Question

I’m experimenting with the following mockable repository pattern:

public interface IEmployeeRepository
{
    ITable<Employee> Employees { get; }
}

This represents a container capable of returning an ITable of Employee objects. I want to be able to create a mock object based off of the repository, as well as have my Linq-to-Sql DataContext implement the repository interface. So I thought I could just use partial class to declare my DataContext type to implement IEmployeeRepository, as it already has an auto-generated member Employees of type Table<Employee>:

public partial class MyDataContext : IEmployeeRepository { }

I get the following error message as a result:

‘MyDataContext’ does not implement interface member
‘IEmployeeRepository.Employees’. ‘MyDataContext.Employees’ cannot
implement ‘IEmployeeRepository.Employees’ because it does not have the
matching return type of ‘System.Data.Linq.ITable’.

But Table<Employee> inherits ITable<Employee>, so shouldn’t it be a suitable return type to implement the interface?

Answer 1

The implementing class’ type must match the interface type exactly. If you can’t change the interface, you could implement it explicitly:

class EmployeeRepository : IEmployeeRepository
{
    // existing property
    public Table<Employee> Employees { get; }

    // explicit IEmployeeRepository property
    ITable<Employee> IEmployeeRepository.Employees { get { return Employees; } }
}