Question

Say I have the following class:

public class MyClass<T>
{
    public MyClass() {
        //construct logic
    }

    public IProperty<T> MyProperty { get; set; }
}

public interface IProperty<T> { }

public DateTimeProperty : IProperty<DateTime>
{

}

If I try to put this code into the constructor of MyClass:

if (typeof(T) == typeof(DateTime))
    MyProperty = new DateTimeProperty();

I get the following compilation error:

Cannot implicitly convert type ‘DateTimeProperty’ to ‘IProperty’

How can I, in the constructor of MyClass, set the value of MyProperty to be a new DateTimeProperty, only if T is of type DateTime?

Answer 1

Note that generics should really be exactly that; generic; what you are doing is specialized (for some T), which is kinda not the point. However, as for how – cast:

MyProperty = (IProperty<T>)(object)new DateTimeProperty();

By casting to object and then back up to (IProperty<T>) you remove the compiler’s ability to apply static type-checking rules. And the downside: you remove the compiler’s ability to apply static type-checking rules.