Implementing an interface with a generic constraint

Question

Bit surprised why this does not work

Is this a limitation of the compiler or does it make good sense not to support it?

public class Class1<T> : IInterface
    where T : Test2
{
    public T Test { get; private set; }
}

public class Test2
{
}

internal interface IInterface
{
    Test2 Test { get; }
}

The error I get is

'ClassLibrary1.Class1<T>' does not implement interface member 'ClassLibrary1.IInterface.Test'. 
'ClassLibrary1.Class1<T>.Test' cannot implement 'ClassLibrary1.IInterface.Test' because it does not have the matching return type of 'ClassLibrary1.Test2'.

Why does Class1 need to be generic? If return type of Test is Test2, then make it return this type and you're done. You will be able to return any types derived from Test2 anyway. — Lou
– Lou, Commented Oct 10, 2012 at 11:50
@Lou just imagine that instead of Class2 we head Interface2. That's the problem I'm facing now. — user3303864
– user3303864, Commented Dec 16, 2018 at 20:52

cuongle · Accepted Answer · 2012-10-10 11:26:55Z

13

For more corrective, implement interface explicitly:

public class Class1<T> : IInterface
where T : Test2
{
    public T Test { get; private set; }

    Test2 IInterface.Test
    {
        get { ... }
    }
}

Then you can avoid compiled error.

edited Oct 10, 2012 at 11:26

answered Oct 10, 2012 at 11:15

cuongle

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4 Comments

Rawling Over a year ago

Or indeed return Test instead of throw new NotImplementedException().

cuongle Over a year ago

@Rawling: it's up to OP, maybe he want to implement in different way

Rawling Over a year ago

Fair enough. Anyway this does explain why the compiler doesn't support this kind of covariance directly - there's a workaround so the compiler team spent their time doing more important stuff :)

buckley Over a year ago

Mr Lippert made the case that every feature, how small, has a big cost. So I accept this answer as I tend to believe the compiler has no other obvious reason to disallow it

Jon · Accepted Answer · 2012-10-10 11:09:32Z

5

Since T can be any type derived from Test2, Class1 does not implement IInterface exactly.

More generally, it is not possible to implement an interface by returning a covariant type:

interface IFoo
{
    object Bar { get; }
}

class Broken : IFoo
{
    string Bar { get; } // you cannot expect to implement IFoo this way!
}

answered Oct 10, 2012 at 11:09

Jon

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2 Comments

Rawling Over a year ago

"Class1 does not implement IInterface exactly" - well, class1.Test does always return a Test2, right? Similarly broken.Bar always returns an object.

Kirill Bestemyanov Over a year ago

Signature of methods in interface and class is not equal. So class is not implement interface.

cauon · Accepted Answer · 2012-10-10 11:14:26Z

4

Change your interface to this and it will compile:

public class Class1<T> : IInterface<T>
    where T : Test2
{
    public T Test { get; private set; }
}

public class Test2
{
}

internal interface IInterface<T>
    where T : Test2
{
    T Test { get; }
}

answered Oct 10, 2012 at 11:14

cauon

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Comments

Charleh · Accepted Answer · 2012-10-10 11:15:06Z

1

Is it feasible for you to make your interface generic e.g.

public class Class1<T> : IInterface<T>
    where T : Test2
{ 
    public T Test { get; private set; } 
} 

public class Test2 
{ 
} 

internal interface IInterface<T>
{ 
    T Test { get; } 
}

Or are you trying to avoid generics on the interface (there are good reasons for that too!)

answered Oct 10, 2012 at 11:15

Charleh

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1 Comment

buckley Over a year ago

Yep, was trying to avoid it so I could write generic code where the compiler otherwise would expect a type parameter

Kirill Bestemyanov · Accepted Answer · 2012-10-10 11:22:26Z

1

Interface say that property Test is of Test2 Type. In your implementation Class1 property Test is of some class that inherited of Test2 but not exactly it. To do what you want, you need write something like this:

public class Class1<T> : IInterface
    where T : Test2
{
    private T _test;
    public Test2 Test { get{return _test} }
}

public class Test2
{ 
}

internal interface IInterface 
{
    Test2 Test { get; }
}

answered Oct 10, 2012 at 11:22

Kirill Bestemyanov

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