Question

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What is the purpose of a marker interface?

Is it bad practice to create a completely empty interface such as:

public interface ISomething
{
}

There is a case where I would like to treat certain objects differently than others, but I don’t require any new behavior.

Answer 1

I personally think that empty interfaces are not a good thing.

Interfaces are meant to describe behavioral contracts. An empty interface doesn’t describe any behavior – you’re not defining any form of contract here.

The design guidelines for interfaces specifically says:

Avoid using marker interfaces (interfaces with no members).

Custom attributes provide a way to mark a type. For more information about custom attributes, see Writing Custom Attributes. Custom attributes are preferred when you can defer checking for the attribute until the code is executing. If your scenario requires compile-time checking, you cannot comply with this guideline.