOptionsBuilder
         return this;
     }
 
+    [Obsolete("Please use more performance `WithUserProperty` with ArraySegment<byte> or ReadOnlyMemory<byte> for the value.")]
     public MqttServerClientDisconnectOptionsBuilder WithUserProperty(string name, string value)
     {
         if (_options.UserProperties == null)
@@ -50,4 +51,26 @@ public sealed class MqttServerClientDisconnectOptionsBuilder
         _options.UserProperties.Add(new MqttUserProperty(name, value));
         return this;
     }
+
+    public MqttServerClientDisconnectOptionsBuilder WithUserProperty(string name, ReadOnlyMemory<byte> value)
+    {
+        if (_options.UserProperties == null)
+        {
+            _options.UserProperties = new List<MqttUserProperty>();
+        }
+
+        _options.UserProperties.Add(new MqttUserProperty(name, value));
+        return this;
+    }
+
+    public MqttServerClientDisconnectOptionsBuilder WithUserProperty(string name, ArraySegment<byte> value)
+    {
+        if (_options.UserProperties == null)
+        {
+            _options.UserProperties = new List<MqttUserProperty>();
+        }
+
+        _options.UserProperties.Add(new MqttUserProperty(name, value));
+        return this;
+    }
 }

Source/MQTTnet.Server/EnhancedAuthentication/ExchangeEnhancedAuthenticationOptionsFactory.cs

@@ -51,6 +51,7 @@ public sealed class ExchangeEnhancedAuthenticationOptionsFactory
         return this;
     }
 
+    [Obsolete("Please use more performance `WithUserProperty` with ArraySegment<byte> or ReadOnlyMemory<byte> for the value.")]
     public ExchangeEnhancedAuthenticationOptionsFactory WithUserProperty(string name, string value)
     {
         ArgumentNullException.ThrowIfNull(name);
@@ -64,4 +65,32 @@ public sealed class ExchangeEnhancedAuthenticationOptionsFactory
 
         return this;
     }
+
+    public ExchangeEnhancedAuthenticationOptionsFactory WithUserProperty(string name, ReadOnlyMemory<byte> value)
+    {
+        ArgumentNullException.ThrowIfNull(name);
+
+        if (_options.UserProperties == null)
+        {
+            _options.UserProperties = new List<MqttUserProperty>();
+        }
+
+        _options.UserProperties.Add(new MqttUserProperty(name, value));
+
+        return this;
+    }
+
+    public ExchangeEnhancedAuthenticationOptionsFactory WithUserProperty(string name, ArraySegment<byte> value)
+    {
+        ArgumentNullException.ThrowIfNull(name);
+
+        if (_options.UserProperties == null)
+        {
+            _options.UserProperties = new List<MqttUserProperty>();
+        }
+
+        _options.UserProperties.Add(new MqttUserProperty(name, value));
+
+        return this;
+    }
 }

Source/MQTTnet.Tests/Clients/MqttClient/MqttClient_Connection_Tests.cs

@@ -6,6 +6,7 @@ using System.Text;
 using MQTTnet.Exceptions;
 using MQTTnet.Formatter;
 using MQTTnet.Internal;
+using MQTTnet.Packets;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
 using MQTTnet.Server.EnhancedAuthentication;
@@ -143,7 +144,7 @@ public sealed class MqttClient_Connection_Tests : BaseTestClass
 
         var client = await testEnvironment.ConnectClient();
 
-        var disconnectOptions = testEnvironment.ClientFactory.CreateClientDisconnectOptionsBuilder().WithUserProperty("test_name", "test_value").Build();
+        var disconnectOptions = testEnvironment.ClientFactory.CreateClientDisconnectOptionsBuilder().WithUserProperty("test_name", Encoding.UTF8.GetBytes("test_value")).Build();
 
         // Perform a clean disconnect.
         await client.DisconnectAsync(disconnectOptions);
@@ -154,7 +155,7 @@ public sealed class MqttClient_Connection_Tests : BaseTestClass
         Assert.IsNotNull(eventArgs.UserProperties);
         Assert.HasCount(1, eventArgs.UserProperties);
         Assert.AreEqual("test_name", eventArgs.UserProperties[0].Name);
-        Assert.AreEqual("test_value", eventArgs.UserProperties[0].Value);
+        Assert.AreEqual("test_value", eventArgs.UserProperties[0].ReadValueAsString());
     }
 
     sealed class TestClientKerberosAuthenticationHandler : IMqttEnhancedAuthenticationHandler
@@ -260,7 +261,7 @@ public sealed class MqttClient_Connection_Tests : BaseTestClass
 
         server.ValidatingConnectionAsync += args =>
         {
-            args.ResponseUserProperties = [new("Property", "Value")];
+            args.ResponseUserProperties = [new("Property", Encoding.UTF8.GetBytes("Value"))];
 
             args.ReasonCode = MqttConnectReasonCode.QuotaExceeded;
 
@@ -274,7 +275,7 @@ public sealed class MqttClient_Connection_Tests : BaseTestClass
         Assert.IsNotNull(response);
         Assert.AreEqual(MqttClientConnectResultCode.QuotaExceeded, response.ResultCode);
         Assert.AreEqual("Property", response.UserProperties[0].Name);
-        Assert.AreEqual("Value", response.UserProperties[0].Value);
+        Assert.AreEqual("Value", response.UserProperties[0].ReadValueAsString());
     }
 
     [TestMethod]

