Generic type constraint for interface?

Question

I can do this:

void MyMethod<T>() where T : class { }

Is there anything like this?

void MyMethod<T>() where T : interface { }

I'd prefer not to specify the interface name explicitly.

The alternative is that I can pass in an argument that's constrained by class and throw an exception if typeof(T).IsInterface returns false, but that's not as clean as a constraint.

This sounds like a solution to an XY Problem - could you show more about what you're trying to solve by wanting to do this? — James Thorpe
– James Thorpe, Commented Jan 20, 2016 at 15:26
Possible duplicate of How can I use interface as a C# generic type constraint? — Ivan Danilov
– Ivan Danilov, Commented Apr 23, 2017 at 11:16

Andrey Korneyev · Accepted Answer · 2016-01-20 15:30:20Z

1

This makes no sense since it doesn't leads to any limitations.

Constraint class means that the type argument must be a reference type.

So what interface without explicit interface name should limit?

Interface itself is just a declaration of methods/properties, so constraint "something is interface" literally mean "something has some declarations" - that is useless.

edited Jan 20, 2016 at 15:30

answered Jan 20, 2016 at 15:25

Andrey Korneyev

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9 Comments

oscilatingcretin Over a year ago

You know what, you're right. There can be no instance of an interface, so nothing will ever be passed in that is only an interface. If only I could delete this question.

Servy Over a year ago

All interface types must be reference types.

Falanwe Over a year ago

Answer is wrong: it would limit T to be an interface type. So MyMethod<IDisposable>() or MyMethod<IEnumerable<string>> would work fine, whereas MyMethod<string>()would not work. OP is right, this constraint does not exist in C# right now. But the real question is: would such a feature be really useful? I can't think of a use case.

Falanwe Over a year ago

@oscilatingcretin: another use case that frequently arises is when you are using some IoC (for instance Autofac). In that case, you will often call scope.Resolve<IMyContract>() and get some instance implementing IMyContract without knowing what is the concrete type of the instance you receive. And most of the time, you just don't care.

Ivan Danilov Over a year ago

@Falanwe I second, it is an incorrect answer. There're even real cases where it is necessary. For example if you want to proxy an instance implementing certain interface - with interface type as T you can be certain that you can proxy everything just fine. But if someone provides sealed class there or class with non-virtual methods - the code will fail miserably at runtime.

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