Type constraints in Attributes

Question

I want to write my enum with custom attributes, for example:

public enum SomeEnum: long
{
    [SomeAttribute<MyClass1>]
    Sms = 1,
    [SomeAttribute<MyClass2>]
    Email = 2
}

but attributes doesn't support generics. Well, the most similar solution is:

public enum SomeEnum: long
{
    [SomeAttribute(typeof(MyClass1))]
    Sms = 1,
    [SomeAttribute(typeof(MyClass2))]
    Email = 2
}

And here is problem: I want Class1 to be inherited from ICustomInterface, so with generics I can write constraint:

[AttributeUsage(AttributeTargets.All)]
class SomeAttribute<T> : Attribute where T: ICustomInterface
{
}

but attributes doesn't support generics.

so finally question is: how can I check in compile time (like T constraints) that type is implementing some interface?

Answer 1

Very simple to your final question:

so finally question is: how can I check in compile time (like T constraints) that type is implementing some interface?

You can not do that.

But you can check it at runtime, with some reflection methods like: Type.IsAssignableFrom

Answer 2

While i've had similar problems you won't get compile time checking for this.

For now this:

public class SomeAttribute : Attribute
{
    public SomeAttribute(Type given)
    {
        Given = given;
        Required = typeof (INotifyDataErrorInfo);
    }

    public Type Given { get; set; }
    public Type Required { get; set; }

    public bool Valid()
    {
        return Required.IsAssignableFrom(Given);
    }
}

public enum TestEnum 
{
    [Some(typeof(string))]
    Sms = 1,
    [Some(typeof(string))]
    Email = 2
}

Is far as you're gonna get sadly.

Though as far as i can recall, if you use PostSharp there is a way to invoke code dependant compile time checks if that's what you're after. That may not point out flaws visually in your IDE, but it still ensures that other devs have to ensure that a certain type is passed.