I was working on some mocking stuff today and I ran into an odd situation. Let's say you have the following class hierarchy:

public abstract class Cup
{
}

public class CoffeeCup : Cup
{
}

public class Shotglass : Cup
{
}

Now, let's define a factory that randomly creates cups:

public static class CupCreator
{
    public static T Create<T>() where T : Cup
    {
        if(DateTime.Now.Second % 2 == 0)
        {
            return new Shotglass();
        }
        else
        {
            return new CoffeeCup();
        }
    }
}

The problem is, the two return statements are in error - here's what the error statement looks like:

Cannot implicitly convert type 'Shotglass' to 'T'

It's error CS0029.

Here's the funny thing. 10 years ago, I would've assumed that there's something wrong with the compiler. I'm older now and I know that I'm probably the idiot :).

When I first did this, I thought, OK, T must be at least a Cup...so returning a CoffeeCup or a Shotglass should be OK, right?

Well, no. What if I call it like this?

var shot = CupCreator.Create<Shotglass>();

Then returning a CoffeeCup is invalid.

Just because T is constrained to be a Cup and the two return types descend from Cup doesn't mean the code is valid. It took me a bit to figure this one out!

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