详解C# Protobuf如何做到0分配内存的序列化

题目很简单, 就是IMessage对象怎么变成Byte[]

答案1:

msg.ToByteArray()

这肯定不符合我们的要求

答案2:

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

这里面memoryStream, codedOutputStream, 还有ToArray都产生了一个对象, MemoryStream内部还会多产生一个byte[]对象

不符合要求

答案3:

有人说你可以给MemoryStream传递一个byte[] slice, 让MemoryStream直接用byte[]

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

这次消息直接被序列化到bytes里面去了, 但是memoryStream对象, codecOutputStream还有memoryStream内部的byte[]都还在, 我就序列化了一个对象, 却产生了3个垃圾对象 

所以, 来仔细看看CodedOutputStream类:

    /// <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
    }

提供了一个byte[]的构造函数, 但是没提供slice的构造函数, 好在有一个私有的构造函数

答案4:

这边就不写代码了, 大概意思就是通过反射私有构造函数来构造一个CodedOutputStream对象, 来省掉MemoryStream和他内部的byte[]

现在离答案已经比较接近了 

那我们的问题是, 能不能连CodedOutputStream也省掉呢?

答案5来了:

经过仔细观察, 发现这个类没有使用Stream的情况下, 就只需要修改buffer, limit, 和position几个成员就行了, 虽然是private成员, 但是C#还是能修改

下来立马实践

    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);
    }

这个实例代码里面, 用了一个static的全局CodedOutputStream, 真正用的时候, 肯定要保证线程安全.

所以接下来的问题是:

1. 如何保证CodedOutputStream对象线程安全

2. 如何把var buffer = new byte[msg.CalculateSize()];这个也省掉

这俩问题就留给读者思考.

Github: http://github.com/egmkang

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