pass c# struct by reference?

Question

In my c# application i receive pointer to c++ struct in callback/delegate. I'm not sure if class can do the trick but just casting c++ pointer to appropriate c# struct works fine, so I'm using c# struct for storing data.

Now I want to pass reference to struct for further processing

• I can't use class because it probably will not "map" perfectly to c++ struct.
• I don't want to copy struct for better latency

How can I do that?

---

This example demonstrates that struct is passed by value, not by reference:

using System;

namespace TestStruct
{
    struct s
    {
        public int a;
    }

    class Program
    {
        static void Main(string[] args)
        {
            s s1 = new s
                       {
                           a = 1
                       };
            Foo(s1);
            Console.WriteLine("outer a = " + s1.a);
        }

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

    }
}

Output is:

inner a = 2
outer a = 1

Answer 1

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.