Any way to have some type parameters inferred from other type parameters' constraints in C#?

Question

Suppose I have the following code:

interface IWidget
{
}

interface IWidgetFactory<TWidget>
    where TWidget : IWidget
{
}

TWidgetFactory CreateFactory<TWidgetFactory, TWidget>()
    where TWidgetFactory : IWidgetFactory<TWidget>
    where TWidget : IWidget
{
    return ...
}

Whenever I call CreateFactory() I must pass in both the TWidgetFactory and TWidget type parameters. This seems unnecessary, because TWidgetFactory already has a constraint such that any specialisation of it must already specify TWidget. Is there any way I can have TWidget inferred automatically when calling CreateFactory() , even if I must add some kind of extra helper methods for it?

(The above is a simple example, but it can get much more complicated in practice, so this could save a lot of complexity.)

Answer 1

Without seeing the body of CreateFactory , it is difficult to tell what you're doing. Based on the return type, I would suspect CreateFactory doesn't actually use TWidget for anything but as the type parameter to the interface. In this case, could you relax your generic constraints a bit?

interface IWidget
{
}

interface IWidgetFactory {} // A new non-generic base interface

interface IWidgetFactory<TWidget> : IWidgetFactory
    where TWidget : IWidget
{
}

// Generic constraints wind up not as specific, 
// but still provide some level of restriction
// TWidget can no longer be used within this method.
TWidgetFactory CreateFactory<TWidgetFactory>()
    where TWidgetFactory : IWidgetFactory
{
    return ...
}