Question

I’m trying to check if something is an interface and I’m not sure I’m going about it the correct way. I have the underlying interface that I want to check against:

interface IName
{
  string Name {get;}
}

I then have a class that implements this interface:

class Person : IName
{
  public string Name {get;} 
}

I then have another interface:

interface IThing<T>
{
  T Thing {get;}
}

I then have another class that implements IThing:

class Teacher : IThing<Person>
{
  public Person Thing {get;}
}

What I want to be able to do is this:

Teacher teacher = new Teacher("Math", "John");
if (teacher is IThing<IName>)
{
  Console.WriteLine((teacher as IThing<IName>).Thing.Name);
}

This doesn’t work however. I’m pretty sure it can’t figure out that IThing<Person> is an IThing<IName>. How do I accomplish this? I basically have several classes that implement the IThing<BaseClassThatIsIName> so I don’t want to explicitly cast to the type, but rather the interface.

Answer 1

The problem is that you’re specifically trying to see if the teacher implements the IThing<IName> interface. It doesn’t however, it implements the IThing<Person> interface where Person implements the IName interface. You’re looking for some covariance here but the way you’ve defined the IThing<> interface doesn’t allow for it. Allow for it:

interface IThing<out T> // note the "out" in the parameter
{
    T Thing { get; }
}

Now the interface will match exact parameter types or more general types. You should be able to do what you want now:

if (teacher is IThing<IName>) // teacher is actually a `IThing<Person>` but
                              // `Person` is an `IName` so we're good
{
    Console.WriteLine((teacher as IThing<IName>).Thing.Name);
}