It might be helpful to see your full example. However, here is a working example with Newtonsoft.Json (do not forget to create your PAT create personal access token):
using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;
namespace ConsoleApp2
{
class Program
{
static void Main(string[] args)
{
string PAT = "<personal access token>"; //https://learn.microsoft.com/en-us/azure/devops/organizations/accounts/use-personal-access-tokens-to-authenticate?view=azure-devops&tabs=preview-page
string requestUrl = "https://dev.azure.com/<my_org>/<my_project>/_apis/wit/workitems/$Task?api-version=5.0";
try
{
List<Object> flds = new List<Object>
{
new { op = "add", path = "/fields/System.Title", value = "Title" }
};
string json = JsonConvert.SerializeObject(flds);
HttpClientHandler _httpclienthndlr = new HttpClientHandler();
using (HttpClient client = new HttpClient(_httpclienthndlr))
{
client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", Convert.ToBase64String(
System.Text.ASCIIEncoding.ASCII.GetBytes(
string.Format("{0}:{1}", "", PAT))));
var request = new HttpRequestMessage(new HttpMethod("PATCH"), requestUrl)
{
Content = new StringContent(json, Encoding.UTF8, "application/json-patch+json")
};
HttpResponseMessage responseMessage = client.SendAsync(request).Result;
}
}
catch (Exception ex)
{
}
}
}
}
Additionally, you can consider to use .NET client libraries for Azure DevOps and TFS. Here is the example: Create a bug in Azure DevOps Services using .NET client libraries
"from": nullelement to the JSON, as outlined in the documentation? Did you set the content-type toapplication/json-patch+json, as indicated in the documentation?