Source/MQTTnet.Tests/Formatter/MqttPacketSerialization_V3_Tests.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Buffers;
+using System.Text;
 using MQTTnet.Exceptions;
 using MQTTnet.Formatter;
 using MQTTnet.Packets;
@@ -45,7 +46,7 @@ public sealed class MqttPacketSerialization_V3_Tests
             SubscriptionIdentifiersAvailable = true,
             TopicAliasMaximum = 963,
             WildcardSubscriptionAvailable = true,
-            UserProperties = [new("Foo", "Bar")]
+            UserProperties = [new("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(connAckPacket, MqttProtocolVersion.V311);
@@ -96,7 +97,7 @@ public sealed class MqttPacketSerialization_V3_Tests
             SubscriptionIdentifiersAvailable = true,
             TopicAliasMaximum = 963,
             WildcardSubscriptionAvailable = true,
-            UserProperties = [new MqttUserProperty("Foo", "Bar")]
+            UserProperties = [new MqttUserProperty("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(connAckPacket, MqttProtocolVersion.V310);
@@ -152,8 +153,8 @@ public sealed class MqttPacketSerialization_V3_Tests
             WillResponseTopic = "WillResponseTopic",
             WillMessageExpiryInterval = 542,
             WillPayloadFormatIndicator = MqttPayloadFormatIndicator.CharacterData,
-            UserProperties = [new("Foo", "Bar")],
-            WillUserProperties = [new("WillFoo", "WillBar")]
+            UserProperties = [new("Foo", Encoding.UTF8.GetBytes("Bar"))],
+            WillUserProperties = [new("WillFoo", Encoding.UTF8.GetBytes("WillBar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(connectPacket, MqttProtocolVersion.V311);
@@ -195,7 +196,7 @@ public sealed class MqttPacketSerialization_V3_Tests
             ReasonString = "ReasonString",
             ServerReference = "ServerReference",
             SessionExpiryInterval = 234,
-            UserProperties = [new("Foo", "Bar")]
+            UserProperties = [new("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(disconnectPacket, MqttProtocolVersion.V311);
@@ -235,7 +236,7 @@ public sealed class MqttPacketSerialization_V3_Tests
             PacketIdentifier = 123,
             ReasonCode = MqttPubAckReasonCode.NoMatchingSubscribers,
             ReasonString = "ReasonString",
-            UserProperties = [new("Foo", "Bar")]
+            UserProperties = [new("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(pubAckPacket, MqttProtocolVersion.V311);
@@ -254,7 +255,7 @@ public sealed class MqttPacketSerialization_V3_Tests
             PacketIdentifier = 123,
             ReasonCode = MqttPubCompReasonCode.PacketIdentifierNotFound,
             ReasonString = "ReasonString",
-            UserProperties = [new("Foo", "Bar")]
+            UserProperties = [new("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(pubCompPacket, MqttProtocolVersion.V311);
@@ -284,7 +285,7 @@ public sealed class MqttPacketSerialization_V3_Tests
             SubscriptionIdentifiers = [123],
             MessageExpiryInterval = 38,
             PayloadFormatIndicator = MqttPayloadFormatIndicator.CharacterData,
-            UserProperties = [new("Foo", "Bar")]
+            UserProperties = [new("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(publishPacket, MqttProtocolVersion.V311);
@@ -313,7 +314,7 @@ public sealed class MqttPacketSerialization_V3_Tests
             PacketIdentifier = 123,
             ReasonCode = MqttPubRecReasonCode.UnspecifiedError,
             ReasonString = "ReasonString",
-            UserProperties = [new("Foo", "Bar")]
+            UserProperties = [new("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(pubRecPacket, MqttProtocolVersion.V311);
@@ -333,7 +334,7 @@ public sealed class MqttPacketSerialization_V3_Tests
             PacketIdentifier = 123,
             ReasonCode = MqttPubRelReasonCode.PacketIdentifierNotFound,
             ReasonString = "ReasonString",
-            UserProperties = [new("Foo", "Bar")]
+            UserProperties = [new("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(pubRelPacket, MqttProtocolVersion.V311);
@@ -353,7 +354,7 @@ public sealed class MqttPacketSerialization_V3_Tests
             PacketIdentifier = 123,
             ReasonString = "ReasonString",
             ReasonCodes = [MqttSubscribeReasonCode.GrantedQoS1],
-            UserProperties = [new("Foo", "Bar")]
+            UserProperties = [new("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(subAckPacket, MqttProtocolVersion.V311);
@@ -383,7 +384,7 @@ public sealed class MqttPacketSerialization_V3_Tests
                     QualityOfServiceLevel = MqttQualityOfServiceLevel.AtMostOnce
                 }
             ],
-            UserProperties = [new("Foo", "Bar")]
+            UserProperties = [new("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(subscribePacket, MqttProtocolVersion.V311);
@@ -407,7 +408,7 @@ public sealed class MqttPacketSerialization_V3_Tests
             PacketIdentifier = 123,
             ReasonCodes = [MqttUnsubscribeReasonCode.ImplementationSpecificError],
             ReasonString = "ReasonString",
-            UserProperties = [new("Foo", "Bar")]
+            UserProperties = [new("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(unsubAckPacket, MqttProtocolVersion.V311);
@@ -425,7 +426,7 @@ public sealed class MqttPacketSerialization_V3_Tests
         {
             PacketIdentifier = 123,
             TopicFilters = ["TopicFilter1"],
-            UserProperties = [new("Foo", "Bar")]
+            UserProperties = [new("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(unsubscribePacket, MqttProtocolVersion.V311);

