Binding data of combobox to Grid, filtering

string course = cboCourse.SelectedItem.ToString();
           //You do your Selection of Student from your table Student
           //you Set the Data Source of the GridView
           string connectionstring = "Data Source=Serveur\\INSTANCESQL;Initial Catalog=Student;Integrated Security=True";
           SqlConnection cnx = new SqlConnection(connectionstring);
           cnx.Open();
           string query = "SELECT [Student] FROM [User].[dbo].[Student] WHERE course = @course";
           SqlCommand cmd = new SqlCommand(query,cnx);
           SqlParameter p1 = new SqlParameter("@course", SqlDbType.Char, 10);
           p1.Value = course;
           cmd.Parameters.Add(p1);

           cmd.ExecuteNonQuery();
           SqlDataAdapter da = new SqlDataAdapter(cmd);
           DataSet ds = new DataSet();
           da.Fill(ds);

           dgStudents.DataSource = ds.Tables[0];
           cnx.Close();