C# Casting Generic Type

Question

I have a generic interface IConstrained which is implemented by the generic Constrained class. When I attempt to do the code below I get an invalid cast exception.

IConstrained<decimal> decimalLimit = new Constrained<decimal>(1);
IConstrained<IComparable> comparableLimit = (IConstrained<IComparable>) decimalLimit;

Why is it not possible to do this if decimal implements IComparable? What would be the correct way to do this? Thanks.

Answer 1

Generic types are not covariant in .NET 2.0. This includes .NET 3.0/3.5 as well since they use the same 2.0 runtime. .NET 4.0 will support covariance, however.