Source/MQTTnet.Tests/Formatter/MqttPacketSerialization_V5_Tests.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Buffers;
+using System.Text;
 using MQTTnet.Formatter;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
@@ -32,7 +33,7 @@ public sealed class MqttPacketSerialization_V5_Tests
             AuthenticationMethod = "AuthenticationMethod",
             ReasonCode = MqttAuthenticateReasonCode.ContinueAuthentication,
             ReasonString = "ReasonString",
-            UserProperties = [new MqttUserProperty("Foo", "Bar")]
+            UserProperties = [new MqttUserProperty("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(authPacket, MqttProtocolVersion.V500);
@@ -68,7 +69,7 @@ public sealed class MqttPacketSerialization_V5_Tests
             SubscriptionIdentifiersAvailable = true,
             TopicAliasMaximum = 963,
             WildcardSubscriptionAvailable = true,
-            UserProperties = [new MqttUserProperty("Foo", "Bar")]
+            UserProperties = [new MqttUserProperty("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(connAckPacket, MqttProtocolVersion.V500);
@@ -124,8 +125,8 @@ public sealed class MqttPacketSerialization_V5_Tests
             WillResponseTopic = "WillResponseTopic",
             WillMessageExpiryInterval = 542,
             WillPayloadFormatIndicator = MqttPayloadFormatIndicator.CharacterData,
-            UserProperties = [new MqttUserProperty("Foo", "Bar")],
-            WillUserProperties = [new MqttUserProperty("WillFoo", "WillBar")]
+            UserProperties = [new MqttUserProperty("Foo", Encoding.UTF8.GetBytes("Bar"))],
+            WillUserProperties = [new MqttUserProperty("WillFoo", Encoding.UTF8.GetBytes("WillBar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(connectPacket, MqttProtocolVersion.V500);
@@ -167,7 +168,7 @@ public sealed class MqttPacketSerialization_V5_Tests
             ReasonString = "ReasonString",
             ServerReference = "ServerReference",
             SessionExpiryInterval = 234,
-            UserProperties = [new MqttUserProperty("Foo", "Bar")]
+            UserProperties = [new MqttUserProperty("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(disconnectPacket, MqttProtocolVersion.V500);
@@ -207,7 +208,7 @@ public sealed class MqttPacketSerialization_V5_Tests
             PacketIdentifier = 123,
             ReasonCode = MqttPubAckReasonCode.NoMatchingSubscribers,
             ReasonString = "ReasonString",
-            UserProperties = [new MqttUserProperty("Foo", "Bar")]
+            UserProperties = [new MqttUserProperty("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(pubAckPacket, MqttProtocolVersion.V500);
@@ -226,7 +227,7 @@ public sealed class MqttPacketSerialization_V5_Tests
             PacketIdentifier = 123,
             ReasonCode = MqttPubCompReasonCode.PacketIdentifierNotFound,
             ReasonString = "ReasonString",
-            UserProperties = [new MqttUserProperty("Foo", "Bar")]
+            UserProperties = [new MqttUserProperty("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(pubCompPacket, MqttProtocolVersion.V500);
@@ -255,7 +256,7 @@ public sealed class MqttPacketSerialization_V5_Tests
             SubscriptionIdentifiers = [123],
             MessageExpiryInterval = 38,
             PayloadFormatIndicator = MqttPayloadFormatIndicator.CharacterData,
-            UserProperties = [new MqttUserProperty("Foo", "Bar")]
+            UserProperties = [new MqttUserProperty("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(publishPacket, MqttProtocolVersion.V500);
@@ -284,7 +285,7 @@ public sealed class MqttPacketSerialization_V5_Tests
             PacketIdentifier = 123,
             ReasonCode = MqttPubRecReasonCode.UnspecifiedError,
             ReasonString = "ReasonString",
-            UserProperties = [new MqttUserProperty("Foo", "Bar")]
+            UserProperties = [new MqttUserProperty("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(pubRecPacket, MqttProtocolVersion.V500);
@@ -303,7 +304,7 @@ public sealed class MqttPacketSerialization_V5_Tests
             PacketIdentifier = 123,
             ReasonCode = MqttPubRelReasonCode.PacketIdentifierNotFound,
             ReasonString = "ReasonString",
-            UserProperties = [new MqttUserProperty("Foo", "Bar")]
+            UserProperties = [new MqttUserProperty("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(pubRelPacket, MqttProtocolVersion.V500);
@@ -322,7 +323,7 @@ public sealed class MqttPacketSerialization_V5_Tests
             PacketIdentifier = 123,
             ReasonString = "ReasonString",
             ReasonCodes = [MqttSubscribeReasonCode.GrantedQoS1],
-            UserProperties = [new MqttUserProperty("Foo", "Bar")]
+            UserProperties = [new MqttUserProperty("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(subAckPacket, MqttProtocolVersion.V500);
@@ -352,7 +353,7 @@ public sealed class MqttPacketSerialization_V5_Tests
                     QualityOfServiceLevel = MqttQualityOfServiceLevel.AtMostOnce
                 }
             ],
-            UserProperties = [new MqttUserProperty("Foo", "Bar")]
+            UserProperties = [new MqttUserProperty("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(subscribePacket, MqttProtocolVersion.V500);
@@ -376,7 +377,7 @@ public sealed class MqttPacketSerialization_V5_Tests
             PacketIdentifier = 123,
             ReasonCodes = [MqttUnsubscribeReasonCode.ImplementationSpecificError],
             ReasonString = "ReasonString",
-            UserProperties = [new MqttUserProperty("Foo", "Bar")]
+            UserProperties = [new MqttUserProperty("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(unsubAckPacket, MqttProtocolVersion.V500);
@@ -395,7 +396,7 @@ public sealed class MqttPacketSerialization_V5_Tests
         {
             PacketIdentifier = 123,
             TopicFilters = ["TopicFilter1"],
-            UserProperties = [new MqttUserProperty("Foo", "Bar")]
+            UserProperties = [new MqttUserProperty("Foo", Encoding.UTF8.GetBytes("Bar"))]
         };
 
         var deserialized = MqttPacketSerializationHelper.EncodeAndDecodePacket(unsubscribePacket, MqttProtocolVersion.V500);

Source/MQTTnet.Tests/MQTTnet.Tests.csproj

@@ -6,13 +6,12 @@
         <LangVersion>default</LangVersion>
         <Nullable>disable</Nullable>
         <NoWarn>$(NoWarn);CA1707</NoWarn>
+        <EnableMSTestRunner>true</EnableMSTestRunner>
     </PropertyGroup>
 
     <ItemGroup>
         <PackageReference Include="Microsoft.IO.RecyclableMemoryStream" Version="3.0.1" />
-        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.0.0" />
-        <PackageReference Include="MSTest" Version="4.0.1" />
-
+        <PackageReference Include="MSTest" Version="4.0.2" />
         <FrameworkReference Include="Microsoft.AspNetCore.App" />
     </ItemGroup>
 
@@ -24,4 +23,10 @@
         <ProjectReference Include="..\MQTTnet.Extensions.TopicTemplate\MQTTnet.Extensions.TopicTemplate.csproj" />
     </ItemGroup>
 
+
+    <ItemGroup>
+    <None Include=".\..\..\global.json"
+            Link="global.json"
+            CopyToOutputDirectory="PreserveNewest"/>
+    </ItemGroup>
 </Project>

Source/MQTTnet.Tests/MQTTv5/Client_Tests.cs

@@ -4,6 +4,7 @@
 
 using System.Buffers;
 using System.Diagnostics.CodeAnalysis;
+using System.Text;
 using Microsoft.IO;
 using MQTTnet.Formatter;
 using MQTTnet.Internal;
@@ -64,8 +65,8 @@ public sealed class Client_Tests : BaseTestClass
         await client.PublishAsync(
             new MqttApplicationMessageBuilder().WithTopic("a")
                 .WithPayload("x")
-                .WithUserProperty("a", "1")
-                .WithUserProperty("b", "2")
+                .WithUserProperty("a", Encoding.UTF8.GetBytes("1"))
+                .WithUserProperty("b", Encoding.UTF8.GetBytes("2"))
                 .WithPayloadFormatIndicator(MqttPayloadFormatIndicator.CharacterData)
                 .WithQualityOfServiceLevel(MqttQualityOfServiceLevel.AtLeastOnce)
                 .Build());
@@ -98,8 +99,8 @@ public sealed class Client_Tests : BaseTestClass
         var applicationMessage = new MqttApplicationMessageBuilder().WithTopic("Hello")
             .WithPayload("World")
             .WithQualityOfServiceLevel(MqttQualityOfServiceLevel.AtMostOnce)
-            .WithUserProperty("x", "1")
-            .WithUserProperty("y", "2")
+            .WithUserProperty("x", Encoding.UTF8.GetBytes("1"))
+            .WithUserProperty("y", Encoding.UTF8.GetBytes("2"))
             .WithResponseTopic("response")
             .WithContentType("text")
             .WithMessageExpiryInterval(50)
@@ -214,8 +215,8 @@ public sealed class Client_Tests : BaseTestClass
         var applicationMessage = new MqttApplicationMessageBuilder().WithTopic("Hello")
             .WithPayload("World")
             .WithQualityOfServiceLevel(MqttQualityOfServiceLevel.AtMostOnce)
-            .WithUserProperty("x", "1")
-            .WithUserProperty("y", "2")
+            .WithUserProperty("x", Encoding.UTF8.GetBytes("1"))
+            .WithUserProperty("y", Encoding.UTF8.GetBytes("2"))
             .WithResponseTopic("response")
             .WithContentType("text")
             .WithMessageExpiryInterval(50)
@@ -250,8 +251,8 @@ public sealed class Client_Tests : BaseTestClass
         var applicationMessage = new MqttApplicationMessageBuilder().WithTopic("Hello")
             .WithPayload(memoryStream.GetReadOnlySequence())
             .WithQualityOfServiceLevel(MqttQualityOfServiceLevel.AtMostOnce)
-            .WithUserProperty("x", "1")
-            .WithUserProperty("y", "2")
+            .WithUserProperty("x", Encoding.UTF8.GetBytes("1"))
+            .WithUserProperty("y", Encoding.UTF8.GetBytes("2"))
             .WithResponseTopic("response")
             .WithContentType("text")
             .WithMessageExpiryInterval(50)

