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: 

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