using System;

using System.Text.Json;

public class Something

{

    public Guid Id {get;set;}

}

​

public class Program

{

    public static void Main()

    {

        var serOptions = new JsonSerializerOptions{

         PropertyNamingPolicy = JsonNamingPolicy.CamelCase  

        };

        var json = "{\"id\":\"12345678-abcd-1234-abcd-000000000000\"}";

        var doc = JsonSerializer.Deserialize<Something>(json,serOptions);

        Console.WriteLine($"met options {doc.Id}");

        doc = JsonSerializer.Deserialize<Something>(json);

        Console.WriteLine($"zonder options {doc.Id}");

    }

}