Building a simple tui app with https://github.com/migueldeicaza/gui.cs (https://asciinema.org/a/189910)

Auswahl.cs

namespace Test
{
    public class Auswahl
    {
        public Auswahl(Produkt pro, int anzahl)
        {
            this.Pro = pro;
            this.Anzahl = anzahl;
            this.Preis = pro.Preis * anzahl;
        }

        public Produkt Pro { get; set; }
        public int Anzahl { get; set; }
        public double Preis { get; set; }
    }
}

Produkt.cs

using System;
using System.Collections.Generic;
using System.IO;
using Terminal.Gui;

namespace Test
{
    public class Produkt
    {
        public Produkt()
        {

        }

        public Produkt (int nummer, string bezeichnung, double preis, int bestand)
        {
            this.Nummer = nummer;
            this.Bezeichnung = bezeichnung;
            this.Preis = preis;
            this.Bestand = bestand;
        }

        public int Nummer { get; set; }
        public string Bezeichnung { get; set; }
        public double Preis { get; set; }
        public int Bestand { get; set; }

        public override string ToString()
        {
            return this.Nummer.ToString() + ";" + this.Bezeichnung + ";" + this.Preis.ToString() + ";" + this.Bestand.ToString();
        }

        public List<Produkt> leseDatei(List<Produkt> produkte)
        {
            FileStream fs = null;
            StreamReader sr = null;
            string[] fields;

            try
            {
                fs = new FileStream(Constants.FILENAME, FileMode.Open, FileAccess.Read);
                sr = new StreamReader(fs);
 
                while (sr.Peek() != -1)
                {
                    fields = sr.ReadLine().Split(';');
                    produkte.Add(new Produkt(int.Parse(fields[0]), fields[1], double.Parse(fields[2]), int.Parse(fields[3])));
                }
            }
            catch (FileNotFoundException fnfeEx)
            {
                MessageBox.ErrorQuery(50, 5, fnfeEx.Message, "ok");
            }
            catch (Exception ex)
            {
                MessageBox.ErrorQuery(50, 5, ex.Message, "ok");
            }
            finally
            {
                if (sr != null)
                    sr.Close();
                if (fs != null)
                    fs.Close();
            }

            return produkte;
        }

        public void schreibeProdukt(Produkt p)
        {
            FileStream fs = null;
            StreamWriter sw = null;

            try
            {
                fs = new FileStream(Constants.FILENAME, FileMode.Append, FileAccess.Write);
                sw = new StreamWriter(fs);
                sw.WriteLine(p.ToString());
            }
            catch (FileNotFoundException fnfeEx)
            {
                MessageBox.ErrorQuery(50, 5, fnfeEx.Message, "ok");
            }
            catch (Exception ex)
            {
                MessageBox.ErrorQuery(50, 5, ex.Message, "ok");
            }
            finally
            {
                if (sw != null)
                    sw.Close();
                if (fs != null)
                    fs.Close();
            }
        }
    }
}

Program.cs

using Terminal.Gui;

namespace Test
{
    static class Program
    {
        static void Main()
        {
            Application.Init();
            var tui1 = new Tui1(Application.Top);
            Application.Run(tui1.Top);
        }
    }
}

Tui1.cs

using Terminal.Gui;
using System;
using System.Collections.Generic;
using System.Text;

namespace Test
{
    static class Constants
    {
        public const string FILENAME = "/tmp/Produkte.dat";
    }

    public class Tui1
    {
        List<Produkt> produkte = new List<Produkt>();
        List<Auswahl> auswahl = new List<Auswahl>();
        List<string> listView1List = new List<string>();
        List<string> listView2List = new List<string>();

        public Tui1(Toplevel top)
        {
            this.Top = top;
            this.Tframe = this.Top.Frame;
            designer();
            var p = new Produkt();
            this.produkte = p.leseDatei(this.produkte);
            aktualisiereListView1List();
            aktualisiereListView2List();
        }

        public Toplevel Top { get; set;}
        public Rect Tframe { get; set;}
        private double Gesamtpreis { get; set; }

