SSSCalculator/Views/CalculatorPage.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"
             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
             x:Class="Calculator.CalculatorPage"
             xmlns:vm="using:Calculator.ViewModels"
             x:DataType="vm:MainWindowViewModel">

  <StackPanel HorizontalAlignment="Center" VerticalAlignment="Center">
    <TextBlock HorizontalAlignment="Center">Выберите арифметическую операцию</TextBlock>
    <ComboBox SelectedIndex="{Binding CP.Func}" Margin="0,10,0,0" Width="200" HorizontalAlignment="Center">
      <ComboBoxItem>Сложение</ComboBoxItem>
      <ComboBoxItem>Вычитание</ComboBoxItem>
      <ComboBoxItem>Умножение</ComboBoxItem>
      <ComboBoxItem>Деление</ComboBoxItem>
    </ComboBox>
    <StackPanel Margin="0,10,0,0"  HorizontalAlignment="Center" Orientation="Horizontal">
      <TextBox Text="{Binding CP.FirstArg}"/>
      <TextBlock VerticalAlignment="Center" Text="{Binding CP.Symb}" Margin="10,0,0,0"/>
      <TextBox Text="{Binding CP.SecondArg}" Margin="10,0,0,0"/>
    </StackPanel>
    <Button Command="{Binding CP.ResultCalc}" Margin="0,10,0,0" HorizontalAlignment="Center" HorizontalContentAlignment="Center" Width="200" Content="Вычислить"/>
    <TextBlock Margin="0,10,0,0" TextAlignment="Center" Width="200">Результат</TextBlock>
    <TextBlock Text="{Binding CP.Result}" Margin="0,10,0,0" TextAlignment="Center" Width="200"/>
    <TextBlock Text="{Binding CP.Message}" Margin="0,10,0,0" TextAlignment="Center"/>
  </StackPanel>
  
</UserControl>