How to get the typeof(T) from a generic method with base type constraint?

Question

I have the following spec to help illustrate the problem:

class when_getting_type_of_generic_argument_using_subtype_instance
{
    static GenericTypeTester _genericTypeTester;
    static IPet _dog;
    static Type _result;

    Establish context =
        () =>
        {
            _genericTypeTester = new GenericTypeTester();

            _dog = new Dog();
        };

    Because of =
        () => _result = _genericTypeTester.Test(_dog);

    It should_return_the_subtype =
        () => _result.ShouldEqual(_dog.GetType());
}

class Dog : IPet
{
}

interface IPet
{
}

class GenericTypeTester
{
    public Type Test<T>(T dog) where T : IPet
    {
        return typeof (T);
    }
}

The spec above fails with the following message:

Expected: System.RuntimeType:[Pepino.ScenarioRunner.Selenium.Specs.Dog] But was: System.RuntimeType:[Pepino.ScenarioRunner.Selenium.Specs.IPet]

I need the result to be of type Dog. Is there something I can do without using reflection?

David · Accepted Answer · 2015-01-05 16:48:32Z

3

The issue here is the type being used at runtime versus compile time.

Because you declared _dog as an IPet, the variable passed into the generic method is an IPet at compile time despite being a Dog at runtime. Thus the compiler uses IPet for the generic parameter, even though the object at runtime is a Dog. Since you used typeof(T), you get the exact type given to the generic method by the compiler.

This can be seen by changing the type of _dog to Dog instead of IPet, which will then cause the compiler to infer the correct type.

This can also be avoided by explicitly casting the object as dynamic:

() => _result = _genericTypeTester.Test(_dog as dynamic);

Which will force the compiler to hold off the type inference until runtime, at which point it will determine that the object is type Dog. This is generally not a good idea for production code, however, since dynamic types are rather slow.

edited Jan 5, 2015 at 16:48

answered Jan 5, 2015 at 16:38

David

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4 Comments

Grax32 Over a year ago

Change static IPet _dog; to static Dog _dog; and you will see the type change to Dog

David Over a year ago

@Grax technically true, so I'll integrate that into my answer

Byron Sommardahl Over a year ago

@Grax is correct, but at runtime, I don't have that luxury. The type has to remain IPet. BUT, your "trick" of casting as dynamic made the spec pass immediately.

Grax32 Over a year ago

Perhaps your GenericTypeTester should return dog.GetType() instead of typeof(T) if you are wanting the runtime type of that variable.

Saul · Accepted Answer · 2015-01-05 16:42:18Z

0

Just add the constrait "class" to your method :

 class GenericTypeTester
 {
   public Type Test<T>(T dog) where T : IPet, class
   {
     return typeof (T);
   }
 }

answered Jan 5, 2015 at 16:42

Saul

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1 Comment

David Over a year ago

This would prevent the rest of the code as-is from compiling, since object _dog is an IPet and thus stops meeting the given constraints. It also doesn't solve the given problem asked

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