title: How do I handle use 100 Continue in a REST web service?
body: <h3>Some background</h3>
  
  <p>I am planning to writing a REST service which helps facilitate collaboration between multiple client systems.  Similar to how git or hg handle things I want the client to perform all merging locally and for the server to reject new changes unless they have been merged with existing changes.</p>
  
  <h3>How I want to handle it</h3>
  
  <p>I don't want clients to have to upload all of their change sets before being told they need to merge first.  I would like to do this by performing a <code>POST</code> with the <code>Expect 100 Continue</code> header.  The server can then verify that it can accept the change sets based on the header information (not hard for me in this case) and either reject the request or send the <code>100 Continue</code> status through to the client who will then upload the changes.</p>
  
  <h3>My problem</h3>
  
  <p>As far as I have been able to figure out so far ASP.NET doesn't support this scenario, by the time you see the request in your controller actions the POST body has normally already been completely uploaded.  I've had a brief look at WCF REST but I haven't been able to see a way to do it there either, their conditional PUT example has the full request body before rejecting the request.</p>
  
  <p>I'm happy to use any alternative framework that runs on .net or can easily be made to run on Windows Azure.</p>