using System;

using System.Text;

using System.Linq;

using System.Collections.Generic;

using System.Net.Http.Headers;

using System.Net.Http;

public class Program

{

    public static void Main()

    {

        HttpClient client = new HttpClient();

        client.DefaultRequestHeaders.Accept.Remove(new MediaTypeWithQualityHeaderValue("application/json"));

        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

        client.DefaultRequestHeaders.Add("Accept", "application/json");

        client.DefaultRequestHeaders.Add("Accept", "application/json");

        Console.WriteLine($"{client.DefaultRequestHeaders.Accept}");

​

    }

}