C# Protobuf如何作到0分配內存的序列化

題目很簡單, 就是IMessage對象怎麼變成Byte[]

 

答案1:

msg.ToByteArray()

這確定不符合咱們的要求

 

答案2:

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

這裏面memoryStream, codedOutputStream, 還有ToArray都產生了一個對象, MemoryStream內部還會多產生一個byte[]對象

不符合要求

 

答案3:

有人說你能夠給MemoryStream傳遞一個byte[] slice, 讓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();

此次消息直接被序列化到bytes裏面去了, 可是memoryStream對象, codecOutputStream還有memoryStream內部的byte[]都還在, 我就序列化了一個對象, 卻產生了3個垃圾對象

 

因此, 來仔細看看CodedOutputStream類:

        /// <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
        }

提供了一個byte[]的構造函數, 可是沒提供slice的構造函數, 好在有一個私有的構造函數

 

答案4:

這邊就不寫代碼了, 大概意思就是經過反射私有構造函數來構造一個CodedOutputStream對象, 來省掉MemoryStream和他內部的byte[]

如今離答案已經比較接近了

 

那咱們的問題是, 能不能連CodedOutputStream也省掉呢?

 

答案5來了:

通過仔細觀察, 發現這個類沒有使用Stream的狀況下, 就只須要修改buffer, limit, 和position幾個成員就好了, 雖然是private成員, 可是C#仍是能修改

下來立馬實踐

        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);
        }

這個實例代碼裏面, 用了一個static的全局CodedOutputStream, 真正用的時候, 確定要保證線程安全.

 

因此接下來的問題是:

1. 如何保證CodedOutputStream對象線程安全

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

 

這倆問題就留給讀者思考.