从.NET 4项目使用.NET 3.5库时出现奇怪的警告C4691

Question

If I have a .NET 3.5, C# project with the following class:

namespace ClassLibrary1
{
    public class Class1
    {
        public static void Method1(Func<int> func)
        {

        }

        public static void Method2()
        {
        }
    }
}

And I attempt to use it from a .NET 4.0, C++/CLI project like so:

#include "stdafx.h"

using namespace System;
using namespace ClassLibrary1;

int main(array<System::String ^> ^args)
{
    Class1::Method2();
    return 0;
}

The C++/CLI project compiles with the following warning:

warning C4691: 'System::Func' : type referenced was expected in unreferenced assembly 'System.Core', type defined in current translation unit used instead 2> This diagnostic occurred while importing type 'ClassLibrary1::Class1 ' from assembly 'ClassLibrary1, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'.

Note that a similar .NET 4.0 C# project using this library will compile without warning, so this is specific to C++/CLI. Referencing System.Core from the project doesn't remove the warning. The warning seems useful in general so I'm reluctant to disable it, is there anything else I can do about it?

This was observed in Visual Studio 2012 and 2013.

Answer 1

This is possibly a framework version mismatch as your C# project is built in .net 3.5 and you are referencing it in a c++ project that's using .net 4.0

My suggestion would be to assign the same framework version for both the projects, either 3.5 or 4.0

Here is a link that explains this warning in a bit more detail.

Answer 2

The most likely reason for this is the covariance changes for .Net 4.0. In this case, the .Net 3.5 definition is:

public delegate TResult Func<TResult>()

While the .Net 4.0 definition is:

public delegate TResult Func<out TResult>()

Using other .Net library items (I tried List<T> ) without any changes does not produce this error.

I would only be concerned about this warning if I was attempting to use the delegate in a covariant manner. With a Func<int> that is unlikely.