        private Window win1 = null;
        private Window win2 = null;
        private ListView listView1 = null;
        private ListView listView2 = null;
        private Label label1 = null;
        private Label label2 = null;
        private Label label3 = null;
        private Label label4 = null;
        private Label label5 = null;
        private Label label6 = null;
        private Button button1 = null;
        private Button button2 = null;
        private Button button3 = null;
        private Button button4 = null;
        private Button button5 = null;
        private Button button6 = null;
        private Button button7 = null;
        private TextField textField1 = null;
        private TextField textField2 = null;
        private TextField textField3 = null;
        private TextField textField4 = null;
        private TextField textField5 = null;

        private void designer()
        {
            this.win1 = new Window("Produkte") {
                X = Pos.Percent(0),
                Y = Pos.Percent(0),
                Width = Dim.Fill(),
                Height = Dim.Percent(50)
            };

            this.win2 = new Window("Auswahl") {
                X = Pos.Percent(0),
                Y = Pos.Percent(50),
                Width = Dim.Fill(),
                Height = Dim.Percent(50)
            };

            // win1

            this.listView1 = new ListView(new Rect (0, 0, 20, 10), listView1List) {
                AllowsMarking = true
            };
            this.listView1.SelectedChanged += new Action(listView1__SelectedChangedEvent);

            this.label1 = new Label("Nummer") {
                X = 22,
                Y = 0,
                Width = 20,
                Height = 1
            };

            this.textField1 = new TextField(22, 1, 20, "int");

            this.label2 = new Label("Bezeichnung") {
                X = 22,
                Y = 2,
                Width = 20,
                Height = 1
            };

            this.textField2 = new TextField(22, 3, 20, "string");

            this.label3 = new Label("Preis") {
                X = 22,
                Y = 4,
                Width = 20,
                Height = 1
            };

            this.textField3 = new TextField(22, 5, 20, "double");

            this.label4 = new Label("Bestand") {
                X = 22,
                Y = 6,
                Width = 20,
                Height = 1
            };

            this.textField4 = new TextField(22, 7, 20, "int");

            this.button1 = new Button(44, 0, "new ") {
                Clicked = () => button1__newAction()
            };

            this.button2 = new Button(44, 1, "save") {
                Clicked = () => button2__saveAction()
            };

            this.button5 = new Button(44, 2, "Auswahl") {
                Clicked = () => this.Top.SetFocus(win2)
            };

            this.button7 = new Button(44, 3, "quit") {
                Clicked = () => Application.RequestStop()
            };

            this.win1.Add(listView1, label1, textField1, label2, textField2, label3, textField3, label4, textField4, button1, button2, button5, button7);

            // win2

            this.listView2 = new ListView(new Rect(0, 0, 20, 10), listView2List) {
                AllowsMarking = true
            };
            this.listView2.SelectedChanged += new Action(listView2__SelectedChangedEvent);

            this.label5 = new Label("Anzahl") {
                X = 22,
                Y = 0,
                Width = 20,
                Height = 1
            };

            this.textField5 = new TextField(22, 1, 20, "int");

            this.button3 = new Button(44, 0, "new") {
                Clicked = () => button3__newAction()
            };

            this.button4 = new Button(44, 1, "buy") {
                Clicked = () => button4__buyAction()
            };

            this.button6 = new Button(44, 2, "Produkte") {
                Clicked = () => this.Top.SetFocus(win1)
            };

            this.label6 = new Label("Gesamtsumme: xxx") {
                X = 44,
                Y = 5,
                Width = 40,
                Height = 1
            };

            this.win2.Add(listView2, label5, textField5, button3, button4, label6, button6);

            // zack
            this.Top.Add(win1, win2);
        }

        private void aktualisiereListView1List()
        {
            this.listView1List.Clear();
            this.listView1List.Add("Nummer, Bezeichnung");

            foreach (Produkt p in this.produkte)
            {
                this.listView1List.Add(p.Nummer + ", " + p.Bezeichnung);
            }
        }

        private void button1__newAction()
        {
            this.textField1.Text = string.Empty;
            this.textField2.Text = string.Empty;
            this.textField3.Text = string.Empty;
            this.textField4.Text = string.Empty;
            this.listView1.SelectedItem = 0;
        }

        private void button2__saveAction()
        {

            if (this.listView1.SelectedItem != 0)
            {
                MessageBox.ErrorQuery(80, 5, "Error", "Es darf kein Eintrag in der Listbox ausgewaehlt sein", "ok");
                return;
            }

            if (this.textField1.Text.IsEmpty || this.textField2.Text.IsEmpty || this.textField3.Text.IsEmpty || this.textField4.Text.IsEmpty)
            {
                MessageBox.ErrorQuery(80, 5, "Error", "Alle Felder ausfuellen (int, string, double, int)", "ok");
                return;
            }

