Calculator/MainWindow.xaml

<Window x:Class="Calculator.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:Calculator"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid Background="Pink">
        <StackPanel Orientation="Horizontal" HorizontalAlignment="Center" VerticalAlignment="Center">
            <TextBox Background="LightBlue" Name="x1" Width="100" Margin="0,0,10,0">1</TextBox>
            <ComboBox SelectionChanged="Fun_SelectionChanged" Name="Fun" Width="100" Margin="0,0,10,0"></ComboBox>
            <TextBox Background="LightBlue" Name="x2" Width="100" Margin="0,0,10,0">2</TextBox>
            <Button  Click="Result" Width="100" Margin="0,0,10,0">=</Button>
            <TextBlock Background="LightBlue" Name="Res" Width="100"></TextBlock>
        </StackPanel>
    </Grid>
</Window>