You can use RequiredFieldValidator control with
InitialValue as the default value for the DropDownList box. You can also
validate the selected value of ASP. Net DropDownList control using
RequiredFieldValidator, when user clicks the list item of dropdownlist control
to select it. A good way of using the InitialValue property is with the
DropDownList server control.
For example
a drop-down list might have a default value
that is not an empty value (by default, some text appears in it). An example of
a drop-down list with the RequiredFieldValidator that uses the without
InitialValue property is illustrated in Below code.
ASPX PAGE:
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:dropdownlist id="dropDowncolor" runat="server">
<asp:ListItem Text="Select Color" Value="none" />
<asp:ListItem Text="Red" Value="Red" />
<asp:ListItem Text="Blue" Value="Blue" />
<asp:ListItem Text="Green" Value="Green" />
</asp:dropdownlist>
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="dropDowncolor" ErrorMessage="Please
select color"></asp:RequiredFieldValidator>
<p>
<asp:button id="btnSubmit" runat="server" text="Submit" onclick="SubmitData" />
</p>
</div>
</form>
</body>
</html>
Now run the application
DropDownList1
Now using InitialValue property in the
RequiredFieldValidator control.
<div>
<asp:dropdownlist id="dropDowncolor" runat="server">
<asp:ListItem Text="Select color" Value="0" />
<asp:ListItem Text="red" Value="18" />
<asp:ListItem Text="blue" Value="19" />
<asp:ListItem Text="pink" Value="20" />
<asp:ListItem Text= "green" Value=">20" />
</asp:dropdownlist>
<asp:requiredfieldvalidator id="reqcolor" runat="server" errormessage="Please
select color" forecolor="Red" controltovalidate="dropDowncolor" initialvalue="0" />
<p>
<asp:button id="btnSubmit" runat="server" text="Submit" onclick="SubmitData" />
</p>
</div>
Now run the application and click on the Button control.
DropDownList2
Now select item from the DropDownList.
DropDownlist3