How to validate DropDownList using ASP.Net RequiredFieldValidator with example

You will have to add a default item and set it as intial value as shown below. Or you can set the Value of first item in intial value

 protected void Page_Load(object sender, EventArgs e)
    {
        DataTable dt = new DataTable();
        dt.Columns.AddRange(new DataColumn[2] { new DataColumn("Item"), new DataColumn("Id") });
        dt.Rows.Add("Shirt", "1");
        dt.Rows.Add("Pant", "2");
        DropDownList1.DataSource = dt;
        DropDownList1.DataTextField = "Item";
        DropDownList1.DataValueField = "Id";
        DropDownList1.DataBind();
        //Add default item
        DropDownList1.Items.Insert(0, new ListItem("Please select", "0"));
}

 <asp:DropDownList ID="DropDownList1" runat="server">
    </asp:DropDownList>
    <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ErrorMessage="Required" ControlToValidate = "DropDownList1" InitialValue = "0"></asp:RequiredFieldValidator>
    <br />
    <asp:Button ID="Button1" runat="server" Text="Button" />