named parameter type constraints

Question

I am designing a custom attribute class.

public class MyAttr: Attribute
{
    public ValueRange ValRange { get; set; }
}

Then I am attempti

Answer 1

Is there any way around this?

Yes.

You can have your attribute use a Type property and then use types that implement a defined interface, for which the code that processes that attribute would have to assume, and as such also create an implicit, but hopefully documented, requirement to its clients:

public interface IValueRange {
  int Start { get; }
  int End { get; }
}
public class MyAttr : Attribute { 
  // The used type must implement IValueRange
  public Type ValueRangeType { get; set; } 
}

// ....

public class Foo { 

  class FooValueRange : IValueRange {
    public int Start { get { return 10; } }
    public int End { get { return 20; } }
  }
  [MyAttr(ValueRangeType = typeof(FooValueRange))]
  public string Prop { get; set; }

}

This is not unlike many classes in the System.ComponentModel namespace, like DesignerAttribute.