Source/MQTTnet.Tests/MqttApplicationMessageBuilder_Tests.cs

@@ -2,7 +2,12 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
+using System.Collections.Generic;
 using System.Text;
+using MQTTnet.Formatter;
+using MQTTnet.Formatter.V5;
+using MQTTnet.Packets;
 using MQTTnet.Protocol;
 
 namespace MQTTnet.Tests;
@@ -59,4 +64,64 @@ public sealed class MqttApplicationMessageBuilder_Tests
         Assert.IsTrue(message.Retain);
         Assert.AreEqual(MqttQualityOfServiceLevel.ExactlyOnce, message.QualityOfServiceLevel);
     }
+
+    [TestMethod]
+    public void CreateApplicationMessage_UserProperty_ReadOnlyMemoryValue()
+    {
+        var value = "utf8";
+        var buffer = Encoding.UTF8.GetBytes(value);
+
+        var message = new MqttApplicationMessageBuilder().WithTopic("topic").WithUserProperty("name", buffer.AsMemory()).Build();
+
+        Assert.IsNotNull(message.UserProperties);
+        Assert.HasCount(1, message.UserProperties);
+
+        var userProperty = message.UserProperties[0];
+        CollectionAssert.AreEqual(buffer, userProperty.ValueBuffer.ToArray());
+        Assert.AreEqual(value, userProperty.ReadValueAsString());
+    }
+
+    [TestMethod]
+    public void CreateApplicationMessage_UserProperty_ArraySegmentValue()
+    {
+        var buffer = Encoding.UTF8.GetBytes("segment");
+        var segment = new ArraySegment<byte>(buffer);
+
+        var message = new MqttApplicationMessageBuilder().WithTopic("topic").WithUserProperty("name", segment).Build();
+
+        Assert.IsNotNull(message.UserProperties);
+        Assert.HasCount(1, message.UserProperties);
+
+        var userProperty = message.UserProperties[0];
+        CollectionAssert.AreEqual(buffer, userProperty.ValueBuffer.ToArray());
+        Assert.AreEqual("segment", userProperty.ReadValueAsString());
+    }
+
+    [TestMethod]
+    public void WriteUserProperty_FromBinaryBuffer_EqualsStringEncoding()
+    {
+        var name = "name";
+        var value = "value";
+        var encoded = Encoding.UTF8.GetBytes(value);
+
+        var binaryWriter = new MqttBufferWriter(32, 256);
+        var binaryPropertiesWriter = new MqttV5PropertiesWriter(binaryWriter);
+        binaryPropertiesWriter.WriteUserProperties(new List<MqttUserProperty> { new(name, encoded.AsMemory()) });
+
+        var stringWriter = new MqttBufferWriter(32, 256);
+        var stringPropertiesWriter = new MqttV5PropertiesWriter(stringWriter);
+#pragma warning disable CS0618 // Type or member is obsolete
+        stringPropertiesWriter.WriteUserProperties(new List<MqttUserProperty> { new(name, value) });
+#pragma warning restore CS0618 // Type or member is obsolete
+
+        CollectionAssert.AreEqual(GetWrittenBytes(stringWriter), GetWrittenBytes(binaryWriter));
+    }
+
+    static byte[] GetWrittenBytes(MqttBufferWriter writer)
+    {
+        var length = writer.Length;
+        var copy = new byte[length];
+        Array.Copy(writer.GetBuffer(), copy, length);
+        return copy;
+    }
 }

Source/MQTTnet.Tests/MqttApplicationMessageValidator_Tests.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Text;
 using MQTTnet.Formatter;
 
 namespace MQTTnet.Tests;
@@ -27,7 +28,7 @@ public sealed class MqttApplicationMessageValidator_Tests
     public void Succeed_When_Using_UserProperties_And_MQTT_500()
     {
         MqttApplicationMessageValidator.ThrowIfNotSupported(
-            new MqttApplicationMessageBuilder().WithTopic("A").WithUserProperty("User", "Property").Build(),
+            new MqttApplicationMessageBuilder().WithTopic("A").WithUserProperty("User", Encoding.UTF8.GetBytes("Property")).Build(),
             MqttProtocolVersion.V500);
     }
 
@@ -35,7 +36,7 @@ public sealed class MqttApplicationMessageValidator_Tests
     public void Succeed_When_Using_WillUserProperties_And_MQTT_311()
     {
         Assert.ThrowsExactly<NotSupportedException>(() => MqttApplicationMessageValidator.ThrowIfNotSupported(
-            new MqttApplicationMessageBuilder().WithTopic("B").WithUserProperty("User", "Property").Build(),
+            new MqttApplicationMessageBuilder().WithTopic("B").WithUserProperty("User", Encoding.UTF8.GetBytes("Property")).Build(),
             MqttProtocolVersion.V311));
     }
 }

Source/MQTTnet.Tests/MqttClientOptionsValidator_Tests.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Text;
 using MQTTnet.Formatter;
 
 namespace MQTTnet.Tests;
@@ -13,27 +14,27 @@ public sealed class MqttClientOptionsValidator_Tests
     [TestMethod]
     public void Succeed_When_Using_UserProperties_And_MQTT_500()
     {
-        new MqttClientOptionsBuilder().WithProtocolVersion(MqttProtocolVersion.V500).WithUserProperty("User", "Property").WithTcpServer("FAKE").Build();
+        new MqttClientOptionsBuilder().WithProtocolVersion(MqttProtocolVersion.V500).WithUserProperty("User", Encoding.UTF8.GetBytes("Property")).WithTcpServer("FAKE").Build();
     }
 
