How to use HttpRequestMessage truely and avoid Error 500 in HttpClient PostAsync?

It is supposed to fill “textbox1” with string value and get another string in textbox3. For this job, I inspected ASP form Elements in the website and checked in “Network” section the “POST” activities when I fill the form and submit it.Therefore I wrote this code for sending the post request and getting the response. But I get the error which tells me “Make sure request headers are used with “HttpRequestMessage” and “HttpResponseMessage”. Before I adding the headers section I got Error 500 in console. please tell me the correct coding.

using System;
using System.Collections.Generic;
using System.Text;
using System.Threading.Tasks;
using System.Net.Http;
using System.Net.Http.Headers;



namespace WebRequest
{
    class myRequest3
    {
      
        public async static void submitpost()
        {
            using (var client = new HttpClient())
            {
                var values = new Dictionary<string, string>
        {
           {"ctl00$ScriptManager1","ctl00$content$ctl00|ctl00$content$Button4" },
           {"__EVENTTARGET","" },
           {"__EVENTARGUMENT","" },
           {"__VIEWSTATE", "/wEPDwUKLTM1MTY1NjE1Nw9kFgJmD2QWAgIJD2QWAgIDD2QWAgIDD2QWAgIBD2QWAmYPZBYCAgEPDxYCHgZfVGl0bGUFGtiz2LHbjNin2YQg2YHYudin2YQg2LPYp9iyZGRkP5ZIUYR1R/p/JMF4Ez0q2psS7pQ=    " },
           {"__VIEWSTATEGENERATOR", "CA0B0334" },
           {"__EVENTVALIDATION", "/wEWBAL6k4rsBQLt4KPyCgL84ejwCALt4KvyCh3brVZZShAeom3oNb8A0uWDpqDK" },
            {"ctl00$content$TextBox1", "hfc0Ac0TIaFPrpef5IGZJQTrCrBdm4S+" },
            {"ctl00$content$TextBox3", "" },
            {"__ASYNCPOST", "true" },
            {"ctl00$content$Button4", "ساخت سریال فعال ساز" },
        };

                var content = new FormUrlEncodedContent(values);
                client.DefaultRequestHeaders.Add("Accept","*/*");
                client.DefaultRequestHeaders.Add("Accept-Encoding","gzip, deflate");
                client.DefaultRequestHeaders.Add("Accept-Language", "en-US,en;q=0.5");
                client.DefaultRequestHeaders.Add("Cache-Control", "no-cache");
                client.DefaultRequestHeaders.Add("Connection","keep-alive");
                client.DefaultRequestHeaders.Add("Content-Length", "641");
                client.DefaultRequestHeaders.Add("Content-Type", "application/x-www-form-urlencoded; charset=utf-8");
                client.DefaultRequestHeaders.Add("Host","address");
                client.DefaultRequestHeaders.Add("Origin","http://address");
                client.DefaultRequestHeaders.Add("Referer","http://address/");
                client.DefaultRequestHeaders.Add("User-Agent", "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:79.0) Gecko/20100101 Firefox/79.0");
                client.DefaultRequestHeaders.Add("X-MicrosoftAjax","Delta=true");


                var response = await client.PostAsync("http://address/default.aspx", content);

                var responseString = await response.Content.ReadAsStringAsync();
                
                Console.WriteLine(responseString);
                

            }

            
        }



    }
}

Answers:

Thank you for visiting the Q&A section on Magenaut. Please note that all the answers may not help you solve the issue immediately. So please treat them as advisements. If you found the post helpful (or not), leave a comment & I’ll get back to you as soon as possible.

Method 1

There are few things you need to notice:
Default request headers – some of them are predefined in content or httpclient class, so you should never do that, e.g. the line that is breaking your code

client.DefaultRequestHeaders.Add(“Content-Type”,
“application/x-www-form-urlencoded; charset=utf-8”);

will always give you error, since that is being done when you’re generating content. So you can check it like this :

var content = new FormUrlEncodedContent(values);
var checkContentType = content.Headers.ContentType;// == "application/x-www-form-urlencoded;

So, if we redactor your headers code a little bit with only necessary one’s that’s what we get :

client.DefaultRequestHeaders.Add("Accept", "*/*"); //ok
//  client.DefaultRequestHeaders.Add("Accept-Encoding", "gzip, deflate"); // unnecessary
//  client.DefaultRequestHeaders.Add("Accept-Language", "en-US,en;q=0.5"); // unnecessary
//  client.DefaultRequestHeaders.Add("Cache-Control", "no-cache"); // unnecessary
//  client.DefaultRequestHeaders.Add("Connection", "keep-alive");
//  client.DefaultRequestHeaders.Add("Content-Length", "641"); // creating content handles that - unnecessary
          
//  client.DefaultRequestHeaders.Add("Content-Type", "application/x-www-form-urlencoded; charset=utf-8"); // already set when creating content
//  client.DefaultRequestHeaders.Add("Host", "address"); // unnecessary
//  client.DefaultRequestHeaders.Add("Origin", "http://address"); // unnecessary
//  client.DefaultRequestHeaders.Add("Referer", "http://address/"); //unnecessary
client.DefaultRequestHeaders.Add("User-Agent", "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:79.0) Gecko/20100101 Firefox/79.0");
//  client.DefaultRequestHeaders.Add("X-MicrosoftAjax", "Delta=true"); // unnecessary

After I run the code now the response is 200 (ok), but server returns “0|error|500||” so you will need to check server side params/form data that are required or missing.

Method 2

Error 500 was solved by revising dictionary values. I have entered “__VIEWSTATE” value incorrectly , besides for the first error, some of the values were unnecessary. by parsing the responseString I could reach to textbox3 value.
Here is the correct code:

                var client = new HttpClient();    
                var values = new Dictionary<string, string>{
                    {"__VIEWSTATE", "/wEPDwUKLTM1MTY1NjE1Nw9kFgJmD2QWAgIJD2QWAgIDD2QWAgIDD2QWAgIBD2QWAmYPZBYCAgEPDxYCHgZfVGl0bGUFGtiz2LHbjNin2YQg2YHYudin2YQg2LPYp9iyZGRkP5ZIUYR1R/p/JMF4Ez0q2psS7pQ=" },
                    {"__EVENTVALIDATION", "/wEWBAL6k4rsBQLt4KPyCgL84ejwCALt4KvyCh3brVZZShAeom3oNb8A0uWDpqDK" },
                    {"ctl00$content$TextBox1", "hfd0Ac0TIaFPrpef5IGZJQTrCrBdm4S+" },
                    {"ctl00$content$Button4", "ساخت سریال فعال ساز" },
                };
                var content = new FormUrlEncodedContent(values);
                client.DefaultRequestHeaders.Add("Accept", "*/*");                    
                HttpResponseMessage response = await client.PostAsync("http://address/default.aspx", content);
                Thread.Sleep(50);
                string responseString = await response.Content.ReadAsStringAsync();
                Thread.Sleep(50);
                string textbox3Value= HTMLValue(responseString, "ctl00_content_TextBox3");
                Console.WriteLine("finished");

Here is a custom code I wrote for extracting textBox3 value:

  public static string HTMLValue(string responseResult, string xElement)
        {
            int x = 0;
            int y = 0;
            string value = "";
            x = responseResult.IndexOf(xElement);
            responseResult = responseResult.Substring(x + 1);
            x = responseResult.IndexOf(">");
            y = responseResult.IndexOf("<");
            if(y-x-1>0)value = responseResult.Substring(x + 1, y - x - 1);
            return (value);
        }

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0