using System;

using System.IO;

using System.Text;

public class Program

{

    public static void Main()

    {

        var json = "\uFEFF{ \"invoiceId\": 123456 }";

        using (var stream = new MemoryStream(Encoding.UTF8.GetBytes(json)))

        using (var reader = new StreamReader(stream))

        using (var memstream = new MemoryStream())

        {

           stream.ToArray().Dump(); // starts with BOM (239,187,191)

​

           reader.BaseStream.CopyTo(memstream);

           var bytes = memstream.ToArray();

            bytes.Dump();           // also starts with BOM (239,187,191)

        }

​

        using (var stream = new MemoryStream(Encoding.UTF8.GetBytes(json)))

        using (var reader = new StreamReader(stream))

        {

           var requestBody = reader.ReadToEnd(); 

           Encoding.UTF8.GetBytes(requestBody).Dump(); // no BOM

        }

    }

}