This has to do with Covariance and Contravariance.
In your question you mentioned the following:
... GenericQueryRepository implements IGenericQueryRepository and Client implements IClient.
Let's make it simpler by using fruits: Fruit implements IFruit. We'll also create a Tree class.
public interface IFruit { }
public class Fruit : IFruit { }
public class Tree<T> where T : IFruit { }
Tree<IFruit> tree = new Tree<Fruit>() // error
This will reproduce the same kind of error you're experiencing. Why? Simple.
Though Fruit implements IFruit, an Fruit Tree doesn't implement a IFruit Tree. There is no cast possible between the Fruit Tree and the IFruit Tree, although you would expect it. They are both Trees, but with a different type parameter. The fact that their type parameters are related to each other, doesn't matter.
In other words: there is no cast possible between the Fruit Tree and the IFruit Tree, because their type parameters don't match.
In general, when casting with generics, make sure their type parameters match. However, there are a few exceptional cases. See Variance in Generic Interfaces.
In your case, you could fix it by using IClient as type parameter for the GenericQueryRepository class. Doing this will allow casting because the type parameters match. But I don't know your application architecture, so this fix might be inapplicable in your case.
EDIT: To make it easier to understand, copy paste the code below and see what the compiler says.
interface IFruit { }
class Fruit : IFruit { }
interface ITree<T> where T : IFruit { }
class Tree<T> : ITree<T> where T : IFruit { }
class Program
{
static void Main(string[] args)
{
ITree<Fruit> test1 = new Tree<Fruit>(); // compiles: type parameters match
ITree<IFruit> test2 = new Tree<Fruit>(); // fails: type parameters don't match
ITree<Fruit> test3 = new Tree<IFruit>(); // fails: type parameters don't match
ITree<IFruit> test4 = new Tree<IFruit>(); // compiles: type parameters match
IEnumerable<IFruit> test5 = new List<Fruit>(); // compiles: this is one of the exceptional cases
}
}
That should clear things up about what is and what is not possible.