How to pass a C# struct by reference?

It sounds like you just want to use ref to pass the struct by reference:

private static void Foo(ref s s1)
{
    s1.a++;
    Console.WriteLine("inner a = " + s1.a);
}

And at the call site:

Foo(ref s1);

See my article on parameter passing in C# for more details.

Note that other than for interop, I would normally strongly recommend against using mutable structs like this. I can understand the benefits here though.