详解C# Protobuf如何做到0分配内存的序列化

msg.ToByteArray()

using var memoryStream = new MemoryStream();
using var codedOutputStream = new CodedOutputStream(memoryStream);
msg.WriteTo(codedOutputStream);
codedOutputStream.Flush();
memoryStream.ToArray();

var bytes = new byte[msg.CalculateSize()];
using var memoryStream = new MemoryStream();
using var codedOutputStream = new CodedOutputStream(memoryStream);
msg.WriteTo(codedOutputStream);
codedOutputStream.Flush();

    /// <summary>
    /// Creates a new CodedOutputStream that writes directly to the given
    /// byte array. If more bytes are written than fit in the array,
    /// OutOfSpaceException will be thrown.
    /// </summary>
    public CodedOutputStream(byte[] flatArray) : this(flatArray, 0, flatArray.Length)
    {
    }

    /// <summary>
    /// Creates a new CodedOutputStream that writes directly to the given
    /// byte array slice. If more bytes are written than fit in the array,
    /// OutOfSpaceException will be thrown.
    /// </summary>
    private CodedOutputStream(byte[] buffer, int offset, int length)
    {
      this.output = null;
      this.buffer = buffer;
      this.position = offset;
      this.limit = offset + length;
      leaveOpen = true; // Simple way of avoiding trying to dispose of a null reference
    }

    delegate void ClearCodedOutputStream(CodedOutputStream stream, byte[] buffer, int offset, int count);
    static ClearCodedOutputStream ResetCodedOutputStream;
    static CodedOutputStream codedOutputStream = new CodedOutputStream(new byte[10]);

    static unsafe void Encode(IMessage msg, byte[] buffer)
    {
      ResetCodedOutputStream(codedOutputStream, buffer, 0, buffer.Length);
      msg.WriteTo(codedOutputStream);
      codedOutputStream.Flush();
    }

    static Action<T, TValue> MakeSetter<T, TValue>(FieldInfo field)
    {
      DynamicMethod m = new DynamicMethod(
        "setter", typeof(void), new Type[] { typeof(T), typeof(TValue) }, typeof(Program));
      ILGenerator cg = m.GetILGenerator();

      cg.Emit(OpCodes.Ldarg_0);
      cg.Emit(OpCodes.Ldarg_1);
      cg.Emit(OpCodes.Stfld, field);
      cg.Emit(OpCodes.Ret);

      return (Action<T, TValue>)m.CreateDelegate(typeof(Action<T, TValue>));
    }

    static void Main(string[] args)
    {
      var bufferField = typeof(CodedOutputStream).GetField("buffer", BindingFlags.NonPublic | BindingFlags.Instance);
      var limitField = typeof(CodedOutputStream).GetField("limit", BindingFlags.NonPublic | BindingFlags.Instance);
      var positionField = typeof(CodedOutputStream).GetField("position", BindingFlags.NonPublic | BindingFlags.Instance);

      var setLimit = MakeSetter<CodedOutputStream, int>(limitField);
      var setPosition = MakeSetter<CodedOutputStream, int>(positionField);
      var setBuffer = MakeSetter<CodedOutputStream, byte[]>(bufferField);

      ResetCodedOutputStream = (stream, buffer, offset, length) => 
      {
        //this.buffer = buffer;
        //this.position = offset;
        //this.limit = offset + length;
        setBuffer(stream, buffer);
        setPosition(stream, offset);
        setLimit(stream, offset + length);
      };

      var buffer = new byte[msg.CalculateSize()];
      Encode(msg, buffer);
    }