calling C++/CLI from C# with out parameter

Question

Need some references to better understand the out parameter (and the '%' operator used) when interfacing C# with C++/CLI. Using VS2012 and this msdn reference:msdn ref

C++ DLL code compiled with /clr

#pragma once
using namespace System;

namespace MsdnSampleDLL {

    public ref class Class1
    {
    public:
        void TestOutString([Runtime::InteropServices::Out] String^ %s) 
        {
            s = "just a string";
        }
        void TestOutByte([Runtime::InteropServices::Out] Byte^ %b) 
        {
            b = (Byte)13;
        }
    };
}

And the C# code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using MsdnSampleDLL;
namespace MsdnSampleApp
{
    class Program
    {
        static void Main(string[] args)
        {
            Class1 cls = new Class1();
            string str;
            cls.TestOutString(out str);

            System.Console.WriteLine(str);

            Byte aByte = (Byte)3;
            cls.TestOutByte(out aByte);            
        }
    }
}

The string portion of this code (copied from msdn) works fine. But when I tried to expand on the idea with passing a Byte to be filled in - I got the following error from compiling the C#

Argument 1: cannot convert from 'out byte' to 'out System.ValueType'

So obviously I'm just not "getting it" from the msdn docs. I'd appreciate links to better documentation to explain this.

David Yaw · Accepted Answer · 2015-02-19 16:04:22Z

4

The problem is in your C++/CLI declaration:

void TestOutByte([Runtime::InteropServices::Out] Byte^ %b)

System::Byte is a value type, not a reference type, therefore it doesn't get the ^. A reference to a value type isn't something that can be represented in C#, so a reference to ValueType is used instead.

Get rid of the ^ on the Byte, and it'll work fine.

answered Feb 19, 2015 at 16:04

David Yaw

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1 Comment

Skippy VonDrake Over a year ago

Thanks. As you said - it works fine now. Time for me to crack a book. :?

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