Alert messages in asp.net

Try using RegisterStartupScript instead of RegisterClientScriptBlock (which might run before page is constructed)

Ii tried this one.But this is not working

What happened?  Same as in the picture?

yaa same in the picture

The TIMING of your script is very important.

The way you had it originally, your alert was called at the top of the page, before anything else was rendered.

With the RegisterStartupScript, "The script block added by the RegisterStartupScript method executes when the page finishes loading but before the page's OnLoad event is raised."

Perhaps you draw a lot of your page in the OnLoad event?

Anyway, you'll want to arrange it so that the alert is called AFTER you set all the important background.  
If your OnLoad event emits the important stuff, you'll want to emit your script AFTER.

Actually after clicking subit button it will come

You mean the OK button on your alert window?

Yes.Please give me solution for that one.With out refreshing that alert should come.Iam using update panel also.please help me it is urgent

This is what you would expect when you run the javascript too early - it pauses until you complete the alert by pressing OK, then it finishes rendering.

You do NOT want to run that Javascript until your page is completely rendered.

It might be best to emit it as a function definition and adding it as an onload as in here:

http://javascript.about.com/library/blonload.htm

Create a JavaScript, say called "displayAlert()" like this:

function displayAlert()
{
   alert("invalid file");
   return true;
}
then change your code to this:
ScriptManager.RegisterClientScriptBlock(this, typeof(Page), "alert", "displayAlert()", true);
The "return ture" in the JavaScript should let your page display after the alert pops up.
Let me know if it works for you.
 

He says it already DOES display after the alert pops up.

He wants it to finish displaying BEFORE the alert pops up, which means he has to complete everything that needs to be done before triggering that alert.  

Using
                    ScriptManager.RegisterClientScriptBlock(this, typeof(Page), "alert", "displayAlert()", true);
triggers the alert BEFORE the page is rendered.


One needs to know everything that is involved in rendering the page (he has indicated at least a part of this is done using AJAX, which could happen AFTER the page is already completely sent over).  

The alert message is a nice way to see where you ARE, but in the end it needs to be triggered AFTER the last significant display operation has occurred.  

Without full access to your code it is not possible to say exactly where that is - if you are displaying pretty much a blank page, and later filling in with AJAX, you have to time the alert carefully.