How to select multiple items in ASP.Net DropDownList

You need to make use of ListBox control

 <asp:ListBox ID="lstFruits" runat="server" SelectionMode = "Multiple">
            <asp:ListItem Text="Mango" Value="1">  </asp:ListItem>
            <asp:ListItem Text="Orange" Value="2"></asp:ListItem>
            <asp:ListItem Text="Apple" Value="3"></asp:ListItem>
        </asp:ListBox>