How to make a derived concrete class implicitly cast a generic base class?

c#, generics

Solution

That is inherently unsafe.

Had that been legal, you would be able to pass a different class inheriting `Entity<int>` to your function that can only accept `IntEntity`s.

Instead, you can use the CRTP:

public abstract class Entity<TEntity, TId> 
    where TId : IEquatable<TId> 
    where TEntity : Entity<TEntity, TId>
{
    protected abstract Expression<Func<TEntity, TId>> GetEntityId();
}

Problem

I have a base abstract generic class, which shoud give an ability to get the identifier of an object through an expression: ``` public abstract class Entity<TId> where TId : IEquatable<TId> { protected abstract Expression<Func<Entity<TId>, TId>> GetEntityId(); } ``` But when I try to derive from it (and implement the abstract method): ``` public class IntEntity : Entity<int> { public int Id { get; set; } protected override Expression<Func<Entity<int>, int>> GetEntityId() { Expression<Func<IntEntity, int>> exp = e => e.Id; return exp; // CS0029 error } } ``` I get CS0029 compile error, saying Cannot implicitly convert type '`System.Linq.Expressions.Expression<System.Func<ConsoleApplication1.IntEntity,int>>`' to '`System.Linq.Expressions.Expression<System.Func<ConsoleApplication1.Entity<int>,int>>`' Is there anything I can do about this?

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