如何在Windows窗体C＃中填充组合框

Question

I try to fill one combobox with MySQL database, but I have a problem. I think everything is OK and I see my old project how to fill the comboboxes. I think all code is the same but I forgot how exactly is filling.

So, this is my code, but combobox is not filled.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using MySql.Data.MySqlClient;
using System.Data.SqlClient;
using System.Data.Common;
using SR.Service;

namespace SR
{
    public partial class allWorkers : Form
    {
        private WorkerService service;
        MySqlConnection connection;
        string MyConnectionString = "Server=localhost;Database=SR_database;Uid=root;Pwd='';";


        public allWorkers()
        {
            InitializeComponent();

            connection = new MySqlConnection(MyConnectionString);
            service = new WorkerService();
        }

        private void btn_Main6_Click(object sender, EventArgs e)
        {
            this.Hide();
            var Main = new Main();
            Main.Show();
        }

        private void btn_createWorker_Click(object sender, EventArgs e)
        {
            //MySqlConnection connection = new MySqlConnection(MyConnectionString);
            MySqlCommand cmd;
            connection.Open();

                cmd = connection.CreateCommand();
                cmd.CommandText = service.CreateWorker(textBox1.Text);
                cmd.ExecuteNonQuery();
                MessageBox.Show("Вие създадохте нов работник с име: " + textBox1.Text);


            MySqlCommand command = new MySqlCommand(service.GetAllWorkers(), connection);
            MySqlDataAdapter da = new MySqlDataAdapter(command);
            using (DataTable dt = new DataTable())
            {
                da.Fill(dt);
                dataGridView1.DataSource = dt;
            }
            connection.Close();
        }

        private void allWorkers_Load(object sender, EventArgs e)
        {

        }

        private void dataGridView1_CellContentClick(object sender, DataGridViewCellEventArgs e)
        {          

        }

        private void btn_deleteWorker_Click(object sender, EventArgs e)
        {


        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {

            MySqlCommand cmd;
            connection.Open();

            cmd = connection.CreateCommand();
            cmd.CommandText = service.selectOnlyWorkerName();
            cmd.ExecuteNonQuery();

            MySqlCommand command = new MySqlCommand(service.selectOnlyWorkerName(), connection);
            using (var reader = command.ExecuteReader())
            {
                SuspendLayout();
                while (reader.Read())
                {
                    comboBox1.Items.Add(reader["worker_name"]);
                }
                ResumeLayout();
            }
            //
            //
            //  MySqlCommand command = new MySqlCommand(service.selectOnlyWorkerName(), connection);
            //  MySqlDataAdapter da = new MySqlDataAdapter(command);
            //
            //  using (DataTable dt = new DataTable())
            //  {
            //      da.Fill(dt);
            //      foreach (DataRow dr in dt.Rows)
            //      {
            //          comboBox1.Items.Add(dr["worker_name"]);
            //      }
            //  }
            //  connection.Close();
        }
    }
}

I think I forgot something, but I don't know what I forgot to write. Thank you guys.

Answer 1

You need to do this,

MySqlDataAdapter da = new MySqlDataAdapter(command);
DataTable dt = new DataTable();
da.Fill(dt);
foreach (DataRow dr in dt.Rows)
{
    cb.Items.Add(dr["yourcol"]);
}

Answer 2

First of all, Always when you're about to change something within UI with WinForms consider wrapping this around SuspendLayout() and ResumeLayout() .

Another thing ... have you ever consider executing your command? Just not to use SqlDataAdapter which would make your work easier.

---

example code:

MySqlCommand command = new MySqlCommand(service.selectOnlyWorkerName(), connection);
using (var reader = command.ExecuteReader())
{
    SuspendLayout();
    while(reader.Read())
    {
        comboBox1.Items.Add(reader["worker_name"]);
    }
    ResumeLayout();
}

or in case you really need to use SqlDataAdapter :

MySqlCommand command = new MySqlCommand(service.selectOnlyWorkerName(), connection);
MySqlDataAdapter da = new MySqlDataAdapter(command);
DataSet ds = new DataSet();
da.Fill(ds,"workers");
SuspendLayout();
foreach ( var row in ds.Tables["workers"].Rows)
{
    comboBox1.Items.Add(row[0]);
}
ResumeLayout();

---

The code posted above should work perfectly fine and in case it won't then debug if your command actually has some results.

Answer 3

I believe you should have handled the DropDown event, rather than SelectedIndexChanged.

DropDown has worked for me in the past.