How to add a RequiredFieldValidator to DropDownList control?

For the most part you treat it as if you are validating any other kind of control but use the InitialValue property of the required field validator.

<asp:RequiredFieldValidator ID="rfv1" runat="server" ControlToValidate="your-dropdownlist" InitialValue="Please select" ErrorMessage="Please select something" />

Basically what it's saying is that validation will succeed if any other value than the 1 set in InitialValue is selected in the dropdownlist.

If databinding you will need to insert the "Please select" value afterwards as follows

this.ddl1.Items.Insert(0, "Please select");

Suppose your drop down list is:

<asp:DropDownList runat="server" id="ddl"><asp:ListItem Value="0" text="Select a Value">....</asp:DropDownList>

There are two ways:

<asp:RequiredFieldValidator ID="re1" runat="Server" InitialValue="0" />

the 2nd way is to use a compare validator:

<asp:CompareValidator ID="re1" runat="Server" ValueToCompare="0" ControlToCompare="ddl" Operator="Equal" />

If you are using a data source, here's another way to do it without code behind.

Note the following key points:

• The ListItem of Value="0" is on the source page, not added in code
• The ListItem in the source will be overwritten if you don't includeAppendDataBoundItems="true" in the DropDownList
• InitialValue="0" tells the validator that this is the value thatshould fire that validator (as pointed out in other answers)

Example:

<asp:DropDownList ID="ddlType" runat="server" DataSourceID="sdsType"                  DataValueField="ID" DataTextField="Name" AppendDataBoundItems="true">    <asp:ListItem Value="0" Text="--Please Select--" Selected="True"></asp:ListItem></asp:DropDownList><asp:RequiredFieldValidator ID="rfvType" runat="server" ControlToValidate="ddlType"                             InitialValue="0" ErrorMessage="Type required"></asp:RequiredFieldValidator><asp:SqlDataSource ID="sdsType" runat="server"                    ConnectionString='<%$ ConnectionStrings:TESTConnectionString %>'                   SelectCommand="SELECT ID, Name FROM Type"></asp:SqlDataSource>