Refactor code

Source/MQTTnet/Internal/MqttMemoryHelper.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Runtime.CompilerServices;
+
+namespace MQTTnet.Internal
+{
+    public static class MqttMemoryHelper
+    {
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static void Copy(byte[] source, int sourceIndex, byte[] destination, int destinationIndex, int length)
+        {
+#if NETCOREAPP3_1_OR_GREATER || NETSTANDARD2_1
+            source.AsSpan(sourceIndex, length).CopyTo(destination.AsSpan(destinationIndex, length));
+#elif NET461_OR_GREATER || NETSTANDARD1_3_OR_GREATER
+            unsafe
+            {
+                fixed (byte* sourceHandle = &source[sourceIndex])
+                {
+                    fixed (byte* destinationHandle = &destination[destinationIndex])
+                    {
+                        System.Buffer.MemoryCopy(sourceHandle, destinationHandle, length, length);
+                    }
+                }
+            }
+#else
+            Array.Copy(source, sourceIndex, destination, destinationIndex, length);
+#endif
+        }
+    }
+}