Q:

ScriptManager.RegisterStartupScript not working on ASP Button click event

what I have tried
I'm trying this since yesterday, to trigger a alert() when an ASP Button_Click. However, when I paste this script in Page_Load it works perfectly fine.
ScriptManager.RegisterStartupScript(this, typeof(Page), "alert", "alert('Fill all fields.');", true);

I have also tried it with below code, ClientScript.RegisterStartupScript which works on page load and not on ASP Button_Click
ClientScript.RegisterStartupScript(this.GetType(), "Alert", "alert('Fill all fields.');", true);

what I need to do
Need to validate the textboxes present in client side, if these textboxes are empty it should display a alert dialog.

A:

ScriptManager.RegisterStartupScript(this, typeof(Page), "alert", "alert('Fill all fields.');", true);

return;

