Question

public T CreateForm<T>() where T: BaseForm, BaseMainForm

I know the above means where T is a BaseForm and a BaseMainForm. But is it also possible to make the constraint that T has to be either a BaseForm or a BaseMainForm?

Answer 1

No, that is not allowed in C#. The compiler uses the generic constraint to determine what operations is available on T within the generic method – so allowing an or expression would not be type safe.

If you need this, consider adding an interface covering the common parts of BaseForm and BaseMainForm, and apply that as the generic constraint. This way, the interface defines the contract of what the method CreateForm<T> needs – and you must simply make sure that the Form’s you pass in implement the interface.

Something like:

public interface IBaseForm 
{
    Foo();
}

class BaseForm : IBaseForm {}
class BaseMainForm : IBaseForm {}

public T CreateForm<T>() where T : IBaseForm