Why I cannot cast derived generic type to base non-generic type (through a constrain)?

问题

Given this fictional example:

class NonGeneric
{
}

class Generic<T> : NonGeneric
    where T : NonGeneric
{
    T DoSomething()
    {
        return this; // **
    }
}

I'd expect it compiles: Generic<T> derives from NonGeneric and T must be a derived class so it satisfies its constrain.

I should be able to do this:

NonGeneric obj = new Generic<NonGeneric>();

Then there should be no problem in this instruction:

return this;

Or at least this:

return (T)this;

Unfortunately it doesn't work and above example doesn't compile with error:

Cannot convert type NonGeneric<T> to 'T'

I'm doing something wrong, and I can't see it, or it's just not allowed? Why this?

I would avoid, if possible, any workaround like the one I describe in this post (Reflection, dynamic compiled methods and so on). I would avoid dynamic objects too (design decision, I can't change that).

回答1:

Have you tried this?

T DoSomething()
{
    return this as T;
}

this is not implicitly the same as T so there is a need to cast it explicitly as in the above example.

回答2:

this is NonGeneric, T is NonGeneric, but this isn't necessarily T. They could be different subclasses, from the perspective of the compiler.

Consider the class:

class Red : NonGeneric
{
}

Then your generic class becomes:

class Generic<Red> : NonGeneric
    where Red : NonGeneric // constraint satisfied
{
    Red DoSomething()
    {
        return this;
    }
}

But this isn't Red, it's Generic<Red>, which is a different subclass of NonGeneric than Red.

来源：https://stackoverflow.com/questions/21432141/why-i-cannot-cast-derived-generic-type-to-base-non-generic-type-through-a-const

标签

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generics

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