Question

I am trying to work around a mocking issue by creating a custom mock of IDbSet.

The custom mock:

public class DbSetMock : IDbSet<Tenant>
{
    /* hidden all other implemented methods/properties */

    public TDerivedEntity Create<TDerivedEntity>() where TDerivedEntity : class, Tenant
    {
        throw new NotImplementedException();
    }
}

The create method gives a build error that I have no clue on how to solve:

cannot specify both a constraint class and the ‘class’ or ‘struct’ constraint

Simply removing class from the constraints results in another build error (which I also don’t understand 🙁 ).

The constraints for type parameter ‘TDerivedEntity’ of method ‘Tests.DAL.Tenants.DbSetMock.Create<TDerivedEntity>()’ must match the constraints for type parameter ‘TDerivedEntity’ of interface method ‘System.Data.Entity.IDbSet<BusinessLayer.DAL.Tenants.Tenant>.Create<TDerivedEntity>()’. Consider using an explicit interface implementation instead.

Can anybody help me successfully building this class?

Answer 1

Since the TDerived type parameter is constrained to be a Tenant, adding the constraints class or struct is redundant. Just remove the class constraint.

UPDATE: Curiously there seems to be a conflict between the compiler errors here. If you “fix” one you get the other, in an infinite loop of despair. Luckily, the second error also gives us a way out: you can use an explicit interface implementation:

public class DbSetMock : IDbSet<Tenant>
{

    TDerivedEntity IDbSet<Tenant>.Create<TDerivedEntity>()
    {
        throw new NotImplementedException();
    }

}

There seems to be no way to implement that method without using explicit interface implementation. If you need it as part of the public interface of the class, I suggest creating another method that the interface implementation forwards to:

public class DbSetMock : IDbSet<Tenant>
{

    TDerivedEntity IDbSet<Tenant>.Create<TDerivedEntity>()
    {
        return Create<TDerivedEntity>();
    }

    public TDerivedEntity Create<TDerivedEntity>() where TDerivedEntity : Tenant
    {
        throw new NotImplementedException();
    }

}