     [TestMethod]
     public void Succeed_When_Using_WillUserProperties_And_MQTT_311()
     {
         Assert.ThrowsExactly<NotSupportedException>(() =>
-            new MqttClientOptionsBuilder().WithProtocolVersion(MqttProtocolVersion.V311).WithWillUserProperty("User", "Property").WithTcpServer("FAKE").Build());
+            new MqttClientOptionsBuilder().WithProtocolVersion(MqttProtocolVersion.V311).WithWillUserProperty("User", Encoding.UTF8.GetBytes("Property")).WithTcpServer("FAKE").Build());
     }
 
     [TestMethod]
     public void Succeed_When_Using_WillUserProperties_And_MQTT_500()
     {
-        new MqttClientOptionsBuilder().WithProtocolVersion(MqttProtocolVersion.V500).WithWillUserProperty("User", "Property").WithTcpServer("FAKE").Build();
+        new MqttClientOptionsBuilder().WithProtocolVersion(MqttProtocolVersion.V500).WithWillUserProperty("User", Encoding.UTF8.GetBytes("Property")).WithTcpServer("FAKE").Build();
     }
 
     [TestMethod]
     public void Throw_When_Using_UserProperties_And_MQTT_311()
     {
         Assert.ThrowsExactly<NotSupportedException>(() =>
-            new MqttClientOptionsBuilder().WithProtocolVersion(MqttProtocolVersion.V311).WithUserProperty("User", "Property").WithTcpServer("FAKE").Build());
+            new MqttClientOptionsBuilder().WithProtocolVersion(MqttProtocolVersion.V311).WithUserProperty("User", Encoding.UTF8.GetBytes("Property")).WithTcpServer("FAKE").Build());
     }
 
     [TestMethod]

Source/MQTTnet.Tests/MqttTcpChannel_Tests.cs

@@ -20,7 +20,8 @@ public class MqttTcpChannel_Tests
 
         try
         {
-            serverSocket.Bind(new IPEndPoint(IPAddress.Any, 50001));
+            serverSocket.Bind(new IPEndPoint(IPAddress.Loopback, 0));
+            var serverPort = ((IPEndPoint)serverSocket.LocalEndPoint).Port;
             serverSocket.Listen(0);
 
 #pragma warning disable 4014
@@ -37,7 +38,7 @@ public class MqttTcpChannel_Tests
                 },
                 ct.Token);
 
-            var remoteEndPoint = new DnsEndPoint("localhost", 50001);
+            var remoteEndPoint = new DnsEndPoint("localhost", serverPort);
             using var clientSocket = new CrossPlatformSocket(AddressFamily.InterNetwork, ProtocolType.Tcp);
             await clientSocket.ConnectAsync(remoteEndPoint, CancellationToken.None);
 

Source/MQTTnet.Tests/Server/Cross_Version_Tests.cs

@@ -43,7 +43,7 @@ public sealed class Cross_Version_Tests : BaseTestClass
 
         var applicationMessage = new MqttApplicationMessageBuilder().WithTopic("My/Message")
             .WithPayload("My_Payload")
-            .WithUserProperty("A", "B")
+            .WithUserProperty("A", Encoding.UTF8.GetBytes("B"))
             .WithResponseTopic("Response")
             .WithCorrelationData(Encoding.UTF8.GetBytes("Correlation"))
             .Build();

Source/MQTTnet.Tests/Server/HotSwapCerts_Tests.cs

@@ -2,6 +2,7 @@ using System.Collections.Concurrent;
 using System.Diagnostics;
 using System.Net;
 using System.Net.Security;
+using System.Net.Sockets;
 using System.Security.Authentication;
 using System.Security.Cryptography;
 using System.Security.Cryptography.X509Certificates;
@@ -26,6 +27,7 @@ public sealed class HotSwapCerts_Tests
         using var client01 = new ClientTestHarness();
         server.InstallNewClientCert(client01.GetCurrentClientCert());
         client01.InstallNewServerCert(server.GetCurrentServerCert());
+        client01.SetServerPort(server.EncryptedPort);
 
         await server.StartServer();
 
@@ -47,6 +49,7 @@ public sealed class HotSwapCerts_Tests
         using var client01 = new ClientTestHarness();
         server.InstallNewClientCert(client01.GetCurrentClientCert());
         client01.InstallNewServerCert(server.GetCurrentServerCert());
+        client01.SetServerPort(server.EncryptedPort);
 
         await server.StartServer();
 
@@ -70,6 +73,7 @@ public sealed class HotSwapCerts_Tests
         using var client01 = new ClientTestHarness();
         server.InstallNewClientCert(client01.GetCurrentClientCert());
         client01.InstallNewServerCert(server.GetCurrentServerCert());
+        client01.SetServerPort(server.EncryptedPort);
 
         await server.StartServer();
         await client01.Connect();
@@ -91,6 +95,7 @@ public sealed class HotSwapCerts_Tests
         using var client01 = new ClientTestHarness();
         server.InstallNewClientCert(client01.GetCurrentClientCert());
         client01.InstallNewServerCert(server.GetCurrentServerCert());
+        client01.SetServerPort(server.EncryptedPort);
 
         await server.StartServer();
         await client01.Connect();
@@ -135,10 +140,11 @@ public sealed class HotSwapCerts_Tests
     sealed class ClientTestHarness : IDisposable
     {
         readonly HotSwappableClientCertProvider _hotSwapClient = new HotSwappableClientCertProvider();
+        int _serverPort = 8883;
 
         IMqttClient _client;
 
-        public string ClientId => _client.Options.ClientId;
+        public string ClientId => _client?.Options?.ClientId;
 
         public Task Connect()
         {
@@ -147,10 +153,15 @@ public sealed class HotSwapCerts_Tests
 
         public void Dispose()
         {
-            _client.Dispose();
+            _client?.Dispose();
             _hotSwapClient.Dispose();
         }
 
+        public void SetServerPort(int port)
+        {
+            _serverPort = port;
+        }
+
         public X509Certificate2 GetCurrentClientCert()
         {
             var result = _hotSwapClient.GetCertificates()[0];
@@ -216,7 +227,7 @@ public sealed class HotSwapCerts_Tests
                     o => o.WithClientCertificatesProvider(_hotSwapClient)
                         .WithCertificateValidationHandler(_hotSwapClient.OnCertificateValidation)
                         .WithSslProtocols(SslProtocols.Tls12))
-                .WithTcpServer("localhost")
+                .WithTcpServer("localhost", _serverPort)
                 .WithCleanSession()
                 .WithProtocolVersion(MqttProtocolVersion.V500);
 
