小玩意，毫無任何難度。

cs：

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 計算器
{
    /// <summary>
    /// MainWindow.xaml 的互動邏輯
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
        
 
private double n1,n2,n3;

        private double CM(double n1,double n2,string fun)
        {
            double n3 = 0;

            switch (fun)
            {
                case "+":
                    n3 = n1 + n2;
                    break;
                case "-":
                    n3 
 
= n1 - n2;
                    break;
                case "x":
                    n3 = n1 * n2;
                    break;
                case "÷":
                    n3 = n1 / n2;
                    break;
                case "%":
                    n3 = n1 % n2;
                    break;
            }

            return n3;
        }

        private void Num_Click(object sender,RoutedEventArgs e)
        {
            var num = (sender as Button).Content;
            if (!string.IsNullOrWhiteSpace(T4.Text))
            {
                ClearFun();
            }
            if (!string.IsNullOrEmpty(T2.Text))
            {
                var d = (T3.Text + num).Replace(" ",string.Empty);

                T3.Text = d;
                n2 = double.Parse(T3.Text);
                return;
            }
            var c = (T1.Text + num).Replace(" ",string.Empty);
            T1.Text = c;
            n1 = double.Parse(T1.Text);

        }

        private void Fun_Click(object sender,RoutedEventArgs e)
        {
            var fun = (sender as Button).Content;
            if (!string.IsNullOrEmpty(T4.Text))
            {
                ClearFun();
                return;
            }
            if(!string.IsNullOrEmpty(T1.Text))
            T2.Text = fun.ToString();

        }

        private void Eq_Click(object sender,RoutedEventArgs e)
        {
            if (string.IsNullOrEmpty(T2.Text) || string.IsNullOrEmpty(T1.Text) || string.IsNullOrEmpty(T3.Text))
                return;
            n3 = CM(n1,T2.Text);
            T4.Text = n3.ToString();
            T5.Text = "=";
        }

        private void Clear_Click(object sender,RoutedEventArgs e)
        {
            ClearFun();

        }
        private void Back_Click(object sender,RoutedEventArgs e)
        {
            if (!string.IsNullOrEmpty(T4.Text))
                return;
            if (!string.IsNullOrEmpty(T2.Text))
            {
                var d = T3.Text;
                T3.Text = d.Substring(0,d.Length-1);
                T3.Text = T3.Text == "" ? "0" : T3.Text;
                n2 = double.Parse(T3.Text == "" ? "0" : T3.Text);
                return;
            }
            var c = T1.Text;
            T1.Text = c.Substring(0,c.Length - 1);
            T1.Text = T1.Text == "" ? "0" : T1.Text;
            n1 = double.Parse(T1.Text);
        }

        private void ClearFun()
        {
            T1.Text = string.Empty;
            T2.Text = string.Empty;
            T3.Text = string.Empty;
            T4.Text = string.Empty;
            T5.Text = string.Empty;
        }
    }
}

xaml：

<Window x:Class="計算器.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:計算器"
        mc:Ignorable="d"
        Title="MainWindow" Height="500" Width="400">
    <Window.Resources>
        <Style TargetType="Border">
            <Style.Setters>
                <Setter Property="BorderThickness" Value="1"/>
                <Setter Property="BorderBrush" Value="Black"/>
            </Style.Setters>
        </Style>
        <Style TargetType="Button">
            <Style.Setters>
                <Setter Property="FontSize" Value="25"/>
            </Style.Setters>
        </Style>
        <Style TargetType="TextBlock">
            <Setter Property="MinHeight" Value="90"/>
            <Setter Property="HorizontalAlignment" Value="Right"/>
            <Setter Property="FontSize" Value="28"/>
        </Style>
       
    </Window.Resources>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <Border>
            <TextBlock>
                <Run  x:Name="T1"/>
              <LineBreak/>
                 <Run x:Name="T2"/>
              <LineBreak/>
                  <Run x:Name="T3"/>
              <LineBreak/>
                 <Run x:Name="T5"/>
                 <Run x:Name="T4"/>
            </TextBlock>
        </Border>
        <Grid Grid.Row="1">
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="*"/>
                <ColumnDefinition Width="*"/>
                <ColumnDefinition Width="*"/>
                <ColumnDefinition Width="*"/>
            </Grid.ColumnDefinitions>
            <Grid.RowDefinitions>
                <RowDefinition Height="*"/>
                <RowDefinition Height="*"/>
                <RowDefinition Height="*"/>
                <RowDefinition Height="*"/>
                <RowDefinition Height="*"/>
            </Grid.RowDefinitions>
            <Button x:Name="Clear" Click="Clear_Click" Content="C"  Grid.Column="0" Grid.Row="0" Background="#FFE7E7E7" />
            <Button Click="Fun_Click" Content="÷" Grid.Column="1" Grid.Row="0" Background="#FFE7E7E7" />
            <Button Click="Fun_Click" Content="x"  Grid.Column="2" Grid.Row="0" Background="#FFE7E7E7"/>
            <Button Click="Back_Click" Content="←" Grid.Column="3" Grid.Row="0" Background="#FFE7E7E7"/>
            <Button Click="Num_Click" Content="1" Grid.Column="0" Grid.Row="1" Background="White"/>
            <Button Click="Num_Click" Content="2" Grid.Column="1" Grid.Row="1" Background="#FFFFC6C6"/>
            <Button Click="Num_Click" Content="3" Grid.Column="2" Grid.Row="1" Background="#FFFFE7C6"/>
            <Button Click="Fun_Click" Content="-" Grid.Column="3" Grid.Row="1" Background="#FFE7E7E7"/>
            <Button Click="Num_Click" Content="4" Grid.Column="0" Grid.Row="2" Background="#FFFFFFC6"/>
            <Button Click="Num_Click" Content="5" Grid.Column="1" Grid.Row="2" Background="#FFC6FFC6"/>
            <Button Click="Num_Click" Content="6" Grid.Column="2" Grid.Row="2" Background="#FFC6FFFF"/>
            <Button Click="Fun_Click" Content="+" Grid.Column="3" Grid.Row="2" Background="#FFE7E7E7"/>
            <Button Click="Num_Click" Content="7" Grid.Column="0" Grid.Row="3" Background="#FFC6C6FF"/>
            <Button Click="Num_Click" Content="8" Grid.Column="1" Grid.Row="3" Background="#FFFFC6FF"/>
            <Button Click="Num_Click" Content="9" Grid.Column="2" Grid.Row="3" Background="#FFFF8484"/>
            <Button Click="Eq_Click" Content="=" Grid.RowSpan="2" Grid.Column="3" Grid.Row="3" Background="#FFE7E7E7"/>
            <Button Click="Fun_Click"  Content="%" Grid.Column="0" Grid.Row="4" Background="#FFFFC684"/>
            <Button Click="Num_Click" Content="0" Grid.Column="1" Grid.Row="4" Background="#FFFF8484"/>
            <Button Click="Num_Click" Content="." Grid.Column="2" Grid.Row="4" Background="#FF84FF84"/>
        </Grid>
    </Grid>
</Window>