Why is a nested struct inside a generic class considered "managed"?

Question

I have the following simple test:

class C<T>
{
    public struct A
    {
        int x;
    }
}

class Program
{
    static unsafe void Main(string[] args)
    {
        IntPtr p = new IntPtr();

        var a = (C<int>.A*)p.ToPointer();
    }
}

The code generates error CS0208:

Cannot take the address of, get the size of, or declare a pointer to a managed
type ('C<int>.A')

Can anyone explain why the struct is considered "managed" in this scenario?

Answer 1

Section 18.2 of the C# 5 spec:

An unmanaged-type is any type that isn’t a reference-type or constructed type, and doesn’t contain reference-type or constructed type fields at any level of nesting.

Now you might wonder if C<int>.A qualifies as a constructed type (its obviously not a reference type). The answer is yes. Section 4.4 defines constructed types. It specifically says nested types are considered constructed. It uses the Outer<T>.Inner below as an example:

class Outer<T>
{
    public class Inner {...}
    public Inner i;             // Type of i is Outer<T>.Inner
}

Answer 2

Because T, which is part of C is also part of A. Which means A is also generic. And all generic types are considered managed.

I guess, it would be possible to check if A uses T and decide on that. But as with all language features, it is feature, which's implementation would not have much return value.