یافتم یافتم اینم جوابش:
GV_Main_Student.DataSource = Search.S_Students.Where(C => (C.S_LastName.Substring(0, txtS_LastName.Text.Length) == txtS_LastName.Text.Substring(0, C.S_LastName.Length)) &&
(C.S_ID.ToString().Substring(0, txtS_ID.Text.Length) == txtS_ID.Text.Substring(0, C.S_ID.ToString().Length)) &&
(C.S_Name.Substring(0, txtS_Name.Text.Length) == txtS_Name.Text.Substring(0, C.S_Name.Length)) &&
(C.S_Father.Substring(0, txtS_Father.Text.Length) == txtS_Father.Text.Substring(0, C.S_Father.Length)) &&
(C.S_Telephone.Substring(0, txtS_Telephone.Text.Length) == txtS_Telephone.Text.Substring(0, C.S_Telephone.Length)) &&
(C.S_Address.Substring(0, txtS_Address.Text.Length) == txtS_Address.Text.Substring(0, C.S_Address.Length)) &&
(C.S_NationNum.ToString().Substring(0, txtS_NationNum.Text.Length) == txtS_NationNum.Text.Substring(0, C.S_NationNum.ToString().Length)) &&
(C.S_IdentifyNum.ToString().Substring(0, txtS_IdetifyNum.Text.Length) == txtS_IdetifyNum.Text.Substring(0, C.S_IdentifyNum.ToString().Length))).Select(C => C);