今天想使用C#，对一个文件Stream进行MD5，同时加入盐值(Salt)。

MD5函数的使用，要传入一个文件流Stream。

如果要在，对一个文件+Salt值进行MD5，就要要把整个文件加载到MemoryStream里，在和Salt合并。

如果文件太大，如果使用一个MemoryStream把FileStream拷贝进来，显然占用太大量的内存了。

这时候，我们需要把MemoryStream和FileStream合并成一个Stream。

逛StackOverflow找到一个MergedStream的方案，整合两个Stream：

http://stackoverflow.com/questions/878837/salting-a-c-sharp-md5-computehash-on-a-stream

整理一下以后，就可以使用既对大文件友好又带盐值的MD5函数了。

#region https://github.com/mr-kelly

// KKUpdater - Robust Resources Package Downloader
// 
// A private module of KSFramework<https://github.com/mr-kelly/KSFramework>
// 
// Author: chenpeilin / mr-kelly
// Email: 23110388@qq.com
// Website: https://github.com/mr-kelly

#endregion

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;

namespace KKUpdater
{
    /// <summary>
    ///     merge two stream
    ///     http://stackoverflow.com/questions/878837/salting-a-c-sharp-md5-computehash-on-a-stream
    /// </summary>
    public class MergedStream : Stream, IDisposable
    {
        private Stream s1;
        private Stream s2;

        public MergedStream(Stream first, Stream second)
        {
            s1 = first;
            s2 = second;
        }

        public override int Read(byte[] buffer, int offset, int count)
        {
            int s1count = (int) Math.Min((long) count, s1.Length - s1.Position);
            int bytesRead = 0;

            if (s1count > 0)
            {
                bytesRead += s1.Read(buffer, offset, s1count);
            }

            if (s1count < count)
            {
                bytesRead += s2.Read(buffer, offset + s1count, count - s1count);
            }

            return bytesRead;
        }

        public override void Write(byte[] buffer, int offset, int count)
        {
            throw new NotImplementedException("Merged stream not support write!");
        }

        public override bool CanRead
        {
            get { return s1.CanRead && s2.CanRead; }
        }

        public override bool CanSeek
        {
            get { return s1.CanSeek && s2.CanSeek; }
        }

        public override bool CanWrite
        {
            get { return s1.CanWrite && s2.CanWrite; }
        }

        public override void Flush()
        {
            s1.Flush();
            s2.Flush();
        }

        public override long Length
        {
            get { return s1.Length + s2.Length; }
        }

        public override long Position
        {
            get { return s1.Position + s2.Position; }
            set { throw new NotImplementedException(); }
        }

        public override long Seek(long offset, SeekOrigin origin)
        {
            throw new NotImplementedException();
        }

        public override void SetLength(long value)
        {
            throw new NotImplementedException();
        }

        void IDisposable.Dispose()
        {
            s1.Dispose();
            s2.Dispose();
        }
    }
}