Default combobox first item mixed with database results

问题

public static void fill_combo(string table, ComboBox cmb, string columns)
    {
        ds = new DataSet();
        try
        {
            conn.Open();

            da = new SqlDataAdapter($"SELECT {columns} FROM [{table}]", conn);

            da.Fill(ds, table);
            cmb.DataSource = ds.Tables[table];
            cmb.ValueMember = ds.Tables[table].Columns[0].ToString();
            cmb.DisplayMember = ds.Tables[table].Columns[1].ToString();

            cmb.SelectedItem = null;
            cmb.Text = "Select...";

        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message, "Error");

        }
        finally
        {
            if (conn != null)
            {
                conn.Close();
            }
        }
    }

Hey guys. With the code above i'm trying to get results from database and bind them to a combobox, but i want the first item to be a random "Select..." that isn't in the database. Thanks in advance.

回答1:

Option 1 - You can insert a DataRow containing desired values to the DataTable:

var connection = @"Your Connection String";
var command = "SELECT Column1, Column2 FROM Table1";
var dt = new DataTable();
using (var da = new SqlDataAdapter(command, connection))
    da.Fill(dt);
var row = dt.NewRow();
row["Column1"] = DBNull.Value;
row["Column2"] = "Select an Item";
dt.Rows.InsertAt(row, 0);
this.comboBox1.DataSource = dt;
this.comboBox1.ValueMember = "Column1";
this.comboBox1.DisplayMember = "Column2";

Option 2 - Instead of adding null data to the DataTable, you can set DropDownStyle to DropDownList and set its DrawMode to OwnerDrawFixed and handle DrawItem event and draw the place holder when selected index is -1:

private void comboBox1_DrawItem(object sender, DrawItemEventArgs e)
{
    var combo = sender as ComboBox;
    var text = "Select an Item";
    if (e.Index > -1)
        text = combo.GetItemText(combo.Items[e.Index]);
    e.DrawBackground();
    TextRenderer.DrawText(e.Graphics, text, combo.Font,
        e.Bounds, e.ForeColor, TextFormatFlags.Left);
}

To show the place holder, it's enough to set comboBox1.SelectedIndex = -1;

来源：https://stackoverflow.com/questions/40568907/default-combobox-first-item-mixed-with-database-results