Why we can’t use sealed classes as generic constraints?

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.

Are you talking about something like this:

class NonSealedClass
{
}

class Test<T> where T : NonSealedClass
{
}

Because it's perfectly legal.

A naked constraint is where one generic type inherits from another e.g.

where X:Y

One generic parameter derives from another generic parameter

class Foo<T>
{
    Foo<S> SubsetFoo<S>() where S : T {  }
}

So the class cannot be sealed.

You can also inherit from generics in the normal way so you would not want them sealed.