Combo box related problem plz solve it window Form

private void Reservation_Load(object sender, EventArgs e)
       {



             string strConnectionstring = @"Data Source=ACER-PC\SQLSERVER; Initial Catalog = hms ; Integrated Security= True"

           string sql3 = "Select Room_Type,Room_Type_ID From Room_Type ";
           SqlConnection con3 = new SqlConnection(connectionString);
           SqlCommand cmd3= new SqlCommand(sql3, con3);
           SqlDataAdapter ad3 = new SqlDataAdapter();
           DataSet ds3 = new DataSet();
           ad3.SelectCommand = cmd3;

           ad3.Fill(ds3, "Room_Type");
           cmbRoomType.DataSource = ds3.Tables[0];

           cmbRoomType.DisplayMember = "Room_Type";
           cmbRoomType.ValueMember = "Room_Type_ID";

       }

       private void cmbRoomType_SelectedIndexChanged(object sender, EventArgs e)
       {
           GetConnectionString getStng = new GetConnectionString();
           string connectionString = getStng.getString();

           string sql = "Select Room_Number,Room_ID,Room_Status From Room where Room_Type='" +  (String)cmbRoomType.SelectedItem + "' and Room_Status = 'Vacant' ";
           SqlConnection con = new SqlConnection(connectionString);
           SqlCommand cmd = new SqlCommand(sql, con);
           SqlDataAdapter ad = new SqlDataAdapter();
           DataSet ds = new DataSet();
           ad.SelectCommand = cmd;

           ad.Fill(ds, "Room");
           cmbRoomNo.DataSource = ds.Tables[0];
           cmbRoomNo.DisplayMember = "Room_Number";
           cmbRoomNo.ValueMember = "Room_ID";
       }