Validate TextBox based on DropDownList selection - asp.net

Question

I need to validate TextBox based on Value selected in DropDownList controls. I have asp:TextBox and asp:DropDownList controls.
If user selects Yes option from the first dropdown he must type value in text box. How can I validate second box? Thanks for help.

Answer 1

The simplest approach is to set the DropDownList's AutoPostBack property to true and handle it's SelectedIndexChanged event. Then you can Enable/Disable the validator there.
Another approach is to use a CustomValidator. This validator is not dependent on a single control. You must write the validation rules on your own. For example the ClientValidationFunction:
<script type="text/javascript" >
function ClientValidate(source, arguments) {
var txt = document.getElementById('TextBox1');
var ddl = document.getElementById('DropDownList1');
var decision = ddl.options[ddl.selectedIndex].text;
if(decision=='Yes'){
arguments.IsValid = txt.value.length > 0;
}else{
arguments.IsValid = true;
}
}
</script>
<asp:DropDownList id="DropDownList1" runat="server">
<asp:ListItem Selected="True">Yes</asp:ListItem>
<asp:ListItem Selected="False">No</asp:ListItem>
</asp:DropDownList>
<asp:TextBox id="TextBox1" runat="server" />
<asp:Button ID="BtnSubmit" runat="server" Text="Submit" />
<asp:CustomValidator id="CustomValidator1"
ValidateEmptyText="true"
ControlToValidate="TextBox1"
ClientValidationFunction="ClientValidate"
OnServerValidate="ServerValidation"
Display="Static"
ErrorMessage="Please enter text!"
runat="server"/>
Remember to always implement a OnServerValidate because you should not rely on javascript only(can be disabled). This is easy:
void ServerValidation(object source, ServerValidateEventArgs args){
args.IsValid = DropDownList1.SelectedIndex == 1 || TextBox1.Text.Length > 0;
}
VB.NET
Protected Sub ServerValidation(source As Object, args As System.Web.UI.WebControls.ServerValidateEventArgs)
args.IsValid = DropDownList1.SelectedIndex = 1 OrElse TextBox1.Text.Length > 0
End Sub

Answer 2

Add this code to your submit button.
if(DropDownList1.SelectedItem.Text.Equals("Yes") && TextBox1.Text.Length==0)
{
Page.ClientScript.RegisterStartupScript(this.GetType(), "myalert", "alert('Enter data in the textbox !');", true);
}

Answer 3

Add a CustomValidator control, that validates the TextBox, from there you'll have to do something like this (assuming C#) in the CustomValidator_ServerValidate event handler:
bool valid = false;
if (dropDownList.SelectedValue.Equals("yes")) {
valid = !String.IsNullOrEmpty(textBox.Text);
}
return valid;