Can you call c# from C? If so, then you can just use any C fuzzing library, and have it call your C# code. I do this with C++ and Objective-C using clang's libfuzzer. You write a single C function that takes a pointer to a buffer and a length and pass it to whomever you want. I just write a C wrapper that calls my Objective-C or C++ functions with the data.

Doesn't libFuzzer only require `extern "C" int LLVMFuzzerTestOneInput(...` to fuzz C++ code? What else does your C wrapper do beyond that? Google puts their fuzz tests right alongside the rest of the Chromium source code, which is C++.

There's a "port" of AFL - https://github.com/Metalnem/sharpfuzz