可能限制通用方法的输出？

Question

I have an interface which defines various filtering on data (queries coming from EF4).

Interface method:

IQueryable<T> filter<T>() where T : class;

Now in a concrete implementation of that interface, I want to be able to do:

public IQueryable<T> filter<T>() {
  if (...) return query.OfType<Foo>().Take(100);
  if (...) return query.OfType<Bar>().Blah();
  // etc
}

But of course that doesn't work as the function signature expects T and not Foo or Bar . Is there some simple way to cast this output, or do I need to forgo the generic approach?

Answer 1

Assuming Foo and Bar classes can both be cast as T , something like this would work:

public IQueryable<T> filter<T>() {
  if (...) return query.OfType<Foo>().Take(100).Cast<T>();
  if (...) return query.OfType<Bar>().Blah().Cast<T>();
  // etc
}

However, you need to make sure that they can both be cast as T , or else you'll obviously get InvalidCastExceptions. So you would be well-served to make sure T can be cast to either type by changing your declaration to someting along the lines of:

public IQueryable<T> filter<T>() where T : IFooBar

Where IFooBar is the base class/interface for both Foo and Bar