C#-Code

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;

namespace _266_Flaechenberechnung
{
    public partial class Form1 : Form
    {
        const string WIDTH = "Breite";
        const string HEIGHT = "Höhe";
        const string RESULT = "Ergebnis";
        const string RECTANGLE = "Rechteck";
        const string TRIANGLE = "Dreieck";
        const string EXIT = "Beenden";

        private Dictionary<string, System.Windows.Forms.Panel> panels;

        private double _widht;
        private double _height;
        private double _result;
        private string _mode;


        public Form1()
        {
            InitializeComponent();

            this._widht = 0.0;
            this._height = 0.0;
            this._result = 0.0;
            this._mode = "";

            this.InitForm();
        }

        private void InitForm()
        {
            this.Width = 500;
            this.Height = 500;

            this.panels = new Dictionary<string, Panel>();

            #region panels

            Panel pTop = new Panel();
            pTop.Name = "panel_top";
            pTop.Width = this.Width;
            pTop.Height = 100;
            pTop.Location = new Point(0, 0);
            //pTop.BackColor = Color.Green;
            this.panels.Add(pTop.Name, pTop);
            this.Controls.Add(pTop);

            Panel pLeft = new Panel();
            pLeft.Name = "panel_left";
            pLeft.Width = this.Width / 2;
            pLeft.Height = this.Height - pTop.Height * 2;
            pLeft.Location = new Point(0, pTop.Height);
            //pLeft.BackColor = Color.Red;
            this.panels.Add(pLeft.Name, pLeft);
            this.Controls.Add(pLeft);

            Panel pRight = new Panel();
            pRight.Name = "panel_right";
            pRight.Width = this.Width / 2;
            pRight.Height = this.Height - pTop.Height * 2;
            pRight.Location = new Point(pRight.Width, pTop.Height);
            //pRight.BackColor = Color.Blue;
            this.panels.Add(pRight.Name, pRight);
            this.Controls.Add(pRight);

            Panel pBottom = new Panel();
            pBottom.Name = "panel_bottom";
            pBottom.Width = this.Width;
            pBottom.Height = pTop.Height;
            pBottom.Location = new Point(0, pLeft.Bottom);
            //pBottom.BackColor = Color.Yellow;
            this.panels.Add(pBottom.Name, pBottom);
            this.Controls.Add(pBottom);

            #endregion

            #region Headline

            Label headline = new Label();
            headline.Text = "Flächenberechnung";
            headline.Width = pTop.Width / 2;
            headline.TextAlign = ContentAlignment.MiddleCenter;
            headline.Font = new Font(headline.Font.Name, 16, FontStyle.Underline);
            headline.Height = (int)headline.Font.Size * 2;
            headline.Location = new Point((this.Width / 2) - (headline.Width / 2), 10);
            pTop.Controls.Add(headline);

            #endregion

            #region description + input + buttons

            for (int i = 0; i < 3; i++)
            {
                string t = this.getTextByIndexAndType(i, new Label());

                Label description = new Label();
                description.Name = "label_" + t;
                description.Text = t;
                description.Width = pLeft.Width;
                description.Height = pLeft.Height / 3;
                description.TextAlign = ContentAlignment.MiddleCenter;
                description.Location = new Point(0, (pLeft.Height / 3) * i);
                pLeft.Controls.Add(description);

                TextBox input = new TextBox();
                input.Name = "input_" + t;
                input.Text = "";
                input.Width = pRight.Width - 50;
                double h = (pRight.Height / 3);
                input.Location = new Point(0, (int)(h * i) + (int)h / 2 - 5);

                if(t == Form1.RESULT)
                {
                    input.ReadOnly = true;
                }
                else
                {
                    input.KeyUp += CustomTextboxInputHandler;
                }

                pRight.Controls.Add(input);

                t = this.getTextByIndexAndType(i, new Button());

                Button b = new Button();
                b.Name = "button_" + t;
                b.Text = t;
                b.BackColor = Color.White;
                b.Width = pBottom.Width / 3 - 10;
                b.Height = pBottom.Height - 50;
                b.Location = new Point(b.Width * i + 5, 0);
                b.Click += CustomButtonHandler;
                pBottom.Controls.Add(b);
            }

