While thinkRF does not provide a C# API that a C# application could use, C# application could use thinkRF's C++ DLL API instead.
In the C# application, the developers would need to write C# wrappers for the structs and functions available in C++ API's wsaInterface.h. DllImport() method is needed. Many examples are provided through a google search using search keywords such as "call C++ DLL in C#".
Once the wrappers of the desired functions are created, reading through C++ API's examples will provide useful information on how to use the functions to interface to the R55x0/R57x0 products, to do data capture or signal processing.