Kapcha/shkola/Views/kalkulator.axaml

<UserControl xmlns="https://github.com/avaloniaui"
             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:vm="clr-namespace:shkola.ViewModels"
			       x:DataType="vm:MainWindowViewModel"
             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
             x:Class="shkola.kalkulator">
  <StackPanel VerticalAlignment="Center" Margin="15">

      <TextBlock HorizontalAlignment="Center" FontSize="20">Калькулятор</TextBlock>
      <TextBlock Margin="0 15 0 0" HorizontalAlignment="Center">Выберите то что хотите сделать:</TextBlock>
      <ComboBox SelectedIndex="{Binding CP.Func}" Margin="0 5 0 0" HorizontalAlignment="Center" Width="400">
        <ComboBoxItem>Сложение</ComboBoxItem>
        <ComboBoxItem>Вычитание</ComboBoxItem>
        <ComboBoxItem>Умножение</ComboBoxItem>
        <ComboBoxItem>Деление</ComboBoxItem>
      </ComboBox>
        <TextBox Width="400" Margin="0 10 0 0" Text="{Binding CP.FirstArg}" TextAlignment="Center" Watermark="Число один"/>
        <TextBlock Width="50"></TextBlock>
        <TextBox Width="400" Text="{Binding CP.SecondArg}" TextAlignment="Center" Watermark="Число два"/>
      <Button Command="{Binding CP.ResultCalc}" Margin="0 10 0 0" HorizontalAlignment="Center" Width="400">Результат</Button>
      <Border  Margin="0 10 0 0" Width="400" Height="30" BorderBrush="#393E46" BorderThickness="1" CornerRadius="3">
        <TextBlock TextAlignment="Center" Text="{Binding CP.Result}"></TextBlock>
      </Border>
  </StackPanel>
</UserControl>