vb.net - Exceptional handling in ASP.NET file handler -

i processing pdf/jpg/gif files using http handler.

a popup receives btnid in query string , detects file extension calls handler btnid , processes request.

files physical stored on server file names in db. when there issue file file not found or error want alert displayed , opened window closed not working right now.

i know reason , writing alert response land in either object tag or image tag.

can 1 please suggest how can achieved. appreciated. thanks!!!

public class viewhelpcontent     inherits system.web.ui.page      'method dynamically decides based on file extension file viewer render     protected sub page_load(byval sender object, byval e system.eventargs) handles me.load         dim embed string = string.empty         dim count integer         dim filename string = string.empty         dim extension string = string.empty          try             if not ispostback                 if not request.querystring("btnid") nothing                     dim btnid string = request.querystring("btnid").tostring()                      count = getfileattachedtobutton(btnid, filename)                     extension = path.getextension(filename)                      if extension.tolower() = ".pdf"                         embed = "<object id=""objpdfviewer"" data=""{0}{1}"" type=""application/pdf"">"                         embed &= "if unable view file, can download <a href = ""{0}{1}&download=1"">here</a>"                         embed &= " or there no file available viewed."                         embed &= "</object>"                     elseif extension.tolower() = ".jpeg" orelse extension.tolower() = ".jpg" orelse extension.tolower() = ".gif"                         embed = "<img id=""objimgviewer"" src=""{0}{1}"" alt=""no file available viewed."" />"                     else                         clientscript.registerstartupscript(me.gettype(), "alert", "alert('no content uploaded button.');", true)                         return                     end if                      helpcontent.text = string.format(embed, resolveurl("~/processhelpcontent.ashx?btnid="), btnid)                  end if             end if         catch ex exception             exceptionhandler.logerror(ex)             jshelper.showsystemerror(me)         end try     end sub  end class      public class processhelpcontent     implements system.web.ihttphandler      'sub processes request , generates requested content based on button id     sub processrequest(byval context httpcontext) implements ihttphandler.processrequest         dim btnid string = string.empty         dim filename string = string.empty         dim contenttype string = string.empty         dim outputdir string = string.empty          dim count integer         dim strpath fileinfo = nothing         dim extension string = string.empty          try             if not context.request.querystring("btnid") nothing                 btnid = context.request.querystring("btnid").tostring()                 outputdir = configurationmanager.appsettings("helpcontentfilesfolder")                  count = getfileattachedtobutton(btnid, filename)                  strpath = new fileinfo(outputdir & filename)                  if not string.isnullorwhitespace(filename)                     if file.exists(strpath.fullname)                         extension = path.getextension(strpath.fullname)                          if context.request.querystring("download") = "1"                             context.response.appendheader("content-disposition", convert.tostring("attachment; filename=") & filename)                         end if                     context.response.cache.setcacheability(httpcacheability.nocache)                          if extension.tolower() = ".pdf"                             context.response.contenttype = "application/pdf"                         elseif extension.tolower() = ".jpg" orelse extension.tolower() = ".jpeg"                             context.response.contenttype = "image/jpeg"                         elseif extension.tolower() = ".gif"                             context.response.contenttype = "image/gif"                         end if                          context.response.transmitfile(strpath.fullname)                         context.response.flush()                     else                         context.response.write("<script>alert('uploaded file missing. please re-upload or contact administrator.'); window.close();</script>")                     end if                 else                     context.response.write("<script>alert('uploaded file missing. please re-upload or contact administrator.'); window.close();</script>")                 end if             end if         catch ex exception             exceptionhandler.logerror(ex)             context.response.write("<script>alert('uploaded file missing. please re-upload or contact administrator.'); window.close();</script>")         end try     end sub      readonly property isreusable() boolean implements ihttphandler.isreusable                     return false         end     end property end class