Mixture of types in generic list

Question

Hello

I'm trying to do the following:

List<IRepository<IBusinessObject, ICriteria>> Repositories { get; }

and call this by

IRepository<ICustomer, ICustomerCriteria> cr = new CustomerRepository();

List.Add(CustomerRepository);

where ICustomer and ICustomerCriteria descend from IBusinessObject and ICriteria respectively.

However, this is not liked by the compiler.

Hmmm, I know I'm pushing it a bit, but I thought this would work? Anyone know why?

Thanks Duncan

Answer 1

C# doesn't support covariance for generics. Consider this Covariance and Contravariance in C#, Part One

Answer 2

This is a generic variance problem:

This will illustrate the issue:

List<string> Strings = ...;
List<object> Objects = Strings; // Should work

Objects.Add(42); // 42 is an object - Should work
// But we would add an integer into a list of strings!!

In .NET FX 4, C# will support a special kind of in and out generic parameters to allow correct behavior.