EntityFramework GroupBy Entity Type

Question

I have a base entity A, and derived entities: AA, AB, AC, AD

context.A.GroupBy(x => x.GetType()).Select(x => new { type=x.GetType(), count = x.Count() });

Of course I get error:

LINQ to Entities does not recognize the method 'System.Type GetType()' method, and this method cannot be translated into a store expression.

How can i group derived entities by type?

Answer 1

如果类型是预定义的，则可以使用Enumerable.OfType方法

var acs = context.A.OfType<AC>().Count();

Answer 2

You need to hydrate your entities to memory before calling the GroupBy - this way the x.GetType() call will be handled by LINQ to Objects and not LINQ to Entities. Try doing this:

context.A.ToList().GroupBy(x => x.GetType()).Select(x => new { type=x.Key, count = x.Count() });

I changed it so you call x.Key and not x.GetType() in your select, otherwise you will get the type of the grouping which will be something like IGrouping<...>