خطا در هنگام ریختن فیلدهای جدول درون تکست باکس

[abdullah20]

            string cnString = @"Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirector  y|\SELLPC.mdf;Integrated Security=True;User Instance=True";

            SqlConnection cn = new SqlConnection(cnString);

            DataSet dataset = new DataSet();

            string query = "SELECT * FROM TBLCPU ";

            SqlDataAdapter datada = new SqlDataAdapter(query, cn);

            datada.Fill(dataset, "TBLCPU");

            for (int i = 1; i <= dataset.Tables["TBLCPU"].Rows.Count; i++)

            {

                string str=(string)(dataset.Tables["TBLCPU"].Rows[i - 1]["MDLCPU "]);

                if (str.Contains(cmbmdlcpu.Text))

                    txtprccpu.Text = (string)(dataset.Tables["TBLCPU"].Rows[i - 1]["heads"]);                        

            }