How to expose a sub section of my stream to a user

问题:

I have a stream that contains many pieces of data. I want to expose just a piece of that data in another stream. The piece of data I want to extract can often be over 100mb. Since I already have stream with the data in it it seems like a waste to copy that data to another stream and return that. What im looking for is a way to reference the data in the first stream while controlling how much of it the second stream can reference. Is this possible

回答1:

There is a good implementation of this by Mark Gravell detailed here. The code posted there is:

using System.IO;
using System;
static class Program
{

 // shows that we can read a subset of an existing stream...
    static void Main()
    {
        byte[] buffer = new byte[255];
        for (byte i = 0; i < 255; i++)
        {
            buffer[i] = i;
        }
        using(MemoryStream ms = new MemoryStream(buffer))
        using (SubStream ss = new SubStream(ms, 10, 200))
        {
            const int BUFFER_SIZE = 17; // why not...
            byte[] working = new byte[BUFFER_SIZE];
            int read;
            while ((read = ss.Read(working, 0, BUFFER_SIZE)) > 0)
            {
                for (int i = 0; i < read; i++)
                {
                    Console.WriteLine(working[i]);
                }
            }
        }
    }
}

class SubStream : Stream
{
    private Stream baseStream;
    private readonly long length;
    private long position;
    public SubStream(Stream baseStream, long offset, long length)
    {
        if (baseStream == null) throw new ArgumentNullException("baseStream");
        if (!baseStream.CanRead) throw new ArgumentException("can't read base stream");
        if (offset < 0) throw new ArgumentOutOfRangeException("offset");

        this.baseStream = baseStream;
        this.length = length;

        if (baseStream.CanSeek)
        {
            baseStream.Seek(offset, SeekOrigin.Current);
        }
        else
        { // read it manually...
            const int BUFFER_SIZE = 512;
            byte[] buffer = new byte[BUFFER_SIZE];
            while (offset > 0)
            {
                int read = baseStream.Read(buffer, 0, offset < BUFFER_SIZE ? (int) offset : BUFFER_SIZE);
                offset -= read;
            }
        }
    }
    public override int Read(byte[] buffer, int offset, int count)
    {
        CheckDisposed();
        long remaining = length - position;
        if (remaining <= 0) return 0;
        if (remaining < count) count = (int) remaining;
        int read = baseStream.Read(buffer, offset, count);
        position += read;
        return read;
    }
    private void CheckDisposed()
    {
        if (baseStream == null) throw new ObjectDisposedException(GetType().Name);
    }
    public override long Length
    {
        get { CheckDisposed(); return length; }
    }
    public override bool CanRead
    {
        get { CheckDisposed(); return true; }
    }
    public override bool CanWrite
    {
        get { CheckDisposed(); return false; }
    }
    public override bool CanSeek
    {
        get { CheckDisposed(); return false; }
    }
    public override long Position
    {
        get {
            CheckDisposed();
            return position;
        }
        set { throw new NotSupportedException(); }
    }
    public override long Seek(long offset, SeekOrigin origin)
    {
        throw new NotSupportedException();
    }
    public override void SetLength(long value)
    {
        throw new NotSupportedException();
    }
    public override void Flush()
    {
        CheckDisposed(); baseStream.Flush();
    }
    protected override void Dispose(bool disposing)
    {
        base.Dispose(disposing);
        if (disposing)
        {
            if (baseStream != null)
            {
                try { baseStream.Dispose(); }
                catch { }
                baseStream = null;
            }
        }
    }
    public override void Write(byte[] buffer, int offset, int count)
    {
        throw new NotImplementedException();
    }
}

回答2:

You need to make your own Stream class that validates its position and returns the desired subset.

I'm not aware of any built-in classes that do this.

回答3:

What exactly are you scared of duplicating? I doubt you have anything super performance critical, parse your Stream on the fly and use a MemoryStream till you find you need something else.

回答4:

Looks like StreamMuxer project was created with similar purpose in mind.