How to handle FileUpload max request length exception

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ASP.NET

file

I know the default upload max is 4MB, if we upload some file larger than this, the ugly error page will pop up.
I find somewhere the solution is enlarge the maxRequestLength is web.config. But I think it's not done, the users can always upload the file larger than the maxRequestLength, I want to know how can I pop up a message box when the upload length is exceed the limit, thanks!


In my asp.net website, I write some code in global.asax file, but it didn't work:

void Application_Error(object sender, EventArgs e)
{
    // Code that runs when an unhandled error occurs
    HttpException h = Server.GetLastError() as HttpException;
    if (h != null && h.WebEventCode == System.Web.Management.WebEventCodes.RuntimeErrorPostTooLarge)
    {
        try
        {
            Server.ClearError();
                        Page page = HttpContext.Current.CurrentHandler as Page;
            if (page is Page)
            {
                page.ClientScript.RegisterStartupScript(this.GetType(), "Error", "<script>alert('error');< / script>");
            }
        }
        catch (Exception)
        {


        }
    }

}