难以从带有参数的存储过程填充数据网格

protected DataTable RetrieveAlumni2()
        {
            {
                MySqlConnection con = new MySqlConnection("server=100.0.0.0;user id=id;password=pass;database=db;persistsecurityinfo=True");
                MySqlCommand cmd = new MySqlCommand("get_alumni_by_city", con);
                MySqlDataAdapter da = new MySqlDataAdapter(cmd);
                DataTable dt = new DataTable();
                try
                {
                    string city = textBox1.Text;
                    con.Open();
                    cmd.CommandType = CommandType.StoredProcedure;
                    cmd.Parameters.AddWithValue("@city", SqlDbType.VarChar).Value = city;
                    da.SelectCommand = cmd;
                    cmd.ExecuteNonQuery();
                    da.Fill(dt);
                    dataGridView1.DataSource = dt;
                }
                catch (System.Exception ex)
                {
                    MessageBox.Show(ex.Message);
                }
                finally
                {
                    cmd.Dispose();
                    con.Close();
                }
                return dt;
            }
        }

给出错误:

"输入字符串格式不正确"

City在mysql服务器中为varchar。