I want to POST from C # to DeepL API

Question

I want to POST from C # to DeepL API. It just doesn't work. Can anyone please tell me how?

var httpWebRequest = (HttpWebRequest)WebRequest.Create("https://api-free.deepl.com/v2/translate");

httpWebRequest.Method = "POST";
httpWebRequest.ContentType = "application/x-www-form-urlencoded";
httpWebRequest.Headers.Add("auth_key", "auth_key");
httpWebRequest.Headers.Add("target_lang", "JA");

using (var streamWriter = new StreamWriter(httpWebRequest.GetRequestStream()))
{
    strSendJson = "{" +
                    "\"text\":\"" + strData.Trim() + "\"," +
                    "}";
}

Answer 1

The DeepL Simpulator is a great place to start out: https://www.deepl.com/docs-api/simulator

When using POST, the auth_key is just another field in the form fields, its not a header.

Because the content type is specified as x-www-form-urlencoded you are expected to send the data as a url encoded form, not JSON. Over the wire this should look something like:

auth_key=[yourAuthKey]&text=Hello, world&target_lang=JA

The following shows how to send this POST request from C# using HttpClient:

string authKey = "~MyAuthKey~";

var client = new HttpClient();
client.DefaultRequestHeaders.Accept.Add(new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/json"));
client.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "application/x-www-form-urlencoded");       
Dictionary<string, string> data = new Dictionary<string, string>
{
    { "auth_key", authKey },
    { "text", strData.Trim() },
    { "target_lang", "JA" }
};
var response = await client.PostAsync("https://api-free.deepl.com/v2/translate", 
    new System.Net.Http.FormUrlEncodedContent(data));
response.EnsureSuccessStatusCode();

var json = response.Content.ReadAsStringAsync().Result;
deeplResponseObj = Newtonsoft.Json.JsonConvert.DeserializeObject<DeepResponse>(json);

foreach(var tx in deeplResponseObj.translations) 
    Console.WriteLine("\"{0}\" (from {1})", tx.text, tx.detected_source_language);

The following class definitions help with deserialization of the response:

public class DeepResponse
{
    List<DeepTranslation> translations { get;set; } = new List<DeepTranslation>();
}

public class DeepTranslation
{
    public string detected_source_language { get;set; }
    public string text { get;set; }
}