如何为相同的LINQ 2 SQL表创建外观?

我想创建一种访问特定表的通用方法,但似乎无法使其正常工作。

 public interface ILetter
 {
     DateTime CreationDate { get; }
 }

 public interface IBill
 {
     DateTime BillDate { get; }
 }

 // The LINQ 2 SQL generated classes already implement the CreationDate property
 public partial class NewLetter : ILetter { }
 public partial class CurrentLetter : ILetter { }
 public partial class HistoricLetter : ILetter { }

然后在我的代码中,我希望能够做到这一点:

 switch (type)
 {
   case 1:
     Table<IBill> specificBill;
     switch (location)      
     {
        ...
     }

   case 2:
     Table<ILetter> specificTable;
     switch (location)
     {
       case 1: specificTable = dataContext.NewLetter as Table<ILetter>; break;
       case 2: specificTable = dataContext.CurrentLetter ...
       case 3: specificTable = dataContext.HistoricLetter ...
      }   
 }

或者类似的东西。不幸的是我不能投 Table<NewLetter> 到 Table<ILetter>