The specified method 'xxx' on the type 'yyy' cannot be translated into a LINQ to Entities store expression because the instance over which it is invoked is not the ObjectContext over which the query in which it is used is evaluated.

I have a method defined in the partial class for one of the entities like this:

public bool IsAnonymous() {
  return (this.Roles_ & UserRoles.Registered) == 0;
 }

This of course doesn't work within LINQ to Entities queries, because the EF provider for LINQ doesn't know how to translate that to SQL so that it could send that to the database. No problem according to the docs. You can define a "Model Defined Function" and tell the EF to use that:

        <Function Name="IsAnonymous" ReturnType="Edm.Boolean" >
          <Parameter Name="user" Type="Kosmas.Models.User"/>
          <DefiningExpression>
            BitWiseAnd(user.Roles, 1) = 0

         </DefiningExpression>

        </Function>

and

[System.Data.Objects.DataClasses.EdmFunction("Kosmas.Models", "IsAnonymous")]
 public bool IsAnonymous() {
  return (this.Roles_ & UserRoles.Registered) == 0;
 }

Or can you?


Well you can't. If you try this you get a very informative error: "The specified method 'Boolean IsAnonymous()' on the type 'Kosmas.Models.User' cannot be translated into a LINQ to Entities store expression because the instance over which it is invoked is not the ObjectContext over which the query in which it is used is evaluated."


A quick Google search did not get anything useful. The only suggestions were to either change the method so that instead of a User instance I call it on the context (making the syntax rather ... silly). Or use Entity SQL (which I'd rather not either).


So I tried whether the example in Programming Entity Framework book actually works and noticed one difference. In the example they were adding the [EdmFunction] attribute to an extension method, while I'm adding it to a plain old ordinary one.


OK, let'ts try that. Let's remove the IsAnonymous method from the partial class and add

public static class Functions {
  [System.Data.Objects.DataClasses.EdmFunction("Kosmas.Models", "IsAnonymous")]
  public static bool IsAnonymous(this User obj) {
   return (obj.Roles_ & UserRoles.Registered) == 0;
  }
 }
et voila, it works.


I would not call this bug ... if it wasn't. But it is!


Anyway, if you do get the nonsensical error message, check whether the method is an ordinary method or an extension one. And see if you can change it from one to the other.