If condition for required field validation

Question

I have a DropDownList and a TextBox in c#.net. If DropDownList value is "No" then some value must enter in TextBox. If dropdownlist value is yes then required field validator is not needed for that TextBox. How to make it possible?

<asp:DropDownList ID="dropdownlist1" runat="server"
    CssClass="NormalText" Width="155px" AutoPostBack="true"
    onselectedindexchanged="ddls_SelectedIndexChanged">
    <asp:ListItem Selected="True" Value=""></asp:ListItem>    
    <asp:ListItem Value="Yes">Yes</asp:ListItem>
    <asp:ListItem Value="No">No</asp:ListItem>
</asp:DropDownList>
<br />
<br />
<asp:TextBox ID="Textbox1" runat="server" CausesValidation="True"></asp:TextBox>
&nbsp;
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server"
    ControlToValidate="Textbox1" ErrorMessage="Explanation needed If you select NO">
</asp:RequiredFieldValidator>
</td>


protected void dropdownlist1_SelectedIndexChanged(object sender, EventArgs e)
{
}

Answer 1

Use this code for conditional validation:

protected void dropdownlist1_SelectedIndexChanged(object sender, EventArgs e)
{
    if(dropdownlist1.SelectedValue == "No")
    {
       RequiredFieldValidator1.Enabled = true;
    }
    else if(dropdownlist1.SelectedValue == "Yes")
    {
       RequiredFieldValidator1.Enabled = false;
    }
}

Optionally, you may also have to set Submit Button Validation group:

 ButtonSubmit.ValidationGroup = string.Empty;

when field validator is not enabled.