c#combobox设置无边框

您可以使用以下代码将 ComboBox 设置为无边框：

comboBox1.FlatStyle = FlatStyle.Flat;
comboBox1.DropDownStyle = ComboBoxStyle.DropDownList;
comboBox1.BackColor = SystemColors.Control;
comboBox1.ForeColor = SystemColors.WindowText;
comboBox1.DrawMode = DrawMode.OwnerDrawFixed;

private void comboBox1_DrawItem(object sender, DrawItemEventArgs e)
{
    e.DrawBackground();
    bool isItemSelected = (e.State & DrawItemState.Selected) == DrawItemState.Selected;

    if (e.Index >= 0)
    {
        Graphics g = e.Graphics;
        Rectangle rect = e.Bounds;

        // 绘制背景色
        g.FillRectangle((isItemSelected) ? Brushes.LightGray : Brushes.White, rect);

        // 绘制文本
        string itemText = comboBox1.Items[e.Index].ToString();
        g.DrawString(itemText, e.Font, (isItemSelected) ? Brushes.White : Brushes.Black, rect.X, rect.Y + 2);

        // 绘制焦点矩形
        if ((e.State & DrawItemState.Focus) == DrawItemState.Focus)
            ControlPaint.DrawFocusRectangle(g, rect);
    }
}

这段代码会将 ComboBox 的外观设置为无边框，并且只能通过下拉列表选择项来选择。另外，还需要在窗体的 `Load` 事件中添加以下代码：