@@ -242,6 +253,7 @@ public sealed class HotSwapCerts_Tests
     sealed class ServerTestHarness : IDisposable
     {
         readonly HotSwappableServerCertProvider _hotSwapServer = new HotSwappableServerCertProvider();
+        readonly int _encryptedPort = GetFreeTcpPort();
 
         MqttServer _server;
 
@@ -261,6 +273,8 @@ public sealed class HotSwapCerts_Tests
             return _server.DisconnectClientAsync(client.ClientId, MqttDisconnectReasonCode.UnspecifiedError);
         }
 
+        public int EncryptedPort => _encryptedPort;
+
         public X509Certificate2 GetCurrentServerCert()
         {
             return _hotSwapServer.GetCertificate();
@@ -281,6 +295,7 @@ public sealed class HotSwapCerts_Tests
             var mqttServerFactory = new MqttServerFactory();
 
             var mqttServerOptions = new MqttServerOptionsBuilder().WithEncryptionCertificate(_hotSwapServer)
+                .WithEncryptedEndpointPort(_encryptedPort)
                 .WithRemoteCertificateValidationCallback(_hotSwapServer.RemoteCertificateValidationCallback)
                 .WithEncryptedEndpoint()
                 .Build();
@@ -290,6 +305,17 @@ public sealed class HotSwapCerts_Tests
             _server = mqttServerFactory.CreateMqttServer(mqttServerOptions);
             return _server.StartAsync();
         }
+
+        static int GetFreeTcpPort()
+        {
+            var listener = new TcpListener(IPAddress.Loopback, 0);
+            listener.Start();
+
+            var port = ((IPEndPoint)listener.LocalEndpoint).Port;
+            listener.Stop();
+
+            return port;
+        }
     }
 
     sealed class HotSwappableClientCertProvider : IMqttClientCertificatesProvider, IDisposable

Source/MQTTnet.Tests/Server/Subscribe_Tests.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Globalization;
+using System.Text;
 using MQTTnet.Exceptions;
 using MQTTnet.Formatter;
 using MQTTnet.Internal;
@@ -98,7 +99,7 @@ public sealed class Subscribe_Tests : BaseTestClass
 
         var client = await testEnvironment.ConnectClient();
 
-        var subscribeOptions = testEnvironment.ClientFactory.CreateSubscribeOptionsBuilder().WithTopicFilter("X").WithUserProperty("A", "1").Build();
+        var subscribeOptions = testEnvironment.ClientFactory.CreateSubscribeOptionsBuilder().WithTopicFilter("X").WithUserProperty("A", Encoding.UTF8.GetBytes("1")).Build();
         await client.SubscribeAsync(subscribeOptions);
 
         CollectionAssert.AreEqual(subscribeOptions.UserProperties.ToList(), eventArgs.UserProperties);

Source/MQTTnet.Tests/Server/Subscription_TopicHash_Tests.cs

@@ -1,4 +1,5 @@
 using System.Diagnostics;
+using System.Linq;
 using System.Text;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;

Source/MQTTnet.Tests/Server/Unsubscribe_Tests.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Text;
 using MQTTnet.Exceptions;
 using MQTTnet.Formatter;
 using MQTTnet.Internal;
@@ -45,7 +46,7 @@ public sealed class Unsubscribe_Tests : BaseTestClass
 
         var client = await testEnvironment.ConnectClient();
 
-        var unsubscribeOptions = testEnvironment.ClientFactory.CreateUnsubscribeOptionsBuilder().WithTopicFilter("X").WithUserProperty("A", "1").Build();
+        var unsubscribeOptions = testEnvironment.ClientFactory.CreateUnsubscribeOptionsBuilder().WithTopicFilter("X").WithUserProperty("A", Encoding.UTF8.GetBytes("1")).Build();
         await client.UnsubscribeAsync(unsubscribeOptions);
 
         CollectionAssert.AreEqual(unsubscribeOptions.UserProperties.ToList(), eventArgs.UserProperties);

Source/MQTTnet.Tests/Server/User_Properties_Tests.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Text;
 using MQTTnet.Formatter;
 using MQTTnet.Internal;
 using MQTTnet.Packets;
@@ -26,10 +27,10 @@ public class Feature_Tests
 
         var message = new MqttApplicationMessageBuilder()
             .WithTopic("A")
-            .WithUserProperty("x", "1")
-            .WithUserProperty("y", "2")
-            .WithUserProperty("z", "3")
-            .WithUserProperty("z", "4"); // z is here two times to test list of items
+            .WithUserProperty("x", Encoding.UTF8.GetBytes("1"))
+            .WithUserProperty("y", Encoding.UTF8.GetBytes("2"))
+            .WithUserProperty("z", Encoding.UTF8.GetBytes("3"))
+            .WithUserProperty("z", Encoding.UTF8.GetBytes("4")); // z is here two times to test list of items
 
         await receiver.SubscribeAsync(new MqttClientSubscribeOptions
         {

Source/MQTTnet/Disconnecting/MqttClientDisconnectOptionsBuilder.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using MQTTnet.Packets;
 
 namespace MQTTnet;
@@ -48,10 +49,25 @@ public sealed class MqttClientDisconnectOptionsBuilder
         return this;
     }
 
+    [Obsolete("Please use more performance `WithUserProperty` with ArraySegment<byte> or ReadOnlyMemory<byte> for the value.")]
     public MqttClientDisconnectOptionsBuilder WithUserProperty(string name, string value)
     {
         _userProperties ??= [];
         _userProperties.Add(new MqttUserProperty(name, value));
         return this;
     }
+
+    public MqttClientDisconnectOptionsBuilder WithUserProperty(string name, ReadOnlyMemory<byte> value)
+    {
+        _userProperties ??= [];
+        _userProperties.Add(new MqttUserProperty(name, value));
+        return this;
+    }
+
+    public MqttClientDisconnectOptionsBuilder WithUserProperty(string name, ArraySegment<byte> value)
+    {
+        _userProperties ??= [];
+        _userProperties.Add(new MqttUserProperty(name, value));
+        return this;
+    }
 }

Source/MQTTnet/Formatter/MqttBufferWriter.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using System.Runtime.CompilerServices;
 using System.Text;
 using MQTTnet.Exceptions;
@@ -193,6 +194,29 @@ public sealed class MqttBufferWriter
         }
     }
 
