c# - 默认组合框第一项与数据库结果混合

Question

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();
            }
        }
    }

大家好。使用上面的代码，我试图从数据库中获取结果并将它们绑定(bind)到一个组合框，但我希望第一个项目是一个随机的“选择...”，它不在数据库中。提前致谢。

Answer 1

选项 1 - 您可以将包含所需值的 DataRow 插入 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";

选项 2 - 您可以将 DropDownStyle 设置为 DropDownList 而不是向 DataTable 添加空数据，并且将其 DrawMode 设置为 OwnerDrawFixed 并处理 DrawItem 事件并在选定索引为 -1 时绘制占位符:

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);
}

要显示占位符，只需设置 comboBox1.SelectedIndex = -1;