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

Now I want to pass a reference to the struct for further processing.

  • I can't use class because it probably will not "map" perfectly to the C++ struct.
  • I don't want to copy the 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
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  • 6
    Beware of your reasoning about struct: struct in C++ is exactly the same as class (barring default accessibility), while in C# they are completely different - value and reference types. Chances are you really would be better of with classes in C# - at very least read and understand how struct behave in C#. Commented May 17, 2013 at 17:27
  • A large number of my interop classes are indeed classes and not structs. If you set up the marshalling correctly (and often the default marshalling works) then you can use a class. If in doubt, I try with a class first. Commented May 17, 2013 at 17:47
  • Mutable structs are evil. Commented Mar 8, 2023 at 0:17

2 Answers 2

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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.

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3 Comments

+1. Side note: code handling struct must be written very carefully since almost all operations with struct create copy (except array, where you still can take a ref of an element).
Why do you strongly recommend against passing structs by ref? As far as I can tell, it's the same thing as passing by reference in C++ (eg. std::string&), and it can eliminate needless copying of potentially large structs.
@cmbasnett: I strongly recommended against using mutable structs. Passing a struct by ref is okay, but a) I would advise against large structs in general; b) I would argue against mutable structs (as do the Microsoft design guidelines) as they can cause a number of surprises.
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You can use c# 7.2 in keyword as follows:

static float Sample(in Vector3 v) { // v.X = 2; // <-- this generate follow compiler error // error CS8332: Cannot assign to a member of variable 'v' // or use it as the right hand side of a ref assignment // because it is a readonly variable return v.X; }

This ensures that struct argument v is:

  • readonly
  • passed by ref

IL details

Vector3 v = Vector3.One; float Sample(Vector3 v) { return v.X; } System.Console.WriteLine(Sample(v)); float ReadonlySample(in Vector3 v) { return v.X; } System.Console.WriteLine(ReadonlySample(v));

produce follow IL:

// Vector3 v2 = Vector3.One; IL_0001: call valuetype [System.Numerics.Vectors]System.Numerics.Vector3 [System.Numerics.Vectors]System.Numerics.Vector3::get_One() IL_0006: stloc.0 // Console.WriteLine(Sample(v2)); IL_0007: nop IL_0008: ldloc.0 IL_0009: call float32 test_console.Sample::'<Main>g__Sample|0_0'(valuetype [System.Numerics.Vectors]System.Numerics.Vector3) IL_000e: call void [System.Console]System.Console::WriteLine(float32) // (no C# code) IL_0013: nop // Console.WriteLine(ReadonlySample(in v2)); IL_0014: nop IL_0015: ldloca.s 0 IL_0017: call float32 test_console.Sample::'<Main>g__ReadonlySample|0_1'(valuetype [System.Numerics.Vectors]System.Numerics.Vector3&) IL_001c: call void [System.Console]System.Console::WriteLine(float32)

you can see that using in we have ldloca in place of ldloc.

In short the struct is passed like it was a ref but is protected to writes by the compiler thanks to the in.

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1 Comment

When I write the post I missed to insert this reference ; you can try yourself simply by changing language version in csproj, for example by setting <LangVersion>6</LangVersion> will produce following error for the same above code error CS8059: Feature 'readonly references' is not available in C# 6. Please use language version 7.2 or greater.

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