Getting an error message saying missing parameter in StringContent, but it is present?

Question

There is a code like this:

 using (var client = new HttpClient())
    {
        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

        HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, url);
        request.Headers.TryAddWithoutValidation("User-Agent", userAgent);
        request.Content = new StringContent("{" +
                    $"\"grant_type\":\"authorization_code\"," +
                    $"\"client_id\":\"*****\"," +
                    $"\"client_secret\":\"****\"," +
                    $"\"code\":\"{autorizationCode}\"," +
                    $"\"redirect_uri\":\"urn:ietf:wg:oauth:2.0:oob\"" +
                    "}", Encoding.UTF8);

        var response = await client.SendAsync(request);
        Token = response.Content.ReadAsStringAsync().Result.ToString();
    }

When I send a request, it gives me an error - "{"error":"invalid_request","error_description":"Missing required parameter: grant_type."}", but grant_type is present.

The request on the site looks like this:

curl -X POST "https://site/oauth/token" \
-H "User-Agent: APPLICATION_NAME" \
-F grant_type="authorization_code" \
-F client_id="CLIENT_ID" \
-F client_secret="CLIENT_SECRET" \
-F code="AUTORIZATION_CODE" \
-F redirect_uri="REDIRECT_URI"

Why is he giving this error? How can I fix it?

Answer 1

The CURL-Parameter -F represents form content, not JSON-Content.

If you want to send FormContent, you cannot use StringContent, you need to use FormUrlEncodedContent, like this:

using (var client = new HttpClient())
    {
        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

        HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, url);
        request.Headers.TryAddWithoutValidation("User-Agent", userAgent);
        request.Content = new FormUrlEncodedContent( new List<KeyValuePair<string, string>>() {
              new KeyValuePair<string,string>("grant_type", "authorization_code"),
              new KeyValuePair<string,string>("client_id", "*****"),
              new KeyValuePair<string,string>("client_secret", "*****"),
              new KeyValuePair<string,string>("code", $"{autorizationCode}"),
              new KeyValuePair<string,string>("redirect_uri", "urn:ietf:wg:oauth:2.0:oob")
       } );

        var response = await client.SendAsync(request);
        Token = response.Content.ReadAsStringAsync().Result.ToString();
    }

/Edit: Regarding your comment, your endpoint also supports JSON and you missed the content-type. I'll leave this here in case anyone gets into the problem with an exact CURL-Request like you mentioned.