<Window x:Class="WpfApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp1"
        mc:Ignorable="d"
        Title="Rechner" Height="415.254" Width="525">
    <Grid>
        <Button x:Name="btSeven" Content="7" HorizontalAlignment="Left" Margin="42,92,0,0" VerticalAlignment="Top" Width="84" Height="45" FontSize="18" Click="btSeven_Click"/>
        <Button x:Name="btEight" Content="8" HorizontalAlignment="Left" Height="45" Margin="131,92,0,0" VerticalAlignment="Top" Width="84" FontSize="18" Click="btEight_Click"/>
        <Button x:Name="btFour" Content="4" HorizontalAlignment="Left" Height="45" Margin="42,142,0,0" VerticalAlignment="Top" Width="84" FontSize="18" Click="btFour_Click"/>
        <Button x:Name="btTwo" Content="2" HorizontalAlignment="Left" Height="45" Margin="131,192,0,0" VerticalAlignment="Top" Width="84" FontSize="18" Click="btTwo_Click"/>
        <Button x:Name="btOne" Content="1" HorizontalAlignment="Left" Height="45" Margin="42,192,0,0" VerticalAlignment="Top" Width="84" FontSize="18" Click="btOne_Click"/>
        <Button x:Name="btFive" Content="5" HorizontalAlignment="Left" Height="45" Margin="131,142,0,0" VerticalAlignment="Top" Width="84" FontSize="18" Click="btFive_Click"/>
        <Button x:Name="btNine" Content="9" HorizontalAlignment="Left" Height="45" Margin="220,92,0,0" VerticalAlignment="Top" Width="84" FontSize="18" Click="btNine_Click"/>
        <Button x:Name="btClear" Content="C" HorizontalAlignment="Left" Height="45" Margin="220,242,0,0" VerticalAlignment="Top" Width="84" FontSize="18" Click="btClear_Click"/>
        <Button x:Name="btSix" Content="6" HorizontalAlignment="Left" Height="45" Margin="220,142,0,0" VerticalAlignment="Top" Width="84" FontSize="18" Click="btSix_Click"/>
        <Button x:Name="btEquals" Content="=" HorizontalAlignment="Left" Height="45" Margin="42,310,0,0" VerticalAlignment="Top" Width="412" FontSize="22" Click="btEquals_Click"/>
        <Button x:Name="btDivision" Content="/" HorizontalAlignment="Left" Height="45" Margin="370,92,0,0" VerticalAlignment="Top" Width="84" FontSize="18" Click="btDivision_Click"/>
        <Button x:Name="btMultiplikation" Content="*" HorizontalAlignment="Left" Height="45" Margin="370,142,0,0" VerticalAlignment="Top" Width="84" FontSize="18" Click="btMultiplikation_Click"/>
        <Button x:Name="btPlus" Content="+" HorizontalAlignment="Left" Height="45" Margin="370,192,0,0" VerticalAlignment="Top" Width="84" FontSize="18" Click="btPlus_Click"/>
        <Button x:Name="btZero" Content="0" HorizontalAlignment="Left" Height="45" Margin="42,242,0,0" VerticalAlignment="Top" Width="84" FontSize="18" Click="btZero_Click"/>
        <Button x:Name="btComma" Content="." HorizontalAlignment="Left" Height="45" Margin="131,242,0,0" VerticalAlignment="Top" Width="84" FontSize="18" Click="btComma_Click"/>
        <Button x:Name="btThree" Content="3" HorizontalAlignment="Left" Height="45" Margin="220,192,0,0" VerticalAlignment="Top" Width="84" FontSize="18" Click="btThree_Click"/>
        <Button x:Name="btMinus" Content="-" HorizontalAlignment="Left" Height="45" Margin="370,242,0,0" VerticalAlignment="Top" Width="84" FontSize="18" Click="btMinus_Click"/>
        <Label x:Name="lbOut" Content="" HorizontalAlignment="Left" Height="60" Margin="42,16,0,0" VerticalAlignment="Top" Width="412" FontSize="36" RenderTransformOrigin="0.502,0.441"/>

    </Grid>
</Window>


















































using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApp1
{
    /// <summary>
    /// Interaktionslogik für MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        string input = string.Empty;
        string operand1 = string.Empty;
        string operand2 = string.Empty;
        char Operation;
        double result = 0.0;

        public MainWindow()
        {
            InitializeComponent();
        }

        private void Calculate()
        {
            lbOut.Content = "";
            operand2 = input;
            double num1, num2;
            double.TryParse(operand1, out num1);
            double.TryParse(operand2, out num2);

            if (Operation == '+')
            {
                result = num1 + num2;
                lbOut.Content = result.ToString();
            }
            else if (Operation == '-')
            {
                result = num1 - num2;
                lbOut.Content = result.ToString();
            }
            else if (Operation == '*')
            {
                result = num1 * num2;
                lbOut.Content = result.ToString();
            }
            else if (Operation == '/')
            {
                if (num2 != 0)
                {
                    result = num1 / num2;
                    lbOut.Content = result.ToString();
                }
                else
                {
                    lbOut.Content = "DIV/Zero!";
                }
            }

            input = string.Empty;
            operand1 = string.Empty;

