You can wrap a C library in a C# class and use it in any .Net application. This is arguable the best thing about C#. Here is an example of how to wrap a callback (function pointer).

Start by writing your C library 
#include 

static const char*(*func)(void);

void set_cb(const char*(*f)(void))
{
    func = f;
}

void call_cb()
{
    printf("%s\n", (*func)());
}

(t.c)

Compile that into a .so library 
gcc -shared -o t.so t.c
Write the C# wrapper 
using System;
using System.Runtime.InteropServices;

public delegate string CallBack();

public class T {

    [DllImport("t")]
    public static extern void set_cb(CallBack x);

    [DllImport("t")]
    public static extern void call_cb();

    public static void Main()
    {
        CallBack mC = new CallBack(T.Ashley);
        set_cb(mC);
        call_cb();
    }

    public static string Ashley()
    {
        System.Console.WriteLine("Ashley called.");
        return "Ha, you wish";
    }
}

(t.cs)

Finally, compile and run that 
mcs t.cs
LD_LIBRARY_PATH=. mono t.exe
You should get the expected output 
Ashley called.
Ha, you wish