c#.net need to resolve "System.InvalidCastException" when we using LINQ

See more:

application

hi friends,
here with i have post my code for your reference.

DataTable dtConfig = TesterSlotTypeGroupDAL.GetConfiguredTSTGroupRows();
           DataTable dt = new DataTable("dTable");
           DataRow dr; int iColCtr = 2;
           var vGroups = from c in dtConfig.AsEnumerable()
                         select new
                         {
                             TST_GroupId = c.Field<decimal>("id"),
                             TST_Group = c.Field<string>("TST Group")
                         };
           foreach (var vGroup in vGroups.Distinct())
           {
               if (dt.Rows.Count == 0)
               {
                   dt.Columns.Add("id");
                   dt.Columns.Add("TST Group");
                   dt.Columns.Add("Machine Size Slot");

                   dt.Columns.Add("Slot");
               }
               dr = dt.NewRow();
               dr[0] = vGroup.TST_GroupId.ToString();
               dr[1] = vGroup.TST_Group.ToString();

               dt.Rows.Add(dr);
           }

           dgv.DataSource = dt;

note:
foreach (var vGroup in vGroups.Distinct()) in this line i am getting error. can you pls give me a idea how to resolve this issue..,

Regards
Siva