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Declaring interface members with generic return types: "The type parameter [generic type] cannot be used with type arguments"
I'm trying to write an interface to define the set of operations I expect my repositories' Unit of Work implementations to have, and I want this interface to be fulfilled by EF Core's DbContext class.
So far this is what I got: (I know the name is awful, suggestions accepted):
public interface IDbContextable<TContext, TEntityEntry>
where TContext : class, IDisposable
where TEntityEntry : class
{
public TEntityEntry Add([NotNullAttribute] object entity);
public TEntityEntry<TEntity> Add<TEntity>([NotNullAttribute] TEntity entity)
where TEntity : class;
public int SaveChanges();
}
The first Add method's signature from EF Core is: public virtual EntityEntry Add([NotNullAttribute] object entity); - and it works fine.
I'm having trouble with the second. The compiler complains:
Error CS0307 The type parameter 'TEntityEntry' cannot be used with type arguments
For completeness, my context class looks like this:
public partial class EntityFrameworkContext : DbContext, IDbContextable<EntityFrameworkContext, EntityEntry>
{
//... some extra methods here
}
How to properly declare the interface to get rid of the error, and so that the signature: public virtual EntityEntry<TEntity> Add<TEntity>([NotNullAttribute] TEntity entity) where TEntity : class; from EF Core becomes valid?
Thanks.
2 answers
The short answer is that it can't be done. C# doesn't support type functions. https://github.com/dotnet/csharplang/issues/339 is probably the issue to follow if you want to track their progress.
Workarounds are going to require another type parameter and probably a factory method: e.g.
public TEntityEntry_TEntity Add<TEntity, TEntityEntry_TEntity>(
[NotNull] TEntity entity,
[NotNull] Func<TEntity, TEntityEntry_TEntity> wrap)
where TEntity : class
where TEntityEntry_TEntity : TEntityEntry;
I believe an implementation of the answer by Peter Taylor above would be:
public override EntityEntry<TEntity> Add<TEntity>([NotNullAttribute] TEntity entity)
where TEntity : class
{
return Add(entity, base.Add);
}
public TEntityEntry_TEntity Add<TEntity, TEntityEntry_TEntity>([NotNullAttribute] TEntity entity,
[NotNullAttribute] Func<TEntity, TEntityEntry_TEntity> wrap)
where TEntity : class
where TEntityEntry_TEntity : EntityEntry
{
return wrap(entity);
}
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