C# Web browser and the save as pop up

The problem is not correctly recognized. You really want to save a file, not to press any button, which would be a typical abuse of technology. If you need to save some data in a file without user intervention do a simple thing — forget about using any dialog box at all.

Now, a dialog box has nothing to do with file/stream I/O, it only provides a file name. Instead, you need to come up some schema with auto-generated file names.

I would suggest to a name a file by the system time. Something like that:

System.DateTime time = System.DateTime.Now;
//to get the time in the Universal Sortable Form without ':',
//such as :2011-06-15T13-45-30
string time = time.ToString("s").Replace(':', '-');
string fileName = string.Format("{0}.xls", time);

It will give you a file name unique with 1-sec resolution.

See http://msdn.microsoft.com/en-us/library/az4se3k1.aspx[^].

If some file come more frequently, you can use system ticks (100-ns intervals, frequent enough for any practical purpose:

System.DateTime time = System.DateTime.Now;
//format good to fit in the biggest hex number of the type long:
string fileName = string.Format("{0:X16}.xls", time.Ticks);

—SA