ScriptManager.RegisterStartupScript not working on ASP Button click event

Question

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);

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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.

Answer 1

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

return;