使用反射在方法调用上获取正确的返回值

Question

I have the following generic method inside my class that works as a repository pattern:

public DbSet<T> GetAll<T>() where T : class
{
  return dbContext.Set<T>();
}

Now, i would like to get a list of all entities in the database that belong to an entity class that implements a specific interface ( IChangeTrackingEntity ). So currently there are around 10 specific tables/classes that conform to this, but i don't want to add 10 hardcoded calls to these tables, so I would like to do it using reflection instead (it might also be that the classes that implement this interface change in the future and I don't want to have to remember to change here as well and make the code dependant on each other).

Example of code that works, but that i don't want:

var result = new List<IChangeTrackingEntity>();
using ( var repository = new DbRepository())
{
  result.AddRange( repository.GetAll<FirstTypeThatImplementsInterface>() );
  result.AddRange( repository.GetAll<SecondTypeThatImplementsInterface>() );
  result.AddRange( repository.GetAll<ThirdTypeThatImplementsInterface>() );
  result.AddRange( repository.GetAll<FourthTypeThatImplementsInterface>() );
  result.AddRange( repository.GetAll<FifthTypeThatImplementsInterface>() );
}
return result;

I am quite close, but I can't get the last part to work of casting the result of the Invoke back to the correct type. Waht i got currently is this:

var result = new List<IChangeTrackingEntity>();
var method = typeof (DbRepository).GetMethod("GetAll");
using ( var repository = new DbRepository())
{
  foreach (var p in typeof(AnchorDbContext).GetProperties())
  {
    if (p.PropertyType.IsGenericType && p.PropertyType.GetGenericTypeDefinition() == typeof(DbSet<>))
    {
      var pType =  p.PropertyType.GetGenericArguments()[0];
      if (pType.GetInterface("IChangeTrackingEntity") != null)
      {
        var genericMethod = method.MakeGenericMethod(new[] {pType});
        result.AddRange(genericMethod.Invoke(repository, null) as DbSet<IChangeTrackingEntity>);
      }
    }
  }
  return result;
}

The problem above it that the Invoke call return a object and I need to cast it to basically DbSet<pType> .

In my current code genericMethod.Invoke(repository, null) as DbSet<IChangeTrackingEntity> returns null indicating that I can't cast the return value as I want, so I need the specific return value type and not the interface I think.

Any idea of how to accomplish this?

Answer 1

Try casting to IEnumerable<IChangeTrackingEntity> . This should work due to co/contravariance.

Answer 2

I don't know much about this specific issue, but you seem to be casting from DbSet<T> where T : IChangeTrackingEntity to DbSet<IChangeTrackingEntity> . This is called covariance or contravariance (I always get confused between them...) and it only works if DbSet<> is an interface. So, casting won't work here. Use an equivalent interface if you can, or make a generic method that accepts DbSet where T: IChangeTrackingEntity and returns DbSet<IChangeTrackingEntity> somehow. I'll try to work out how to do that, and post an answer, if no one has answered before me (unlikely on this site :P)

Answer 3

I'm thinking you need to see this question :

MethodInfo method = typeof(Sample).GetMethod("GenericMethod");
MethodInfo generic = method.MakeGenericMethod(myType);
generic.Invoke(this, null);