            Produkt p = null;

            try
            {
                p = new Produkt(Convert.ToInt32(this.textField1.Text.ToString()), this.textField2.Text.ToString(), Convert.ToDouble(this.textField3.Text.ToString()), Convert.ToInt32(this.textField4.Text.ToString()));
            }
            catch (Exception ex)
            {
                MessageBox.ErrorQuery(80, 5, "Error", ex.Message, "ok");
                return;
            }

            p.schreibeProdukt(p);

            this.produkte.Add(p);
            aktualisiereListView1List();
            this.listView1.Redraw(new Rect(0, 0, 20, 10));

            this.listView1.SelectedItem = this.listView1List.Count - 1;
        }

        private void listView1__SelectedChangedEvent()
        {
            if (this.listView1.SelectedItem == 0)
            {
                this.textField1.Text = "int";
                this.textField2.Text = "string";
                this.textField3.Text = "double";
                this.textField4.Text = "int";
            }
            else
            {
                this.textField1.Text = produkte[this.listView1.SelectedItem - 1].Nummer.ToString();
                this.textField2.Text = produkte[this.listView1.SelectedItem - 1].Bezeichnung;
                this.textField3.Text = produkte[this.listView1.SelectedItem - 1].Preis.ToString();
                this.textField4.Text = produkte[this.listView1.SelectedItem - 1].Bestand.ToString();
            }
        }

        private void aktualisiereListView2List()
        {
            this.listView2List.Clear();
            this.listView2List.Add("Bezeichnung, Anzahl, Preis");

            foreach (Auswahl a in this.auswahl)
            {
                this.listView2List.Add(a.Pro.Bezeichnung + ", " + a.Anzahl + "," + a.Preis);
            }
        }

        private void button3__newAction()
        {
            this.auswahl.Clear();
            aktualisiereListView2List();
            this.listView2.Redraw(new Rect(0, 0, 20, 10));

            this.Gesamtpreis = 0.0;
            this.textField5.Text = "int";
            this.label6.Text = "Gesamtpreis: xxx";

            //this.listView2.SelectedItem = 0;
        }

        private void button4__buyAction()
        {
            if (this.listView1.SelectedItem == 0)
            {
                MessageBox.ErrorQuery(80, 5, "Error", "Es muss ein Produkt ausgewaehlt sein", "ok");
                return;
            }

            try
            {
                if (Convert.ToInt32(this.textField5.Text.ToString()) == 0 || this.textField5.Text.IsEmpty)
                {
                    MessageBox.ErrorQuery(80, 5, "Error", "Auswahl muss korrekt sein (int). Und groesser 0", "ok");
                    return;
                }
            }
            catch (Exception ex)
            {
                MessageBox.ErrorQuery(80, 5, "Error", ex.Message, "ok");
                return;
            }

            if (Convert.ToInt32(this.textField5.Text) > this.produkte[listView1.SelectedItem - 1].Bestand)
            {
                MessageBox.ErrorQuery(80, 5, "Error", "Lagerbestand zu klein", "ok");
            }

            this.auswahl.Add(new Auswahl(this.produkte[listView1.SelectedItem - 1], Convert.ToInt32(this.textField5.Text)));

            aktualisiereListView2List();
            this.listView2.Redraw(new Rect(0, 0, 20, 30));
            this.listView2.SelectedItem = this.listView2List.Count - 1;

            this.Gesamtpreis = 0.0;
            foreach (Auswahl a in auswahl)
                this.Gesamtpreis += a.Preis;

            this.label6.Text = "Gesamtpreis: " + this.Gesamtpreis.ToString();
        }

        private void listView2__SelectedChangedEvent()
        {
            if (this.listView2.SelectedItem == 0)
            {
                this.textField5.Text = "int";
            }
            else
            {
                this.textField5.Text = this.auswahl[this.listView2.SelectedItem - 1].Anzahl.ToString();
            }
        }
    }
}

TODO