How to make Newtonsoft.Json return pure json?

Question

JSON

There is a C# service that gives a json string to a js script. I am using the Newtonsoft.Json library. But in my implementation, the service wraps the json in the <string></string> tag, which, accordingly, prevents me from working with it normally on the client. How can you get rid of it?
JS code:

function VerifySignature() {
  var keyInfo = GetKeyInfo();
  // отправляем данные в контроллер для валидации сертификата
  $.ajax({
    type: "POST",
    url: "VerifySignatureService.asmx/VerifySignature",
    data: { certInfo: JSON.stringify(keyInfo) },
    success: function (result) {
      alert(result);
      console.log(result);
    }
  });
}

C# code:

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.ComponentModel.ToolboxItem(false)]
[System.Web.Script.Services.ScriptService]
public class VerifySignatureService : System.Web.Services.WebService
{
  [WebMethod]
  [ScriptMethod(UseHttpGet = false, ResponseFormat = ResponseFormat.Json)]
  public string VerifySignature(string certInfo)
  {
    ResponseType response = new ResponseType();
    if (string.IsNullOrEmpty(certInfo) || certInfo == "[]")
    {
      response.isError = true;
      response.isValid = false;
      response.message = "Пришел пустой keyInfo";
      return JsonConvert.SerializeObject(response);
    }
    var keyInfo = JsonConvert.DeserializeObject<KeyInfo>(certInfo);
    if (CertificateVerifier.VerifyCertificateSign(true, keyInfo))
    {
      response.isError = false;
      response.isValid = true;
      response.message = Resources.SuccessfullySigned;
    }
    else
    {
      response.isError = false;
      response.isValid = false;
      response.message = "Ваш ключ не прошел проверку";
    }
    return JsonConvert.SerializeObject(response, Formatting.Indented);
  }
}
    
public class ResponseType
{
  public string message;
  public bool isError;
  public bool isValid;
}

Console.log(result) output:

<string xmlns="http://tempuri.org/">{ "message": "Документ успешно подписан", "isError": false, "isValid": true }</string>

Answer 1

Newtonsoft.Json has nothing to do with it. The point is in the request, from the client side. You need to specify the application/json content type and pass the JSON to data :

$.ajax({
  type: "POST",
  url: "WebService1.asmx/VerifySignature",
  contentType: "application/json",
  data: JSON.stringify({ certInfo: keyInfo }), // обернуть все параметры
  success: function (result) {
    alert(result);
    console.log(result);
  }
});