how we get max substring value using LINQ

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C#

respected,i want to use LINQ to SQL,

by simple programming in my application my code is

private string getuniqueID()
        {
                con.OpenConnection();
                int regcount = -1;
                string strregister = "";
                string str = "select (MAX(SUBSTRING(Registration_Id,7,3))) from tbl_registration";
                SqlCommand cmd = new SqlCommand(str, DBConnect.connection);
                SqlDataReader dr = null;
                try
                {
                    dr = cmd.ExecuteReader();
                    if (dr.HasRows)
                    {
                        while (dr.Read())
                        {
                            if (!(dr.GetValue(0).ToString() == ""))
                            {
                                regcount = Convert.ToInt32(dr.GetString(0));
                                int regcountinc = regcount + 1;
                                if (regcountinc <= 9)
                                {
                                    strregister = "STUREG00" + regcountinc;
                                }
                                else
                                {
                                    strregister = "STUREG0" + regcountinc;
                                }
                            }
                            else
                            {
                                strregister = "STUREG001";
                            }
                        }
                    }
                    else
                    {
                        strregister = "STUREG001";
                    }
                    txtregid.ReadOnly = true;
                }
                catch (SqlException ex)
                {
                    MessageBox.Show(ex.Message);
                }
                finally
                {
                    con.CloseConnection();
                }
                return strregister;

but please tell me what should i write for get max substring value using LINQ instead of
string str = "select (MAX(SUBSTRING(Registration_Id,7,3))) from tbl_registration";
i want to use linq for this concept,