HttpWebRequest in c# do not work with .net 4.5 -

i'm working on c# project send xml server , receives xml response.
.net framework 4.0 installed works fine.
.net framework 4.5 installed throws exception:

system.nullreferenceexception: der objektverweis wurde nicht auf eine objektinstanz festgelegt.   bei system.domainnamehelper.idnequivalent(string hostname)   bei system.uri.get_idnhost()   bei system.net.httpwebrequest.getsafehostandport(uri sourceuri, boolean adddefaultport, boolean forcepunycode)   bei system.net.httpwebrequest.generateproxyrequestline(int32 headerssize)   bei system.net.httpwebrequest.serializeheaders()   bei system.net.httpwebrequest.endsubmitrequest()   bei system.net.httpwebrequest.checkdeferredcalldone(connectstream stream)   bei system.net.httpwebrequest.begingetresponse(asynccallback callback, object state)   bei fahrzeugverwaltungsserver.outsideworld.man_integrationsserver.rawcommunication.isserver.dopostandget()`   

i use method begingetresponse , parameters there not null.
know what's wrong?
why work 4.0 not 4.5?
did forget set up?

edit 1

private void dopostandget()     {         try         {             //caching               inform(systemicons.information, translations.isserver_postandget_0);               trace.traceinformation("out:\n" + beautify(inputxml));                string c = cache.get(inputxml.outerxml);             if (c != null)             {                 xmldocument docl = new xmldocument();                 docl.loadxml(c);                 inform(systemicons.information, translations.isserver_postandget_1);                 printindocument(docl, "aus cache.");                 this.doc = docl;             }              //read access information:             uribuilder urib = new uribuilder("http", manhaendlerdaten.is_host, 9005, manhaendlerdaten.is_path);              urib.username = manhaendlerdaten.is_user;             urib.password = manhaendlerdaten.is_password;              string proxyuser = manhaendlerdaten.is_proxy_user;             string proxypassword = manhaendlerdaten.is_proxy_password;              // create credentials request's header:             var proxy =             convert.tobase64string(             encoding.utf8.getbytes(proxyuser + ":" + proxypassword));              var user =             convert.tobase64string(             encoding.utf8.getbytes(urib.username + ":" + urib.password));              //set proxy when needed:             try             {                 webrequest.defaultwebproxy = new webproxy(manhaendlerdaten.is_proxy_ip, manhaendlerdaten.is_proxy_port);                 if (webrequest.defaultwebproxy == null)                     trace.writeline(string.format("webrequest.defaultwebproxy ist null. {0}, {1}", manhaendlerdaten.is_proxy_ip, manhaendlerdaten.is_proxy_port));             }             catch (exception e)             {                 trace.traceerror("1\n" + e.tostring());                 debug.writeline(translations.isserver_postandget_3);                 webrequest.defaultwebproxy = null; //speed further request avoiding proxy-auto-detect                 //pass when no proxy specified             }              // system.net.servicepointmanager.expect100continue = false //this nasty 1 if not set false                          client = (httpwebrequest)webrequest.create(urib.uri);              //encodings:             client.headers.add("accept-encoding", "deflate");              client.contenttype = "text/xml; charset=utf-8";              client.accept = "text/xml; charset=utf-8";              client.headers.add("soapaction", "\"\"");              //authentification:                     client.headers.add("proxy-authorization", "basic " + proxy);              client.headers.add("authorization", "basic " + user);               //connection , protocol:             client.host = urib.host;              client.useragent = translations.fullservicename;              client.protocolversion = httpversion.version10;              client.keepalive = true;              client.method = webrequestmethods.http.post;              client.timeout = 60000;              client.proxy = new webproxy(manhaendlerdaten.is_proxy_ip, manhaendlerdaten.is_proxy_port);              if (client.proxy == null)                 trace.writeline(string.format("client.proxy ist null. {0}, {1}", manhaendlerdaten.is_proxy_ip, manhaendlerdaten.is_proxy_port));              client.readwritetimeout = 60000;               //accept cookies within isserver-instance             if (this.cookiecont == null)             {                 this.cookiecont = new cookiecontainer();             }              client.cookiecontainer = cookiecont;               inform(systemicons.information, translations.isserver_postandget_7);              //post request:             using (stream to_request = client.getrequeststream())             {                 inputxml.save(to_request);                 to_request.flush();             }               requeststate myrequeststate = new requeststate();             myrequeststate.request = client;               webrequestresponse = false;             iasyncresult asyncresult = client.begingetresponse(new asynccallback(finishwebrequest), myrequeststate);             while (webrequestresponse == false)             {                 thread.sleep(100);             }         }         catch (exception e)         {             trace.traceerror(e.tostring());             throw e;         } }   

edit 2
in config file use appsettings individual settings. like:
<add key="database_connection" value="firebird"/>

to honest targeting .net 4.5 have using httpclient instead of httpwebrequest.

httpclient