Why Does Casting `MyEntityRepository` to `RepositoryBase` Fail in C#?

Generics and Casting: Understanding the Casting Limitation

Despite its common occurrence, casting inherited classes to base classes can be tricky, as exemplified by a runtime exception encountered when attempting the following code:

public abstract class EntityBase { }
public class MyEntity : EntityBase { }

public abstract class RepositoryBase<T> where T : EntityBase { }
public class MyEntityRepository : RepositoryBase<MyEntity> { }

MyEntityRepository myEntityRepo = GetMyEntityRepo(); // whatever
RepositoryBase<EntityBase> baseRepo = (RepositoryBase<EntityBase>)myEntityRepo;

This casting fails because RepositoryBase is not actually a base class of MyEntityRepository. As such, casting from MyEntityRepository to RepositoryBase is invalid.

The underlying reason for this limitation lies in the concept of generic variance. Generic variance refers to the ability of a type to change its parameterization in a covariant or contravariant manner. However, this form of generic variance is only partially supported in C#, primarily for generic interfaces and delegates.

In a more generic context, a covariant variation allows for a derived type to replace its base type in a given scenario. This means that MyEntityRepository, a type derived from RepositoryBase, could replace RepositoryBase in most instances. However, this assumption is not universally valid due to potential runtime conflicts.

For example, consider a method like this in RepositoryBase:

void Add(T entity) { ... }

Casting MyEntityRepository to RepositoryBase would open the door to adding non-MyEntity instances to the repository, which is undesirable. This demonstrates that unrestricted covariant generic variance can compromise the integrity of a program's architecture.

In C# 4, generic variance is permitted for reference types within generic interfaces and delegates, but not for classes. For more detailed information, refer to Microsoft's MSDN documentation, Eric Lippert's blog series, or the video of a presentation given at NDC in July 2010.

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