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?

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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.