Calculator in WPF application C#.NET

C# Code:
Download here: source code
Concatenate of two numbers:

namespace calculator
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, RoutedEventArgs e)
        {
            string numberBTN  = String.Concat(button1.Content.ToString(), textBox1.Text);
            textBox1.Text = numberBTN;
        }
    }
}

XAML COde:



<Window x:Class="calculator.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Calculator" Height="350" Width="525">
    <Grid>
        <TextBox Height="27" HorizontalAlignment="Center" Margin="153,35,0,0" Name="textBox1" VerticalAlignment="Top" Width="200" />
        <Button Content="1" Height="23" Margin="234,109,0,0" Name="button1" VerticalAlignment="Top" HorizontalAlignment="Left" Width="36" Click="button1_Click" />
    </Grid>
</Window>