Windows Forms + System.Xml

Form1.cs

using System;
using System.Collections.Generic;
using System.Windows.Forms;
using System.Xml;

namespace XmlExp
{
    public partial class Form1 : Form
    {
        public const string XML_DATA = "http://www.ecb.int/stats/eurofxref/eurofxref-daily.xml";
        //public const string XML_DATA = "x.xml";
        private List<Kurs> k = new List<Kurs>();

        public Form1()
        {
            InitializeComponent();
            parseXml();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            fillListBox1();
        }

        private void fillListBox1()
        {
            foreach (Kurs kk in k)
                listBox1.Items.Add($"{kk.Currency}\t{kk.Rate}");
        }

        private void parseXml()
        {
            XmlReader xmlReader = XmlReader.Create(XML_DATA);
            k.Clear();

            while (xmlReader.Read())
            {
                if (xmlReader.NodeType == XmlNodeType.Element && xmlReader.Name == "Cube")
                {
                    if (xmlReader.HasAttributes)
                        if (xmlReader.AttributeCount == 2)
                            k.Add(new Kurs(xmlReader.GetAttribute("currency"), double.Parse(xmlReader.GetAttribute("rate").Replace(".", ","))));
                }
            }
        }

        private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            textBox1.Text = k[listBox1.SelectedIndex].Currency;
            textBox2.Text = k[listBox1.SelectedIndex].Rate.ToString();
        }

        private void textBox3_Leave(object sender, EventArgs e)
        {
            if (textBox3.Text == "" || listBox1.SelectedIndex == -1)
                return;

            try
            {
                textBox4.Text = Math.Round(double.Parse(textBox3.Text) * k[listBox1.SelectedIndex].Rate, 2).ToString();
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
    }
}

Kurs.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace XmlExp
{
    class Kurs
    {
        public Kurs(string currency, double rate)
        {
            this.Currency = currency;
            this.Rate = rate;
        }

        public string Currency { get; set; }
        public double Rate { get; set; }
    }
}