+    public void WriteString(ReadOnlyMemory<byte> value)
+    {
+        var span = value.Span;
+        var length = span.Length;
+
+        if (length > EncodedStringMaxLength)
+        {
+            throw new MqttProtocolViolationException($"The maximum string length is 65535. The current string has a length of {length}.");
+        }
+
+        EnsureAdditionalCapacity(length + 2);
+
+        _buffer[_position] = (byte)(length >> 8);
+        _buffer[_position + 1] = (byte)length;
+
+        if (length > 0)
+        {
+            span.CopyTo(_buffer.AsSpan(_position + 2));
+        }
+
+        IncreasePosition(length + 2);
+    }
+
     public void WriteTwoByteInteger(ushort value)
     {
         EnsureAdditionalCapacity(2);

Source/MQTTnet/Formatter/V5/MqttV5PropertiesReader.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using MQTTnet.Exceptions;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
@@ -59,14 +60,14 @@ public struct MqttV5PropertiesReader
             if (CurrentPropertyId == MqttPropertyId.UserProperty)
             {
                 var name = _body.ReadString();
-                var value = _body.ReadString();
+                var valueBuffer = ReadUserPropertyValueBuffer();
 
                 if (CollectedUserProperties == null)
                 {
                     CollectedUserProperties = new List<MqttUserProperty>();
                 }
 
-                CollectedUserProperties.Add(new MqttUserProperty(name, value));
+                CollectedUserProperties.Add(new MqttUserProperty(name, valueBuffer));
                 continue;
             }
 
@@ -190,6 +191,11 @@ public struct MqttV5PropertiesReader
         return _body.ReadTwoByteInteger();
     }
 
+    public ReadOnlyMemory<byte> ReadUserPropertyValueBuffer()
+    {
+        return _body.ReadBinaryData();
+    }
+
     public bool ReadWildcardSubscriptionAvailable()
     {
         return _body.ReadByte() == 1;

Source/MQTTnet/Formatter/V5/MqttV5PropertiesWriter.cs

@@ -260,7 +260,7 @@ public sealed class MqttV5PropertiesWriter
         {
             _bufferWriter.WriteByte((byte)MqttPropertyId.UserProperty);
             _bufferWriter.WriteString(property.Name);
-            _bufferWriter.WriteString(property.Value);
+            _bufferWriter.WriteString(property.ValueBuffer);
         }
     }
 

Source/MQTTnet/MqttApplicationMessageBuilder.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using System.Buffers;
 using System.Runtime.InteropServices;
 using System.Text;
@@ -263,10 +264,33 @@ public sealed class MqttApplicationMessageBuilder
     ///     Adds the user property to the message.
     ///     <remarks>MQTT 5.0.0+ feature.</remarks>
     /// </summary>
+    [Obsolete("Use the WithUserProperty accepting a ReadOnlyMemory<byte> for better performance.")]
     public MqttApplicationMessageBuilder WithUserProperty(string name, string value)
     {
         _userProperties ??= [];
         _userProperties.Add(new MqttUserProperty(name, value));
         return this;
     }
+
+    /// <summary>
+    ///     Adds the user property to the message using a pre-encoded UTF-8 value buffer.
+    ///     <remarks>MQTT 5.0.0+ feature.</remarks>
+    /// </summary>
+    public MqttApplicationMessageBuilder WithUserProperty(string name, ReadOnlyMemory<byte> value)
+    {
+        _userProperties ??= [];
+        _userProperties.Add(new MqttUserProperty(name, value));
+        return this;
+    }
+
+    /// <summary>
+    ///     Adds the user property to the message using a pre-encoded UTF-8 value buffer.
+    ///     <remarks>MQTT 5.0.0+ feature.</remarks>
+    /// </summary>
+    public MqttApplicationMessageBuilder WithUserProperty(string name, ArraySegment<byte> value)
+    {
+        _userProperties ??= [];
+        _userProperties.Add(new MqttUserProperty(name, value));
+        return this;
+    }
 }

Source/MQTTnet/Options/MqttClientOptionsBuilder.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using System.Net;
 using System.Net.Sockets;
 using System.Text;
@@ -357,6 +358,7 @@ public sealed class MqttClientOptionsBuilder
         return this;
     }
 
+    [Obsolete("Please use more performance `WithUserProperty` with ArraySegment<byte> or ReadOnlyMemory<byte> for the value.")]
     public MqttClientOptionsBuilder WithUserProperty(string name, string value)
     {
         if (_options.UserProperties == null)
@@ -368,6 +370,28 @@ public sealed class MqttClientOptionsBuilder
         return this;
     }
 
+    public MqttClientOptionsBuilder WithUserProperty(string name, ReadOnlyMemory<byte> value)
+    {
+        if (_options.UserProperties == null)
+        {
+            _options.UserProperties = new List<MqttUserProperty>();
+        }
+
+        _options.UserProperties.Add(new MqttUserProperty(name, value));
+        return this;
+    }
+
+    public MqttClientOptionsBuilder WithUserProperty(string name, ArraySegment<byte> value)
+    {
+        if (_options.UserProperties == null)
+        {
+            _options.UserProperties = new List<MqttUserProperty>();
+        }
+
+        _options.UserProperties.Add(new MqttUserProperty(name, value));
+        return this;
+    }
+
     public MqttClientOptionsBuilder WithWebSocketServer(Action<MqttClientWebSocketOptionsBuilder> configure)
     {
         ArgumentNullException.ThrowIfNull(configure);
@@ -462,10 +486,25 @@ public sealed class MqttClientOptionsBuilder
         return this;
     }
 
+    [Obsolete("Please use more performance `WithWillUserProperty` with ArraySegment<byte> or ReadOnlyMemory<byte> for the value.")]
     public MqttClientOptionsBuilder WithWillUserProperty(string name, string value)
     {
         _options.WillUserProperties ??= [];
         _options.WillUserProperties.Add(new MqttUserProperty(name, value));
         return this;
     }
+
+    public MqttClientOptionsBuilder WithWillUserProperty(string name, ReadOnlyMemory<byte> value)
+    {
+        _options.WillUserProperties ??= [];
+        _options.WillUserProperties.Add(new MqttUserProperty(name, value));
+        return this;
+    }
+
+    public MqttClientOptionsBuilder WithWillUserProperty(string name, ArraySegment<byte> value)
+    {
+        _options.WillUserProperties ??= [];
+        _options.WillUserProperties.Add(new MqttUserProperty(name, value));
+        return this;
+    }
 }

