Combine Alert Popup In Save As File Function In C#?

I'm tring to write a function that need to save a file (in csv) and has a condition.

it's something like that:

If the file is small then 1MB then I want to pop up an alert in javascript and then continue with the save as file commands.

What I did was:

if(...)
{
}
else
{
ClientScript.RegisterStartupScriptBlock(...alert('aaa')...);
}
Response.ContentType = "text/csv";
Response.AppendHeader("Content-Disposition","attachment; filename=file.csv");
Response.TransmitFile("FILE.csv");
Response.End();

the save as works fine. but the alert wont popup in the else case. I tried RegisterStartupScript. I tried many options. But it seems that the response.end is stops the clientscript from happening.

I am writing in c# asp.net (ajax enabled website, but not in use), visual studio 2005.