This is a question that we get most often, in this category.
For example, have a look at the answer that I provided a few weeks ago:
Android TCP client and C# visual studio server how to do it.[
^].
The thing is you need to understand that Android as well as .NET framework, both support TCP communication, you just need to know how to make them work together — you can make a TCP client on Android and TCP listener (server) on the .NET, and vice versa. The primary thing to learn and understand is the way that a TCP network works. Rest would be simple as both the platforms provide the tools for network programming.