Multi-select dropdown list in ASP.NET
You could use the System.Web.UI.WebControls.CheckBoxList control or use the System.Web.UI.WebControls.ListBox control with the SelectionMode property set to Multiple.
You could use the System.Web.UI.WebControls.CheckBoxList control or use the System.Web.UI.WebControls.ListBox control with the SelectionMode property set to Multiple.