揭示指定的转换无效。

提示指定的转换无效。。。
class Northwind : DataContext
  {
  public Table<Depart> T_Dept;
  public Northwind(string conn) : base(conn) { }
  }
  [Table(Name="T_Depart")]
  public class Depart
  {
  [Column(IsPrimaryKey = true, IsDbGenerated = true)]
  public int DeptId { set; get; }

  [Column(Name="DeptManId")]
  public int DeptManId { set; get; }
  [Column]
  public string DeptName { set; get; }
  [Column]
  public string DeptTel { set; get; }
  [Column]
  public string DeptFax { set; get; }

  //public Table<Employee> emps { set; get; }
  }
  static void Main(string[] args)
  {
  Northwind nw = new Northwind(@"*****");
  var select = (from td in nw.T_Dept
  where td.DeptId == 1
  select new
  {
  DeptId = td.DeptId,
  DeptFax = td.DeptFax,
  DeptTel = td.DeptTel,
  DeptName = td.DeptName,
  DeptManId = td.DeptManId
  });
  Depart dept = nw.T_Dept.First();
  Console.ReadKey();
  }


Depart dept = nw.T_Dept.First();提示指定的转换无效。。。
请问怎么解决？？？？

[解决办法]

C# code

Northwind nw = new Northwind(@"*****");  var select = (from td in nw.T_Dept  where td.DeptId == 1  select new Depart  //注意此处  {  DeptId = td.DeptId,  DeptFax = td.DeptFax,  DeptTel = td.DeptTel,  DeptName = td.DeptName,  DeptManId = td.DeptManId  }).FirstOrDefault();  Depart dept = (select ==null? new Depart(): select);  Console.ReadKey();
[解决办法]
http://www.cnblogs.com/JeffreyZhao/archive/2008/02/19/using-translate-method-and-modify-command-text-before-query-in-linq-to-sql.html

C# code
Northwind nw = new Northwind(@"*****");  var select = (from td in nw.T_Dept  where td.DeptId == 1  select new    {  DeptId = td.DeptId,  DeptFax = td.DeptFax,  DeptTel = td.DeptTel,  DeptName = td.DeptName,  DeptManId = td.DeptManId  }).FirstOrDefault(); using (dataContext.Connection)    {         Depart dept =nw.ExecuteQuery<Depart>(query, true).FirstOrDefault();    }
[解决办法]
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    class Northwind : DataContext
    {
        public Table<Depart> T_Dept;
        public Northwind(string conn) : base(conn) { }
    }
    [Table(Name="T_Depart")]
    public class Depart
  ……