Async download file in c# -

i need improve code not understand made mistake. file downloaded in corrupted format. using cookies required part of method.

/*downlod task */ public static async task<int> createdownloadtask(string urltodownload,string sdestinationpath, string cookiedstr) {     int buffersize = 4096;     int receivedbytes = 0;     int totalbytes = 0;     webclient client = new webclient();     client.headers.add(httprequestheader.cookie, cookiedstr);      using (var stream = await client.openreadtaskasync(urltodownload))     {         using (memorystream ms = new memorystream())         {             int read = 0;             totalbytes = int32.parse(client.responseheaders[httpresponseheader.contentlength]);             var buffer = new byte[buffersize];             while ((read = await stream.readasync(buffer, 0, buffer.length)) > 0)             {                 ms.write(buffer, 0, read);                 filestream file = new filestream(sdestinationpath, filemode.append, fileaccess.write);                 ms.position = 0;                 ms.writeto(file);                 file.flush();                 file.close();                 buffer = new byte[buffersize];                 receivedbytes += read;                 console.writeline(receivedbytes + " " + totalbytes);                 // downloadbytesprogress args = new downloadbytesprogress(urltodownload, receivedbytes, totalbytes);             }             ms.close();         }         return receivedbytes;     } }