Extension methods with interface

Question

Assume we have this model :

public abstract class AbstractTableReferentielEntity {}
public class EstimationTauxReussite : AbstractTableReferentielEntity { }

I created a extension method for all classes that inherit from AbstractTableReferentielEntity.

public static EntityItemViewModel ToEntityItem<T>(this T entity)
    where T : AbstractTableReferentielEntity {}

But for one specific type of AbstractTableReferentielEntity (like EstimationTauxReussite), I would like to perform a specific action, so I created a second extension method.

 public static EntityItemViewModel ToEntityItem(this EstimationTauxReussite entity) {}

All extensions methods are declared in the same namespace.

After that, I retrieve some data from a DB with Entity Framework :

protected List<EntityItemViewModel> GetAllActifEntityItem<T>()
    where T : AbstractTableReferentielEntity
{
    return Context
        .Set<T>()
        .Where(item => item.IsActif)
        .Select(item => item.ToEntityItem())
        .ToList();
}

It compiles.

When T at runtime is a EstimationTauxReussite type, it goes into the wrong method ToEntityItem when I call Select(item => item.ToEntityItem()). It doesn't go into the most specific extension method. Any ideas ?

Answer 1

The reason is that extension methods are "syntactic sugar", ie they are a compiler trick. Your line:

.Select(item => item.ToEntityItem())

is effectively converted by the compiler to:

.Select(item => StaticClassWithExtensionMethod.ToEntityItem(item))

and then turned into IL. This means that the type of item has to be determined at compile-time, not runtime. So the AbstractTableReferentielEntity version of the extension method is used as that's the one tht matches the type at compile time.