previous Request.Form["EventTarget"] not clearing on page load

See more:

ASP.NET

Hello Guys,
I have a issue.i am a making gridview double clikable. doubleclick on row i am calling a function name selectrow(rowindex) and inside function i am calling a javascript __dopostback('selectrow',rowindex) function.on pageload i am taking the value of Request.Form["__EVENTTARGET"] and Request.Form["__EVENTTARGET"] and check if Request.Form["__EVENTTARGET"]&&Request.Form["__EVENTTARGET"] not null then take the index of gridview selected row and call a function to download file listed in colum.till here fine.but after clicking on any button same vale of Request.Form["__EVENTTARGET"]&&Request.Form["__EVENTTARGET"] comes and downloadin start again.if button is not inside updatepannel if button is inside updatepannel javascript runtime error comes.



this is my code to function download

System.IO.FileInfo FileName = new System.IO.FileInfo(_Filename);
if (FileName.Exists)
{
   FileStream myFile = new FileStream(_Filename, FileMode.Open,FileAccess.Read,FileShare.Write);
                    BinaryReader _BinaryReader = new BinaryReader(myFile);
                    long startBytes = 0;
                    string lastUpdateTiemStamp = File.GetLastWriteTimeUtc(_Filename).ToString("r");
                    string _EncodedData = HttpUtility.UrlEncode(_Filename, Encoding.UTF8) + lastUpdateTiemStamp;
                    Response.Clear();
                    Response.Buffer = false;
                    Response.AddHeader("Accept-Ranges", "bytes");
                    Response.AppendHeader("ETag", "\"" + _EncodedData + "\"");
                    Response.AppendHeader("Last-Modified", lastUpdateTiemStamp);
                    Response.ContentType = "application/octet-stream";
                    Response.AddHeader("Content-Disposition", "attachment;filename=" + FileName.Name);
                    Response.AddHeader("Content-Length", (FileName.Length - startBytes).ToString());
                    Response.AddHeader("Connection", "Keep-Alive");

                    //Set the Content Encoding type
                    Response.ContentEncoding = Encoding.UTF8;

                    //Send data

                    _BinaryReader.BaseStream.Seek(startBytes, SeekOrigin.Begin);
                    int maxCount = (int)Math.Ceiling((FileName.Length - startBytes + 0.0) / 1024);
                    int i=0;
                    try
                    {
                        for (i = 0; i < maxCount && Response.IsClientConnected; i++)
                        {
                            Response.BinaryWrite(_BinaryReader.ReadBytes(1024));
                            Response.Flush();
                            if (i == maxCount - 1)
                            {
                                _BinaryReader.Close();
                               
                            }

                        }
                    }
                    catch (Exception ex)
                    {
                        MessageBox.Show(ex.Message);
                    }
                    finally
                    {
                        _BinaryReader.Close();
                        File.Delete(outputFile);
                    }
                  
                    if (i < maxCount)
                    {
                        MessageBox.Show("Error in downloading");
                        //Response.Write("<script language=javascript>confirm('Error in downloading')");
                    }
}

and on pageload this is my code

//////////////////

string source = Request.Form["__EVENTTARGET"];
       string value = Request.Form["__EVENTTARGET"];

if (!string.IsNullOrEmpty(source) && !string.IsNullOrEmpty(value))
       {

           if (source.ToLower().Equals("selectrow"))
           {

               int SelectedRow = int.Parse(value);
               string text = path + "\\" + GridView1.Rows[SelectedRow].Cells[0].Text;
               Download(text);