.NET Fiddle

CSharp · Console

/Program.cs

using System;
using Newtonsoft.Json.Linq;
using System.IO;
using System.Text;

namespace Demo
{
    public class Program
    {
        public static void Main(string[] args)
        {
			var json = "\uFEFF{ \"invoiceId\": 123456 }";
			
			// works
			Console.WriteLine("works:");
			using (var stream = new MemoryStream(Encoding.UTF8.GetBytes(json)))
			using (var reader = new StreamReader(stream))
			{
			   var requestBody = reader.ReadToEnd(); 
			   var parsed1 = JObject.Parse(requestBody);
				Console.WriteLine(parsed1["invoiceId"]);
			}
			
			// fails
			Console.WriteLine("fails:");
			var parsed2 = JObject.Parse(json);
			Console.WriteLine(parsed2["invoiceId"]);
        }
    }
}