How to validate default value in dropdown list

Use required field validator & set its Initialvalue property to 0 th postion value of dropdown. (here --Select-- is that value)

<asp:dropdownlist id="ddlstatus" runat="server"   >

<asp:listitem>--Select--</asp:listitem> 
</asp:dropdownlist>

<asp:requiredfieldvalidator id="RequiredFieldValidator2" runat="server" errormessage="Please select status" controltovalidate="ddlstatus" validationgroup="btnAdd" initialvalue="--Select--" ></asp:requiredfieldvalidator>

Hi,

using Javascript : Clientside validation
=========================================

<script type="text/javascript">
function validate()
{
var ddllist= document.getElementById(<%=DropDownList1.ClientID %>).value;
if (ddllist.options[ddllist.selectedIndex].value== 'Select') {
alert('Select a value');
return false;
}
}
</script>

call this validate function onclick of dropdown or onclientclick of button

---------------------------------------------------------------------------------

On Server side validation :
============================

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
   {
if (DropDownList1.SelectedItem.Text == "Select")
       {
Lable.Text="Select any value";
       }

   }

suppose your drop down list id is ddlItems.

protected bool ValidateItems(){
    //Dropdwon list selected index property will give you currently selected item index
    // if user have selected default value in you case it is "Select" then that property will return 0 because you are inserting default value at positioning 0 in your dropdownlist items. so only you need to check the current selected item index based on your requirement you can validate your selection.

    if(ddlItems.SelectedIndex > 0)
       retrurn true;
    
    retrun false;
}

<script type="text/javascript" language="javascript">
        function Validate() {
 var country = document.getElementById("ddlCountry").selectedIndex;
if (country <= 0) {
                alert("Please choose country");
                return false;
            }
}
<asp:button id="btnSubmit" onclientclick="return Validate()" text="Submit" runat="server" onclick="btnSubmit_Click" xmlns:asp="#unknown" />

page.cs file

 string Conn = ConfigurationManager.ConnectionStrings["Conn"].ConnectionString; 
protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            FillCountrys();           
        }
    }

protected void FillCountrys()
    {
        SqlConnection cnn = new SqlConnection(Conn);
        SqlDataAdapter ada = new SqlDataAdapter("select * from tbl_Countrys", cnn);
        DataSet ds = new DataSet();
        ada.Fill(ds);
        ddlCountry.DataSource = ds;
        ddlCountry.DataTextField = "CountryName";
        ddlCountry.DataValueField = "CountryCode";
        ddlCountry.DataBind();
        ddlCountry.Items.Insert(0, "Select Country");        
    }