C#-Code

using System;
using System.Collections.Generic;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Windows.Forms;

namespace _274_Taschenrechner
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            this.InitControls();
        }

        private void InitControls()
        {
            this.Width = 350;
            this.Height = 400;
            this.BackColor = Color.Green;
            this.MaximizeBox = false;
            this.FormBorderStyle = FormBorderStyle.Fixed3D;
            this.Text = "A Simple Green Calculator";

            TextBox t = new TextBox();
            t.Name = "output";
            t.Width = this.Width - 40;
            t.ReadOnly = true;
            t.Location = new Point(10, 10);

            this.Controls.Add(t);

            int size = 50;
            int x = this.Width - 100, y = t.Bottom + 20;

            Button b;

            for (int i = 9; i >= 1; i--)
            {
                b = new Button();
                b.Name = "" + i;
                b.Text = b.Name;
                b.Width = size;
                b.Height = size;
                b.FlatStyle = FlatStyle.Flat;
                b.BackColor = Color.LightGreen;
                b.Click += Click_Digit;

                if (i % 3 == 0 && i != 9)
                {
                    y += 75;
                    x = this.Width - 175;
                }
                else
                {
                    x -= 75;
                }

                b.Location = new Point(x, y);

                this.Controls.Add(b);
            }

            x = 28;

            b = new Button();
            b.Name = "" + 0;
            b.Text = b.Name;
            b.Width = size;
            b.Height = size;
            b.FlatStyle = FlatStyle.Flat;
            b.BackColor = Color.LightGreen;
            b.Location = new Point(x, y + 75);
            b.Click += Click_Digit;
            this.Controls.Add(b);

            x += 75;

            b = new Button();
            b.Name = "C";
            b.Text = b.Name;
            b.Width = size;
            b.Height = size;
            b.FlatStyle = FlatStyle.Flat;
            b.BackColor = Color.LightPink;
            b.Location = new Point(x, y + 75);
            b.Click += Click_Reset;
            this.Controls.Add(b);

            x += 75;

            b = new Button();
            b.Name = "=";
            b.Text = b.Name;
            b.Width = size;
            b.Height = size;
            b.FlatStyle = FlatStyle.Flat;
            b.BackColor = Color.Gold;
            b.Location = new Point(x, y + 75);
            b.Click += Click_Calculate;
            this.Controls.Add(b);

            x = this.Width - 100;
            y = t.Bottom + 20;

            List<string> operators = new List<string>() { "+", "-", "*", "/" };

            foreach(string s in operators)
            {
                b = new Button();
                b.Name = s;
                b.Text = b.Name;
                b.Width = size;
                b.Height = size;
                b.FlatStyle = FlatStyle.Flat;
                b.BackColor = Color.LightSkyBlue;
                b.Location = new Point(x, y);
                b.Click += Click_Operator;

                y += 75;

                this.Controls.Add(b);
            }
        }

        private TextBox getOutputTextbox()
        {
            return this.Controls.Find("output", true).FirstOrDefault() as TextBox;
        }

        private void Click_Digit(object sender, EventArgs e)
        {
            Button b = sender as Button;
            TextBox tb = this.getOutputTextbox();
            tb.Text += b.Name;
        }

        private void Click_Reset(object sender, EventArgs e)
        {
            TextBox tb = this.getOutputTextbox();
            tb.Clear();
        }

        private void Click_Calculate(object sender, EventArgs e)
        {
            TextBox tb = this.getOutputTextbox();
            tb.Text = new DataTable().Compute(tb.Text, null).ToString();
        }
        

        private void Click_Operator(object sender, EventArgs e)
        {
            Button b = sender as Button;
            TextBox tb = this.getOutputTextbox();
            tb.Text += b.Name;
        }
    }
}

C#-Code

namespace _274_Taschenrechner
{
    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(9F, 20F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(182, 25);
            this.Font = new System.Drawing.Font("Microsoft Sans Serif", 12F, System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point, ((byte)(0)));
            this.Margin = new System.Windows.Forms.Padding(4, 5, 4, 5);
            this.Name = "Form1";
            this.Text = "Form1";
            this.ResumeLayout(false);

        }

        #endregion
    }
}