为什么不能将通用类型转换为继承类型？

Question

I have some method with the following definition;

 public void SomeMethod<T>() where T : BaseClass, new()
 {
      InheritedClass instance = (InheritedClass)instanceOfT; //won't compile
 }

Why is that cast not allowed? How can you get around this? I have about 200 lines of code which is all suited for generics except I need a one LINQ query for each of the inheriting classes (there are 3). I was trying to do a typeof check, then follow that with a cast, then execute the appropriate query but the compiler won't let me do the cast... Given I just checked to ensure that T is in fact an instance of type InheritedClass there is no way it would fail, why won't the compiler let me do it?

Answer 1

You need to cast the variable to object and then back down to do this. As it is the compiler believes that there is no possible way that the cast could succeed, but it doesn't ever apply that check to variables of type object .

InheritedClass instance = (InheritedClass)(object)instanceOfT;

Answer 2

Because the actual instanceOfT's type (let's call it InheritedClassB ) may not be related to InheritedClass .

Given I just checked to ensure that T is in fact an instance of type InheritedClass

Actually, you're only checking that T extends BaseClass.

Edit: This looks like a code smell.. Can't you let the concrete class handle this method? So each concrete class would have its own implementation, and know how to perform SomeMethod .