MainWindow.xaml

<Window x:Class="inlinetest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525"
        x:Name="root">
    <StackPanel HorizontalAlignment="Center">
        <StackPanel Orientation="Horizontal" HorizontalAlignment="Center" Margin="0,100,0,20">
            <TextBox Text="{Binding LeftSideNumber, ElementName=root}" 
                     PreviewTextInput="ValidateIsNumber" Width="50" Margin="0,0,10,0"/>
            <TextBlock>X</TextBlock>
            <TextBox Text="{Binding RightSideNumber, ElementName=root}" 
                     PreviewTextInput="ValidateIsNumber" Width="50" Margin="10,0,10,0"/>
            <TextBlock Margin="0,0,10,0">=</TextBlock>
            <TextBlock x:Name="Answer"/>
            <x:Code>
                <![CDATA[
                    private System.Text.RegularExpressions.Regex isNumberRegex = 
                      new System.Text.RegularExpressions.Regex(@"^-*[0-9,\.]+$");

                    public string LeftSideNumber { get; set; }
                    public string RightSideNumber { get; set; }
                    
                    void CalculateClicked(object sender, RoutedEventArgs e)
                    {
                        var left = Convert.ToDouble(LeftSideNumber);
                        var right = Convert.ToDouble(RightSideNumber);
                        Answer.Text = Convert.ToString(left * right);
                    }

                    void ValidateIsNumber(object sender, TextCompositionEventArgs e)
                    {
                        e.Handled = !isNumberRegex.IsMatch(e.Text);
                    }
                ]]>
            </x:Code>
        </StackPanel>
        <Button Click="CalculateClicked" Width="100">Calculate</Button>
    </StackPanel>
</Window>