Where is the cast here? LINQ to Entities only supports casting Entity Data Model primitive types

Question

I have a number of entity framework tables that I have made support an interface `IHistoricEntity` using their parital classes. `IHistoricEntity` has `ActiveTo` `Datetime?` property. ``` // Auto generated LINQ to Entities domain service: [EnableClientAccess()] public partial class ProductService : LinqToEntitiesDomainService { public IQueryable GetANALYSIS_CODES() { return this.ObjectContext.ANALYSIS_CODES; } ... } // My Partial class to add interface public partial class ANALYSIS_CODES : IHistoricEntity {} ``` I am trying to refactor this working code to a method: ``` List filtered = (from rec in ps.GetANALYSIS_CODES() where rec.ActiveTo == null select rec).ToList() ``` Like so: ``` private List Filter(IQueryable queryable) where T : IHistoricEntity { return (from rec in queryable where rec.ActiveTo == null select rec).ToList(); } // called like this: List filtered = Filter(ps.GetANALYSIS_CODES()); ``` This giv…

Accepted Answer

`rec.ActiveTo` refers to a property defined in your interface. Therefore Linq needs to cast `rec` to `IHistoricEntity` before being able to access that property.

Don't be fooled by the exception being raised in `.ToList()`: the Linq query is only evaluated and executed when the records are needed, in this case, when the collection is to be transformed into a `List<>`.

Update: I verified @hvd's comment, and indeed, adding a `where T: class` clause changes the Linq expression from

```
System.Collections.Generic.List`1[MyType]
    .Where(x => (Convert(x).ActiveTo == Convert(null)))
```

to

```
System.Collections.Generic.List`1[MyType]
    .Where(x => (x.ActiveTo == Convert(null)))
```

Where is the cast here? LINQ to Entities only supports casting Entity Data Model primitive types

Solution

Problem