How to make a Textbox required IF a Checkbox is checked

Question

How can I make a textbox required if a checkbox is checked?

I figure I could write a custom validator, but I was hoping to avoid a full post back to check the valida

Answer 1

To solve this all within ASP.Net, make the checkbox do a postback:

Then on the code-behind, enable or disable the validators:

protected void Existing_CheckedChanged(object sender, EventArgs e)
{
     RequiredFieldValidator1.Enabled =! Existing.Checked;
}