How can I implement IRandomAccessStream in C#?

问题:

I would like to implement an instance of IRandomAccessStream in C# (it will be returning data generated in realtime). The stream does not actually need to be writable or seekable, but I want to return my own data in the ReadAsync method (which is actually part of IInputStream).

public IAsyncOperationWithProgress<IBuffer, uint> ReadAsync(IBuffer buffer, uint count, InputStreamOptions options)
{
    throw new NotImplementedException("To be done");
}

My two main questions are:

1. how do I return something that implements IAsyncOperationWithProgress? Is there anything built into the framework to help with this?
2. how do I write data into the buffer? IBuffer only has Length and Capacity properties (the concrete Buffer class doesn't offer any more either).

回答1:

How to Convert byte Array to IRandomAccessStream

I've found this blog article, hopefully this realization of IRandomAccessStream can be a starting point for you.

class MemoryRandomAccessStream : IRandomAccessStream
{
    private Stream m_InternalStream;

    public MemoryRandomAccessStream(Stream stream)
    {
        this.m_InternalStream = stream;
    }

    public MemoryRandomAccessStream(byte[] bytes)
    {
        this.m_InternalStream = new MemoryStream(bytes);
    }

    public IInputStream GetInputStreamAt(ulong position)
    {
        this.m_InternalStream.Seek((long)position, SeekOrigin.Begin);

        return this.m_InternalStream.AsInputStream();
    }

    public IOutputStream GetOutputStreamAt(ulong position)
    {
        this.m_InternalStream.Seek((long)position, SeekOrigin.Begin);

        return this.m_InternalStream.AsOutputStream();
    }

    public ulong Size
    {
        get { return (ulong)this.m_InternalStream.Length; }
        set { this.m_InternalStream.SetLength((long)value); }
    }

    public bool CanRead
    {
        get { return true; }
    }

    public bool CanWrite
    {
        get { return true; }
    }

    public IRandomAccessStream CloneStream()
    {
        throw new NotSupportedException();
    }

    public ulong Position
    {
        get { return (ulong)this.m_InternalStream.Position; }
    }

    public void Seek(ulong position)
    {
        this.m_InternalStream.Seek((long)position, 0);
    }

    public void Dispose()
    {
        this.m_InternalStream.Dispose();
    }

    public Windows.Foundation.IAsyncOperationWithProgress<IBuffer, uint> ReadAsync(IBuffer buffer, uint count, InputStreamOptions options)
    {
        var inputStream = this.GetInputStreamAt(0);
        return inputStream.ReadAsync(buffer, count, options);
    }

    public Windows.Foundation.IAsyncOperation<bool> FlushAsync()
    {
        var outputStream = this.GetOutputStreamAt(0);
        return outputStream.FlushAsync();
    }

    public Windows.Foundation.IAsyncOperationWithProgress<uint, uint> WriteAsync(IBuffer buffer)
    {
        var outputStream = this.GetOutputStreamAt(0);
        return outputStream.WriteAsync(buffer);
     }
}

回答2:

1. Use AsyncInfo.Run(Func<CancellationToken, IProgress<uint>, Task<IBuffer>>) method to create IAsyncOperationWithProgress instance from a delegate.

   public IAsyncOperationWithProgress<IBuffer, uint> ReadAsync(IBuffer buffer, uint count, InputStreamOptions options)
   {    
       if (buffer == null) throw new ArgumentNullException("buffer");
   
       Func<CancellationToken, IProgress<uint>, Task<IBuffer>> taskProvider =
       (token, progress) => ReadBytesAsync(buffer, count, token, progress, options);
   
       return AsyncInfo.Run(taskProvider);
   }
   
   private async Task<IBuffer> ReadBytesAsync(IBuffer buffer, uint count, CancellationToken token, IProgress<uint> progress, InputStreamOptions options)
   {
   ... Fill the buffer here. Report the progress.
       return buffer;
   }
   

2. Usually you do not need to access the buffer data directly. But in case you need to do this in c# you can use System.Runtime.InteropServices.WindowsRuntime.WindowsRuntimeBufferExtensions class to copy data to/from a buffer.