Are there any rules for type conversion between C# generics?

Question

I'm having an interface IJob from which all Job classes inherits and I have generic repo class which will process all kind of IJob's.

The problem im facing is im not able to able convert Repository<Job1> to Repository<IJob> even though Job1 is a type of IJob.

Code

internal class Program
{
    public interface IJob { }
    public class Job1 : IJob { }
    public class Job2 : IJob { }

    public class Repository<TJob>
        where TJob : IJob
    {
        public List<TJob> GetJobs()
        {
            return new List<TJob>();
        }
    }

    private static void Main(string[] args)
    {
        IJob iJob = new Job1(); // Valid
        Repository<IJob> repo = new Repository<Job1>(); // Not Valid
    }
}

Can someone let me know why this wont be possible in C# and are there any other rules type conversion in C# generics?

Answer 1

If you want covariance, then you'll have to introduce a generic interface for the repository and change the return type of the GetJobs method to IEnumerable<TJob>:

internal class Program
{
    public interface IJob { }
    public class Job1 : IJob { }
    public class Job2 : IJob { }

    public interface IRepository<out TJob> where TJob : IJob
    {
        IEnumerable<TJob> GetJobs();
    }

    public class Repository<TJob> : IRepository<TJob> where TJob : IJob
    {
        public IEnumerable<TJob> GetJobs()
        {
            return new List<TJob>();
        }
    }

    private static void Main(string[] args)
    {
        IJob iJob = new Job1(); // Valid
        IRepository<IJob> repo = new Repository<Job1>(); // Also valid
    }
}

For more information about covariance and contravariance, see: https://msdn.microsoft.com/en-us/library/dd799517%28v=vs.110%29.aspx

Answer 2

Repository<IJob> repo = new Repository<Job1>(); // Not Valid

Yes, because it isn't an Repository<IJob>. With an Repository<IJob>, this line would be valid:

repo.GetJobs().Add(new Job2());

Now, with a Repository<Job1>, this line is invalid. It won't compile.

So the error you get is correct. A Repository<Job1> is not a Repository<IJob>.