We use RequiredFieldValidator control to make a field mandatory in ASP.Net. This little tip will help us to use a DropDownList control with RequiredFieldValidator control. This code snippet will insert a default item as the first Item to the DropDownList with value as empty. In this scenario, When a RequiredFieldValidator is associated with DropDownList through its ControlToValidate property; it will make the DropDownList as a mandatory field when it is left with default value selection.

ASPX

 <asp:DropDownList ID="ddlRank" runat="server">

        </asp:DropDownList>

        <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="ddlRank"

            ErrorMessage="Select a Rank"></asp:RequiredFieldValidator>

        <asp:Button ID="btnSave" runat="server" Text="Save" />

CodeBehind

protected void Page_Load(object sender, EventArgs e)

    {

        if (!IsPostBack)

        {

            ddlRank.Items.Add(new ListItem("Select",""));

            ddlRank.Items.Add(new ListItem("1", "1"));

            ddlRank.Items.Add(new ListItem("2", "2"));

            ddlRank.Items.Add(new ListItem("3", "3"));

        }

    }

Execute the page. You will receive an error message when you click Save without selecting a value in DropDownList control.