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    <p>I'm <em>fairly</em> certain that there's no combination of WCF bindings that will allow you to <em>literally</em> write to the client's stream. It does seem logical that there should be, given that somewhere beneath the surface there's definitely going to be a <code>NetworkStream</code>, but once you start adding encryption and all that fun stuff, WCF would have to know how to wrap this stream to turn it into the "real" message, which I don't think it does.</p> <p>However, the I-need-to-return-a-stream-but-component-X-wants-to-write-to-one scenario is a common one, and I have a catch-all solution that I use for this scenario, which is to create a two-way stream and spawn a worker thread to write to it. The <em>easiest</em> and <em>safest</em> way to do this (by which I mean, the way that involves writing the least code and therefore least likely to be buggy) is to use anonymous pipes.</p> <p>Here's an example of a method that returns an <a href="http://msdn.microsoft.com/en-us/library/system.io.pipes.anonymouspipeclientstream(VS.90).aspx" rel="nofollow noreferrer"><code>AnonymousPipeClientStream</code></a> that can be used in WCF messages, MVC results, and so on - anywhere you want to invert the direction:</p> <pre><code>static Stream GetPipedStream(Action&lt;Stream&gt; writeAction) { AnonymousPipeServerStream pipeServer = new AnonymousPipeServerStream(); ThreadPool.QueueUserWorkItem(s =&gt; { using (pipeServer) { writeAction(pipeServer); pipeServer.WaitForPipeDrain(); } }); return new AnonymousPipeClientStream(PipeDirection.In, pipeServer.ClientSafePipeHandle); } </code></pre> <p>An example usage of this would be:</p> <pre><code>static Stream GetTestStream() { string data = "The quick brown fox jumps over the lazy dog."; return GetPipedStream(s =&gt; { StreamWriter writer = new StreamWriter(s); writer.AutoFlush = true; for (int i = 0; i &lt; 50; i++) { Thread.Sleep(50); writer.WriteLine(data); } }); } </code></pre> <p>Just two caveats about this approach:</p> <ol> <li><p>It will fail if the <code>writeAction</code> does anything to dispose the stream (that's why the test method doesn't actually dispose the <code>StreamWriter</code> - because that would dispose the underlying stream);</p></li> <li><p>It might fail if the <code>writeAction</code> doesn't actually write any data, because <code>WaitForPipeDrain</code> will return immediately and create a race condition. (If this is a concern, you can code a little more defensively to avoid this, but it complicates things a lot for a rare edge case.)</p></li> </ol> <p>Hopefully that helps in your specific case.</p>
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    1. COP.S. I should have added in the answer - take stock of how much data really needs to be streamed out. This is useful for several MB or more, but if you've only got a few KB to send then it would cost less to just create a `MemoryStream` for your component to write to and then return that.
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    2. COWhoa, yes!. Now let me understand this. Please confirm; (1) The AnonymousPipe{Client,Server}Stream is a pair of classes that will be used server-side only. (2) What's returned to the client is a System.IO.Stream.
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    3. COps: I've asked this question before, see http://stackoverflow.com/questions/1499520 . I'm very pleased to get a general, simple answer.
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