是否可以在没有HttpClient缓冲整个请求的情况下将PushStreamContent添加到MultipartFormDataContent？

Question

I am trying to send a large file in an HTTP POST, along with some other data, using a MultipartFormDataContent . To send the file in chunks, I am using a PushStreamContent which writes chunks of data from my file stream to output stream, flushing after each write:

PushStreamContent pushStreamContent = new PushStreamContent((stream, content, context) => {
    byte[] buffer = new byte[4096];
    int bytesRead;
    do {
        bytesRead = fileStream.Read(buffer, 0, 4096);
        stream.Write(buffer, 0, bytesRead);
        stream.Flush();
    }
    while(bytesRead != 0);
    stream.Close();
});

If I POST this HttpContent using the following code, everything works fine:

new HttpClient().PostAsync(destinationUrl, pushStreamContent);

If, however, this PushStreamContent is added to a MultipartFormDataContent , as follows:

MultipartFormDataContent postForm = new MultipartFormDataContent {
    {stringContent, "atom"},
    {pushStreamContent, "binary"}
};

and that is posted, then I receive an OutOfMemoryException within the lambda of the PushStreamContent . Presumably, the HttpClient is buffering the whole of the MultipartFormDataContent into memory, which fails as the request content is very large. I would like the client to defer to the flushing of the contained PushStreamContent . Is this possible? I am using .NET 4.5.

Answer 1

The request is being buffered because HttpClient needs to know the total length of the request in order to set the Content-Length header. A PushStreamContent object always returns false for TryComputeLength(out long) .

Thus the solution is to create a sub-class of PushStreamContent which does know its length:

class PushStreamContentWithLength : PushStreamContent {
    public PushStreamContentWithLength(Action<Stream, HttpContent, TransportContext> onStreamAvailable, long streamLength) : base(onStreamAvailable) {
        mStreamLength = streamLength;
    }

    readonly long mStreamLength;

    protected override bool TryComputeLength(out long length) {
        length = mStreamLength;
        return true;
    }
}

If one of these is added to a MultipartFormDataContent , the data is pushed to the stream during an HTTP post, rather than being pre-loaded into memory.