Question

Can you guess what is the reason to not allow sealed classes for type-constraints in generics? I only have one explanation is to give opportunity to use naked constraints.

Answer 1

If the class is sealed it cannot be inherited. If it cannot be inherited it’d be the only type valid for the generic type argument [assuming if allowed to be a type argument]. If it is the only generic type argument then there’s no point in making it generic! You can simply code against the type in non-generic class.

Here’s some code for this.

public class A
{
    public A() { }
}

public sealed class B : A
{
    public B() { }
}

public class C<T>
        where T : B
{
    public C() { }
}

This will give compiler error: ‘B’
is not a valid constraint. A type used
as a constraint must be an interface,
a non-sealed class or a type
parameter.

In addition to this, You can also not have a static class as generic type-constraint. The reason is simple. Static classes are marked as abstract and sealed in compiled IL which can be neither instantiated nor inherited.

Here’s the code for this.

public class D<T>
        where T : X
{
    public D() { }
}

public static class X
{
}

This will give compiler error:‘X’:
static classes cannot be used as
constraints.