Try this example:
public void TheDownload(string path)
{
System.IO.FileInfo toDownload = new System.IO.FileInfo(HttpContext.Current.Server.MapPath(path));
HttpContext.Current.Response.Clear();
HttpContext.Current.Response.AddHeader("Content-Disposition",
"attachment; filename=" + toDownload.Name);
HttpContext.Current.Response.AddHeader("Content-Length",
toDownload.Length.ToString());
HttpContext.Current.Response.ContentType = "application/octet-stream";
HttpContext.Current.Response.WriteFile(patch);
HttpContext.Current.Response.End();
}
The implementation is done in the follows:
TheDownload("@"c:\Temporal\Test.txt"");
Source: http://www.systemdeveloper.info/2014/03/force-downloading-file-from-c.html