        }

        private void btSeven_Click(object sender, RoutedEventArgs e)
        {
            input += "7";
            if (operand1 == string.Empty)
            {
                lbOut.Content = input;
            }
            else
            {
                lbOut.Content += "7";
            }
        }

        private void btEight_Click(object sender, RoutedEventArgs e)
        {
            input += "8";
            if (operand1 == string.Empty)
            {
                lbOut.Content = input;
            }
            else
            {
                lbOut.Content += "8";
            }
        }

        private void btNine_Click(object sender, RoutedEventArgs e)
        {
            input += "9";
            if (operand1 == string.Empty)
            {
                lbOut.Content = input;
            }
            else
            {
                lbOut.Content += "9";
            }
        }

        private void btDivision_Click(object sender, RoutedEventArgs e)
        {
            if (operand1 == string.Empty)
            {
                if (lbOut.Content.ToString() != "DIV/Zero!")
                {
                    if (input == string.Empty)
                    {
                        operand1 = result.ToString();
                    }
                    else
                    {
                        operand1 = input;
                    }
                }
                else
                {
                    lbOut.Content = "";
                    operand1 = string.Empty;
                }
                Operation = '/';
                result = 0.0;
                input = string.Empty;
            }
            else
            {
                Calculate();
                Operation = '/';
                operand1 = result.ToString();
                result = 0.0;
            }
            lbOut.Content += "/";
        }

        private void btFour_Click(object sender, RoutedEventArgs e)
        {
            input += "4";
            if (operand1 == string.Empty)
            {
                lbOut.Content = input;
            }
            else
            {
                lbOut.Content += "4";
            }
        }

        private void btFive_Click(object sender, RoutedEventArgs e)
        {
            input += "5";
            if (operand1 == string.Empty)
            {
                lbOut.Content = input;
            }
            else
            {
                lbOut.Content += "5";
            }
        }

        private void btSix_Click(object sender, RoutedEventArgs e)
        {
            input += "6";
            if (operand1 == string.Empty)
            {
                lbOut.Content = input;
            }
            else
            {
                lbOut.Content += "6";
            }
        }

        private void btMultiplikation_Click(object sender, RoutedEventArgs e)
        {
            if (operand1 == string.Empty)
            {
                if (lbOut.Content.ToString() != "DIV/Zero!")
                {
                    if (input == string.Empty)
                    {
                        operand1 = result.ToString();
                    }
                    else
                    {
                        operand1 = input;
                    }
                }
                else
                {
                    lbOut.Content = "";
                    operand1 = string.Empty;
                }
                Operation = '*';
                result = 0.0;
                input = string.Empty;
            }
            else
            {
                Calculate();
                Operation = '*';
                operand1 = result.ToString();
                result = 0.0;
            }
            lbOut.Content += "*";
        }

        private void btOne_Click(object sender, RoutedEventArgs e)
        {
            input += "1";
            if (operand1 == string.Empty)
            {
                lbOut.Content = input;
            }
            else
            {
                lbOut.Content += "1";
            }
        }

        private void btTwo_Click(object sender, RoutedEventArgs e)
        {
            input += "2";
            if (operand1 == string.Empty)
            {
                lbOut.Content = input;
            }
            else
            {
                lbOut.Content += "2";
            }
        }

        private void btThree_Click(object sender, RoutedEventArgs e)
        {
            input += "3";
            if (operand1 == string.Empty)
            {
                lbOut.Content = input;
            }
            else
            {
                lbOut.Content += "3";
            }
        }

        private void btPlus_Click(object sender, RoutedEventArgs e)
        {
            if (operand1 == string.Empty)
            {
                if (lbOut.Content.ToString() != "DIV/Zero!")
                {
                    if (input == string.Empty)
                    {
                        operand1 = result.ToString();
                    }
                    else
                    {
                        operand1 = input;
                    }
                }
                else
                {
                    lbOut.Content = "";
                    operand1 = string.Empty;
                }
                Operation = '+';
                result = 0.0;
                input = string.Empty;
            }
            else
            {
                Calculate();
                Operation = '+';
                operand1 = result.ToString();
                result = 0.0;
            }
            lbOut.Content += "+";
        }

        private void btZero_Click(object sender, RoutedEventArgs e)
        {
            input += "0";
            if (operand1 == string.Empty)
            {
                lbOut.Content = input;
            }
            else
            {
                lbOut.Content += "0";
            }
        }

        private void btComma_Click(object sender, RoutedEventArgs e)
        {
            lbOut.Content = "";
            if (input == string.Empty)
            {
                input += "0.";
            }
            else
            {
                input += ".";
            }
            lbOut.Content = input;
        }

        private void btClear_Click(object sender, RoutedEventArgs e)
        {
            lbOut.Content = "";
            input = string.Empty;
            operand1 = string.Empty;
            operand2 = string.Empty;
        }

        private void btMinus_Click(object sender, RoutedEventArgs e)
        {
            if (operand1 == string.Empty)
            {
                if (lbOut.Content.ToString() != "DIV/Zero!")
                {
                    if (input == string.Empty)
                    {
                        operand1 = result.ToString();
                    }
                    else
                    {
                        operand1 = input;
                    }
                }
                else
                {
                    lbOut.Content = "";
                    operand1 = string.Empty;
                }
                Operation = '-';
                result = 0.0;
                input = string.Empty;
            }
            else
            {
                Calculate();
                Operation = '-';
                operand1 = result.ToString();
                result = 0.0;
            }
            lbOut.Content += "-";
        }

        private void btEquals_Click(object sender, RoutedEventArgs e)
        {
            Calculate();
        }
    }
}