Form1.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Diagnostics;
using System.Globalization;

using Newtonsoft.Json;
using Newtonsoft.Json.Linq;

namespace CurrencyExchange
{
    public partial class Form1 : Form
    {
        private ApiConn api_connection;
        private JToken list_cur;
        public Form1()
        {
            InitializeComponent();
            api_connection = new ApiConn();


            list_cur = api_connection.getAllCurrency();
            addCurrencyToComboBox();

            resetUI();
        }

        private void resetUI()
        {
            this.curr1_label.Text = "";
            this.curr2_label.Text = "";
            this.curr2_name_label.Text = "";
        }

        private void addCurrencyToComboBox()
        {
            foreach (var cur_item in list_cur)
            {
                foreach (var curr_detail_item in cur_item)
                {
                    string curr_name = curr_detail_item["id"] + " - " + curr_detail_item["currencyName"];
                    this.curr1_comboBox.Items.Add(curr_name);
                    this.curr2_comboBox.Items.Add(curr_name);
                    //Debug.WriteLine(curr_name);
                }
                
            }
        }

        private void curr2_label_Click(object sender, EventArgs e)
        {
            Console.WriteLine("Api conn created");
        }

        private void konversi_button_Click(object sender, EventArgs e)
        {
            if (this.curr1_comboBox.SelectedItem == null || this.curr2_comboBox.SelectedItem == null)
            {
                Debug.WriteLine("BELUM PILIH");
                return;
            }
            string from = this.curr1_comboBox.SelectedItem.ToString().Substring(0, 3);
            string to = this.curr2_comboBox.SelectedItem.ToString().Substring(0,3);

            double rate = api_connection.getCurrencyRate(from, to);
            double conv_res = Double.Parse(input_curr_textbox.Text) * rate;

            Debug.WriteLine("HASIL ", conv_res);

            this.curr1_label.Text = Double.Parse(input_curr_textbox.Text).ToString("C3", CultureInfo.CurrentCulture).Substring(1) + " " + curr1_comboBox.SelectedItem.ToString().Substring(6) + " =";
            this.curr2_label.Text = conv_res.ToString("C3", CultureInfo.CurrentCulture).Substring(1);
            this.curr2_name_label.Text = curr2_comboBox.SelectedItem.ToString().Substring(6) + "s";


            this.curr_rate.Text = "1 " + curr1_comboBox.SelectedItem.ToString().Substring(0, 3) + " = " + rate.ToString("C5", CultureInfo.CurrentCulture).Substring(1) + " "
                + curr2_comboBox.SelectedItem.ToString().Substring(0, 3);
        }

        private void curr1_comboBox_SelectedIndexChanged(object sender, EventArgs e)
        {
            this.symbol_curr_label.Text = list_cur[this.curr1_comboBox.SelectedItem.ToString().Substring(0, 3)]["currencySymbol"].ToString();
        }
    }
}