Question: How do I build a XML-RPC web service in C#?

Answer: 
I've seen some people produce "web services" that are just simple .aspx pages that spits out Xml instead of html.

The "proper" way to do this is to probably to [implement your own custom http handler](http://www.developer.com/net/asp/article.php/3565541/Use-Custom-HTTP-Handlers-in-Your-ASPNET-Applications.htm) though.

That said, **you should have a really good reason to not use SOAP based services** before you go to all that effort.

**UPDATE:** Have you seen [XML-RPC.NET](http://xml-rpc.net/)?

> 
> [It] is a library for implementing XML-RPC Services and clients in the .NET environment.
> 
> 
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if you mean RPC where you cliend simply sends raw XML via form variables, etc, and you parse it in the implementation of your service, .net 3.5 and wcf support out of the box POX services.

<http://msdn.microsoft.com/en-us/library/aa395208.aspx>