How to pass const int from managed C++ DLL to C# for use as field value in an attribute

Question

I have a const int that is computed at compile time in my Managed C++ DLL. I need to use this value in an attribute within the C# program that calls it. Initially I had created a static method that returns the const int value but C# doesn't see this as a compile time const. I also tried declaring it as a const int within the DLL namespace

// C++
namespace MyNameSpace {
    const int AttributeConstValue = 15 + sizeof(int);
 . . .
}

Attempts to access MyNameSpace.AttributeConstValue from C# returns "does not exist in namespace MyNameSpace"

Is there a way to pass a const to C# and have it see it as a const expression?

score 2 · Accepted Answer · answered Jun 26 '12 at 21:31

You must use the C++/CLI literal keyword to declare public constants that are visible to other managed compilers. And it must appear inside a ref class. Like this:

namespace Example {
    public ref class Constants {
    public:
        literal int AttributeConstValue = 15 + sizeof(int);
    };
}

Sample C# usage:

[MyAttribute(Example.Constants.AttributeConstValue)]
// etc..

Beware that this is fairly dangerous. The literal value gets compiled into the C# assembly's metadata without a reference to your C++/CLI assembly. So if you make a change to this declaration but don't recompile the C# project then you'll have a nasty mismatch. But as long as you need to use it in an attribute declaration then there's no fix for this.

The unsafe condition is mitigated in my case by both the C++ DLL project and C# projects being built within the same solution with the C# project depending on C++ DLL. — JonN, Jun 26 '12 at 22:38

score 0 · Answer 2 · answered Jun 26 '12 at 21:19

0

Const is different then other declaration. When code start to compile - the compiler just replace all places where you use the const with it's value - because const value could not be changed. It's resolved once - at compilation time and stays that way. I don't really remember but I think optimization flag even delete const declaration from code. So the situation is like this: when you done compiling you can not change it - so you first attempt - to return it in some function or getter/setter is right one. Obviously you can not return it to const variable because to-return-it you must compile the program and when you compile the program... you get the point :)

P.S. Don't know about C++ but in C# const is class members and could not be declared in namespace

answered Jun 26 '12 at 21:19

Alex F

3,180
2
28
40

You missed that I need to use this constant as the value of a field in an C# **attribute** and using my first technique the C# compiler complained that it was not a constant expression. See my answer for a solution to this. Here is the C# usage [MarshalAs(UnmanagedType.ByValArray, SizeConst=MyNameSpace.ConstClass.AttributeConstValue] – JonN Jun 26 '12 at 21:29
@JonN: A `const` variable is *not* a constant expression. The compiler wants a `constant expression` as it must have this available at compile time. – Ed S. Jun 26 '12 at 21:35
@Eds That's true but I wasn't sure how to create a symbolic constant that was usable from C#. Using the **literal** keyword as Hans points out, in managed C++ makes AttributeConstValue a symbol that is a constant expression. – JonN Jun 26 '12 at 22:30
@JonN: Yep, I didn't know about `literal` (I know little to nothing of C++/CLI) – Ed S. Jun 26 '12 at 22:37

score 0 · Answer 3 · edited May 23 '17 at 12:24

Found answer HERE

// C++ Const declaration
namespace MyNameSpace {
public ref class ConstClass {
public:
     literal int AttributeConstValue = 15 + sizeof(int);
. . .
}

// C# Usage
[MarshalAs(UnmanagedType.ByValArray, SizeConst=MyNameSpace.ConstClass.AttributeConstValue)]
public byte [] results;

Referencing MyNameSpace.ConstClass.AttributeConstValue as field value for C# attribute works fine using this technique. The value is seen as a compile time constant expression.

How to pass const int from managed C++ DLL to C# for use as field value in an attribute

3 Answers3