如何使用分部类扩展ADO.NET实体框架对象？

我创建了一个Visual Basic WPF应用程序项目,其中包含Toy.edmx,这是一个从名为Toy的数据库生成的ADO.NET实体数据模型.

它的Window1.xaml.vb文件如下所示:

1   Class Window1
2   
3       Private Sub Window1_Loaded( _
4       ByVal sender As System.Object, _
5       ByVal e As System.Windows.RoutedEventArgs) _
6       Handles MyBase.Loaded
7   
8           Dim dc As New ToyEntities1
9           Label1.Content = (From c As Client In dc.ClientSet _
10                            Select c).First.FirstName
11  
12      End Sub
13  
14  End Class

运行得很好.

但是,如果我添加文件Client.vb ...

1   Partial Public Class Client
2       Function IsWashington() As Boolean
3           Return Me.LastName = "Washington"
4       End Function
5   End Class

...并在我的Window1.xaml.vb查询中添加WHERE子句...

9           Label1.Content = (From c As Client In dc.ClientSet _
10                            Where c.IsWashington _
11                            Select c).First.FirstName

...然后我得到这个NotSupportedException:

LINQ to Entities无法识别方法'Boolean IsWashington()'方法,并且此方法无法转换为存储表达式.

如何使用分部类扩展ADO.NET实体框架对象？