How to implement generic GetById() where Id can be of various types

Question

I am trying to implement a generic GetById(T id) method which will cater for types which may have differing ID types. In my example, I have an entity which has an ID of type int, and one of type string.

However, I keep getting an error and I have no idea why:

The type 'int' must be a reference type in order to use it as parameter 'TId' in the generic type of method IEntity

Entity Interface:

To cater for my domain models which can have Id's of type int or string.

public interface IEntity<TId> where TId : class
{
    TId Id { get; set; }
}

Entity Implementations:

public class EntityOne : IEntity<int>
{
    public int Id { get; set; }

    // Other model properties...
}

public class EntityTwo : IEntity<string>
{
    public string Id { get; set; }

    // Other model properties...
}

Generic Repository Interface:

public interface IRepository<TEntity, TId> where TEntity : class, IEntity<TId>
{
    TEntity GetById(TId id);
}

Generic Repository Implementation:

public abstract class Repository<TEntity, TId> : IRepository<TEntity, TId>
    where TEntity : class, IEntity<TId>
    where TId : class
{
    // Context setup...

    public virtual TEntity GetById(TId id)
    {
        return context.Set<TEntity>().SingleOrDefault(x => x.Id == id);
    }
}

Repository Implementations:

 public class EntityOneRepository : Repository<EntityOne, int>
    {
        // Initialise...
    }

    public class EntityTwoRepository : Repository<EntityTwo, string>
    {
        // Initialise...
    }

Answer 1

You should remove the constraint on TId from your Repository class

public abstract class Repository<TEntity, TId> : IRepository<TEntity, TId>
where TEntity : class, IEntity<TId>
{
    public virtual TEntity GetById(TId id)
    {
        return context.Set<TEntity>().Find(id);
    }
}

Answer 2

public interface IEntity<TId> where TId : class
{
    TId Id { get; set; }
}

The where TId : class constraint requires every implementation to have a Id which derives from object which is not true for value types like int.

Thats what the error message tells you: The type 'int' must be a reference type in order to use it as parameter 'TId' in the generic type of method IEntity

Just remove the constraint where TId : class from IEntity<TId>