I think this question should be off-topic as it kind of circumvents the entire [No asking for Source Code rule][1].  Why does it matter what language the application is in? Also, there is really only one way to know what an application is written in, by looking at the source code of the application.  So, if the OP is asking for any application that has a Bitcoin Payment API in it and specifically is asking for the application to be in C#, he wants the source code.

For the question to be on-topic on this site, the OP should remove the C# requirement. It seems by his last comment:

> you misunderstood my questions. I know all the API's. For example cryptopay C# payment gateway API. I am just looking for **any application that has implementet the btc payment API**.

If you want any application, then you should be fine with any application written in any language.  Again, the only reason (unless I'm missing something) to have a requirement of "Must be written in C#" is to get the source code.  This then provides a circumvention around the rule that you can't ask for source code.  Currently (in my opinion), the question is off-topic until the C# requirement is removed because it shouldn't matter.


  [1]: http://meta.softwarerecs.stackexchange.com/a/905/133