Source/MQTTnet/Packets/MqttUserProperty.cs

@@ -2,19 +2,37 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Text;
+
 namespace MQTTnet.Packets;
 
 public sealed class MqttUserProperty
 {
+    readonly ReadOnlyMemory<byte> _valueBuffer;
+
+    [Obsolete("Please use more performance constructor with ArraySegment<byte> or ReadOnlyMemory<byte> for the value.")]
     public MqttUserProperty(string name, string value)
+        : this(name, new ReadOnlyMemory<byte>(Encoding.UTF8.GetBytes(value ?? throw new ArgumentNullException(nameof(value)))))
+    {
+    }
+
+    public MqttUserProperty(string name, ArraySegment<byte> value)
+        : this(name, CreateMemory(value))
+    {
+    }
+
+    public MqttUserProperty(string name, ReadOnlyMemory<byte> value)
     {
         Name = name ?? throw new ArgumentNullException(nameof(name));
-        Value = value ?? throw new ArgumentNullException(nameof(value));
+        _valueBuffer = value;
     }
 
     public string Name { get; }
 
-    public string Value { get; }
+    public ReadOnlyMemory<byte> ValueBuffer => _valueBuffer;
+
+    [Obsolete("Please use more performance property ValueBuffer or the MqttUserPropertyExtensionMethod `ReadValueAsString`")]
+    public string Value => this.ReadValueAsString();
 
     public override bool Equals(object obj)
     {
@@ -33,16 +51,46 @@ public sealed class MqttUserProperty
             return true;
         }
 
-        return string.Equals(Name, other.Name, StringComparison.Ordinal) && string.Equals(Value, other.Value, StringComparison.Ordinal);
+        if (!string.Equals(Name, other.Name, StringComparison.Ordinal))
+        {
+            return false;
+        }
+
+        return _valueBuffer.Span.SequenceEqual(other._valueBuffer.Span);
     }
 
+
     public override int GetHashCode()
     {
-        return Name.GetHashCode() ^ Value.GetHashCode();
+        var hashCode = new HashCode();
+
+        if (!string.IsNullOrEmpty(Name))
+        {
+            hashCode.Add(Name, StringComparer.Ordinal);
+        }
+        else
+        {
+            hashCode.Add(0);
+        }
+
+        hashCode.AddBytes(_valueBuffer.Span);
+
+        return hashCode.ToHashCode();
     }
 
+
     public override string ToString()
     {
-        return $"{Name} = {Value}";
+        return $"{Name} = {this.ReadValueAsString()}";
+    }
+
+    static ReadOnlyMemory<byte> CreateMemory(ArraySegment<byte> value)
+    {
+        if (value.Array == null)
+        {
+            throw new ArgumentNullException(nameof(value));
+        }
+
+        return new ReadOnlyMemory<byte>(value.Array, value.Offset, value.Count);
     }
 }

Source/MQTTnet/Packets/MqttUserPropertyExtensions.cs

@@ -0,0 +1,27 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Text;
+
+namespace MQTTnet.Packets;
+
+public static class MqttUserPropertyExtensions
+{
+    /// <summary>
+    ///     Reads the value of the user property as a UTF-8 string.
+    /// </summary>
+    public static string ReadValueAsString(this MqttUserProperty userProperty)
+    {
+        ArgumentNullException.ThrowIfNull(userProperty);
+
+        var buffer = userProperty.ValueBuffer;
+        if (buffer.IsEmpty)
+        {
+            return string.Empty;
+        }
+
+        return Encoding.UTF8.GetString(buffer.Span);
+    }
+}

Source/MQTTnet/Subscribing/MqttClientSubscribeOptionsBuilder.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using MQTTnet.Exceptions;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
@@ -77,6 +78,7 @@ public sealed class MqttClientSubscribeOptionsBuilder
     ///     Adds the user property to the subscribe options.
     ///     <remarks>MQTT 5.0.0+ feature.</remarks>
     /// </summary>
+    [Obsolete("Use the WithUserProperty accepting a ReadOnlyMemory<byte> for better performance.")]
     public MqttClientSubscribeOptionsBuilder WithUserProperty(string name, string value)
     {
         _subscribeOptions.UserProperties ??= [];
@@ -84,4 +86,28 @@ public sealed class MqttClientSubscribeOptionsBuilder
 
         return this;
     }
+
+    /// <summary>
+    ///     Adds the user property to the subscribe options.
+    ///     <remarks>MQTT 5.0.0+ feature.</remarks>
+    /// </summary>
+    public MqttClientSubscribeOptionsBuilder WithUserProperty(string name, ReadOnlyMemory<byte> value)
+    {
+        _subscribeOptions.UserProperties ??= [];
+        _subscribeOptions.UserProperties.Add(new MqttUserProperty(name, value));
+
+        return this;
+    }
+
+    /// <summary>
+    ///     Adds the user property to the subscribe options.
+    ///     <remarks>MQTT 5.0.0+ feature.</remarks>
+    /// </summary>
+    public MqttClientSubscribeOptionsBuilder WithUserProperty(string name, ArraySegment<byte> value)
+    {
+        _subscribeOptions.UserProperties ??= [];
+        _subscribeOptions.UserProperties.Add(new MqttUserProperty(name, value));
+
+        return this;
+    }
 }

Source/MQTTnet/Unsubscribing/MqttClientUnsubscribeOptionsBuilder.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using MQTTnet.Packets;
 
 namespace MQTTnet;
@@ -36,11 +37,30 @@ public sealed class MqttClientUnsubscribeOptionsBuilder
     ///     Adds the user property to the unsubscribe options.
     ///     <remarks>MQTT 5.0.0+ feature.</remarks>
     /// </summary>
+    [Obsolete("Use the WithUserProperty accepting a ReadOnlyMemory<byte> for better performance.")]
     public MqttClientUnsubscribeOptionsBuilder WithUserProperty(string name, string value)
     {
         return WithUserProperty(new MqttUserProperty(name, value));
     }
 
+    /// <summary>
+    ///     Adds the user property to the unsubscribe options.
+    ///     <remarks>MQTT 5.0.0+ feature.</remarks>
+    /// </summary>
+    public MqttClientUnsubscribeOptionsBuilder WithUserProperty(string name, ReadOnlyMemory<byte> value)
+    {
+        return WithUserProperty(new MqttUserProperty(name, value));
+    }
+
+    /// <summary>
+    ///     Adds the user property to the unsubscribe options.
+    ///     <remarks>MQTT 5.0.0+ feature.</remarks>
+    /// </summary>
+    public MqttClientUnsubscribeOptionsBuilder WithUserProperty(string name, ArraySegment<byte> value)
+    {
+        return WithUserProperty(new MqttUserProperty(name, value));
+    }
+
     /// <summary>
     ///     Adds the user property to the unsubscribe options.
     ///     <remarks>MQTT 5.0.0+ feature.</remarks>

global.json

@@ -0,0 +1,5 @@
+{
+    "test": {
+        "runner": "Microsoft.Testing.Platform"
+    }
+}