            #endregion
        }

        private Control getControl(Panel p, string controlName, Type t)
        {
            var controls = p.Controls;

            foreach(Control c in controls)
            {
                if(c.Name == controlName)
                {
                    return c;
                }
            }

            return null;
        }

        private void CustomButtonHandler(object sender, EventArgs e)
        {
            Button b = sender as Button;

            string bName = b.Name.Replace("button_", "");

            if(bName == Form1.EXIT)
            {
                if(MessageBox.Show("Wirklich beenden?", "Beenden?", MessageBoxButtons.YesNo, MessageBoxIcon.Question) == DialogResult.Yes)
                {
                    Application.Exit();
                }

                return;
            }

            // Farbe aller Buttons zurücksetzen
            var buttons = this.panels["panel_bottom"].Controls;

            foreach (Button tempButton in buttons)
            {
                tempButton.BackColor = Color.White;
            }

            b.BackColor = Color.LightGreen;
            this._mode = bName;

            if (this._mode != "")
            {
                this.CalcResult();
            }
        }

        private void CustomTextboxInputHandler(object sender, EventArgs e)
        {
            TextBox t = sender as TextBox;

            double result;

            if(Double.TryParse(t.Text, out result))
            {
                t.BackColor = Color.White;

                string tName = t.Name.Replace("input_", "");

                switch (tName)
                {
                    case Form1.WIDTH:
                        this._widht = result;
                        break;
                    case Form1.HEIGHT:
                        this._height = result;
                        break;
                    default:
                        break;
                }

                if(this._mode != "")
                {
                    this.CalcResult();
                }
            }
            else
            {
                t.BackColor = Color.LightPink; // Ungültiger Input markieren
            }
        }

        private void CalcResult()
        {
            TextBox tResult = this.getControl(this.panels["panel_right"], "input_Ergebnis", typeof(TextBox)) as TextBox;
            double result = 0.0;


            switch (this._mode)
            {
                case Form1.RECTANGLE:
                    result = this._widht * this._height;
                    break;
                case Form1.TRIANGLE:
                    result = 0.5 * this._widht * this._height;
                    break;
                default:
                    break;
            }

            tResult.Text = result.ToString();
        }

        private string getTextByIndexAndType(int i, object o)
        {
            if(o.GetType() == typeof(Label))
            {
                switch (i)
                {
                    case 0:
                        return Form1.WIDTH;
                    case 1:
                        return Form1.HEIGHT;
                    case 2:
                        return Form1.RESULT;
                    default:
                        return "";
                }
            }
            else if(o.GetType() == typeof(Button))
            {
                switch (i)
                {
                    case 0:
                        return Form1.RECTANGLE;
                    case 1:
                        return Form1.TRIANGLE;
                    case 2:
                        return Form1.EXIT;
                    default:
                        return "";
                }
            }
            else
            {
                return "";
            }
        }
    }
}

C#-Code

namespace _266_Flaechenberechnung
{
    partial class Form1
    {
        /// <summary>
        /// Erforderliche Designervariable.
        /// </summary>
        private System.ComponentModel.IContainer components = null;

        /// <summary>
        /// Verwendete Ressourcen bereinigen.
        /// </summary>
        /// <param name="disposing">True, wenn verwaltete Ressourcen gelöscht werden sollen; andernfalls False.</param>
        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        #region Vom Windows Form-Designer generierter Code

        /// <summary>
        /// Erforderliche Methode für die Designerunterstützung.
        /// Der Inhalt der Methode darf nicht mit dem Code-Editor geändert werden.
        /// </summary>
        private void InitializeComponent()
        {
            this.SuspendLayout();
            // 
            // Form1
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(184, 11);
            this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.FixedToolWindow;
            this.MaximizeBox = false;
            this.Name = "Form1";
            this.Text = "266";
            this.ResumeLayout(false);

        }

        #endregion
    }
}