问题
Why the following code get the error?
Invalid variance: The type parameter 'T' must be invariantly valid on 'UserQuery.IItem<T>.GetList()'. 'T' is covariant.
public interface IFoo {}
public interface IBar<T> where T : IFoo {}
public interface IItem<out T> where T: IFoo
{
IEnumerable<IBar<T>> GetList();
}
回答1:
The interfaces IBar and IItem do not agree on variance: in your IBar declaration, the T is not covariant, as there is no out keyword, whereas in IITem the T is covariant.
回答2:
The following code will get rid of the error.
public interface IFoo {}
public interface IBar<out T> where T : IFoo {}
public interface IItem<out T> where T: IFoo
{
IEnumerable<IBar<T>> GetList();
}
来源：https://stackoverflow.com/questions/12484293/invalid-variance-the-type-parameter-t-must-be-invariantly-valid-on-xxx-iitem