Visual Studio 2015扩展方法调用方法组

Question

I have an extension method like

public static void RemoveDetail<TMaster, TChild>(this TMaster master, TChild child)
        where TMaster : class, IMaster<TChild>
        where TChild : class, IDetail<TMaster>;

And I have two classes

public class Principal : IMaster<Permission>
{
        public virtual IEnumerable<Permission> Permissions { get; }
}

and

public class Permission : IDetail<Principal>

I call RemoveDetail from an Action accepted by method public static void Foreach<T>(this IEnumerable<T> source, Action<T> action); :

aPrincipal.Permissions.Foreach(x => aPrincipal.RemoveDetail(x));

ReSharper suggest me to replace this call with method group like

aPrincipal.Permissions.Foreach(aPrincipal.RemoveDetail);

This worked fine in VS2013 and previous but it fails on compilation in VS2015 with

'Principal' does not contain a definition for 'RemoveDetail' and no extension method 'RemoveDetail' accepting a first argument of type 'Principal' could be found (are you missing a using directive or an assembly reference?)

Anyone has a suggestion? Do I have to update all of the usages and make ReSharper to avoid this replacement?

Answer 1

I was able to reproduce your problem

public interface IDetail<T>
{

}

public interface IMaster<T>
{

}

public class MyClass
{
    public void method()
    {
        Principal aPrincipal = new Principal();
        aPrincipal.Permissions.Foreach(x => aPrincipal.RemoveDetail(x)); // No suggestion from resharper
    }
}

public class Permission : IDetail<Principal>
{

}

public class Principal : IMaster<Permission>
{
    public virtual IEnumerable<Permission> Permissions { get; }
}

public static class Class
{
    public static void RemoveDetail<TMaster, TChild>(this TMaster master, TChild child)
        where TMaster : class, IMaster<TChild>
        where TChild : class, IDetail<TMaster>
    {

    }

    public static void Foreach<T>(this IEnumerable<T> source, Action<T> action)
    {

    }
}

The resharper does not suggest anything. so probably you have to update your Resharper to newer version. probably was a bug which is fixed now.

If this is not like the way you do. then you have to put more information.

Also Compiler gives same error when i try to convert it to method group. So this question may remain: Why Compiler cant do this?

Edit:

To fix this problem you have to call the RemoveDetail method from inside Principal. so make your Principal class like this.

public class Principal : IMaster<Permission>
{
    public virtual IEnumerable<Permission> Permissions { get; }

    public void RemoveDetail(Permission p)
    {
        Class.RemoveDetail(this, p);
    }
}

I think there is kind of ambiguous inside compiler (Possibly bug) that cant recognize RemoveDetail method. how ever compiler tries to find it inside Principal . so you can fix it with creating RemoveDetail inside Principal and call a static RemoveDetail from there.

Edit 2:

The problem is generic type constraint.

where TMaster : class, IMaster<TChild>

This makes compiler to look at class which implements IMaster<TChild> and thats Principal . if you remove this where clause it will solve the problem. otherwise the compiler expects that it should be Principal .

If you use lambda you remove this ambiguity.

aPrincipal.RemoveDetail // compiler expects property/field/method in Principal
                        // but compiler forgets method group and static generic method!
x => aPrincipal.RemoveDetail(x) // this is no more like property or field
                                //but a method that is considered static generic method!

So thats a C#6 bug

Answer 2

I gave a feedback on this subject to Microsoft on September the 3rd 2015 here:
https://connect.microsoft.com/VisualStudio/feedback/details/1747835/visual-studio-2015-extension-method-call-as-method-group

Today I got this feedback from Neal [MSFT]:

This is fixed in VS2015 Update 1.
It is tracked here: https://github.com/dotnet/roslyn/issues/4970

Answer 3

I don't see how it worked in VS2013. RemoveDetail is a method of TMethod not your Action MyClass .

It should be something like

z.ForEach(x => master.RemoveDetail(x);