Pass int by reference from C++/CLI to C#

Question

It seems like there must be a duplicate question, but I haven't been able to find it.

I'm writing a bridge to let an old C program access some C# objects. The bridge is written in C++/CLI.

In one case there is a C# function that's defined as:

public static string GetNameAndValue(out int value);

My C++/CLI wrapper function:

char* GetNameAndValue(int* value);

Which is easy enough to call from C. But how do I call the C# method from C++/CLI?

I first tried the obvious:

String ^ str;
str = TheObject::GetNameAndValue(value);

That gives me error C2664: Cannot convert parameter 2 from 'int *' to 'int %'.

Okay, then, how about GetNameAndValue(%value)? That gives me error C3071: operator '%' can only be applied to an instance of a ref class or a value-type.

Fair enough. So if I create an int I can pass it with %??

int foo;
str = TheObject::GetNameAndValue(%ix);

No dice. C3071 again. Which I find odd because int definitely is a value-type. Or is it?

There must be some magic combination of %, ^, &, or some other obscenity that will do what I want, but after swearing at this thing for 30 minutes I'm stumped. And searching Google for different combinations of C++/CLI and C# mostly gives information about how to call C++ from C#.

SO, my question: How do you pass an int from C++/CLI to a C# method that expects an out int (or ref int, if I have to)?

You've got things badly backwards. C++/CLI is pretty useless to allow a C program to call a C# method. It is a language extension that was made to go the other way, from managed code to native code. Running managed code from a native program requires much bigger weapons, it has to load and initialize the CLR first. That's going to take more than 30 minutes to sort out. — Hans Passant
– Hans Passant, Commented May 12, 2014 at 22:30
@HansPassant: That's not been my experience in the short time I've been working with this. My C program calls a DLL that was compiled with C++/CLI, which in turn calls C# code in another assembly. Works quite well. As I understand it, the C++/CLI DLL loads the runtime. Whatever the case, I've been able to get information from the C# code back to the C code. — Jim Mischel
– Jim Mischel, Commented May 12, 2014 at 23:19
C++/CLI supports __declspec(dllexport), that's the only scenario I can think of where you could have good luck with it. Same idea as Giesecke's [DllExport] hack. As posted, your question makes very little sense to me and the problems you are having are entirely normal. Good luck with it. — Hans Passant
– Hans Passant, Commented May 12, 2014 at 23:36

Tomasz Malik · Accepted Answer · 2014-05-12 22:17:33Z

6

C#:

class MyClass 
{ 
    public static string GetNameAndValue(out int value);
}

C++/CLI:

int value;
String^ x = MyClass::GetNameAndValue(value);

C++/CLI wrapper:

CString GetNameAndValue(int* value)
{
    String^ x = MyClass::GetNameAndValue(*value);
    return CString(x);
}

C++/CLI wrapper 2:

CString GetNameAndValue(int& value)
{
    String^ x = MyClass::GetNameAndValue(value);
    return CString(x);
}

It's the same for C# "ref".

edited May 12, 2014 at 22:17

answered May 12, 2014 at 22:07

Tomasz Malik

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4 Comments

Jim Mischel Over a year ago

That is totally non obvious! So when I do MyClass::GetNameAndValue(*value), the compiler is automatically marshaling it as something like [out] System::Int32? It'll take me a bit to test this, but if it works I'll accept the answer.

Tomasz Malik Over a year ago

It's just like calling C++: "void x(int&);" by passing variable "int v;". Mechanism is the same. There is no cast. For compiler the desired prototype is the same as defined one.

Tomasz Malik Over a year ago

What's more the only difference between ref and out is that out force you to assign the variable inside method. Otherwise you can think of ref as C++ &. By the way I use C++, C++/CLI, C# for a while and there is really no need to use % symbol, you can define it's equivalent in C# or C++. So in C++/CLI the only additional reference type symbol left to care about is ^.

IInspectable Over a year ago

There is no way to define anything like a tracking reference in C++. % in C++/CLI is unique in that it follows objects around when they move during a heap compaction. Objects moving around during their lifetime is a unique feature of .NET. It doesn't exist in C++, hence there is no facility for a matching reference.

Pifcnt · Accepted Answer · 2015-05-12 16:06:23Z

1

I know this is answered already but only to clarify what the compiler was suggesting with "C2664: Cannot convert parameter 2 from 'int *' to 'int %'".
I think it wanted this(C++/CLI):

int a=5
int% foo=a;
str = TheObject::GetNameAndValue(foo);

answered May 12, 2015 at 16:06

Pifcnt

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2 Comments

Ben Voigt Over a year ago

Not really. It's a reference parameter, and can bind to an lvalue. You don't need to create another reference in the caller... and the parameter won't bind to the caller's reference anyway.

Pifcnt Over a year ago

The whole point & purpose of my post was to be explicit and clear, without fancy terms. You are of course correct, but very academic. I am of the opinion that simple even over-trivial examples are better. This is actually the problem with MSDN : it is written in a very academic way, so it is counter-intuitive and difficult to grasp in a short time. That is why we have sites like this one.

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