using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

        }
        int count = 1;

        private void button_Click(object sender, RoutedEventArgs e)
        {
            string start = textBox.Text;
            double tal = Convert.ToDouble(start);


            textBlock.Text = "Sum:" + " ";
            textBlock1.Text = "Count:" + " " + count++;
            textBlock2.Text = "Average:" + " ";
        }
    }
}



XML:

<Window x:Class="WpfApplication1.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:WpfApplication1"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <TextBox x:Name="textBox" HorizontalAlignment="Left" Height="23" Margin="10,131,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="120"/>
        <Button x:Name="button" Content="Add" HorizontalAlignment="Left" Height="23" Margin="153,131,0,0" VerticalAlignment="Top" Width="66" Click="button_Click"/>
        <TextBlock x:Name="textBlock" HorizontalAlignment="Left" Margin="10,197,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Height="23" Width="88"><Run Text="Sum"/><Run Text=": 0"/></TextBlock>
        <TextBlock x:Name="textBlock1" HorizontalAlignment="Left" Margin="10,167,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="74"><Run Text="Count:"/><Run Text=" 0"/></TextBlock>
        <TextBlock x:Name="textBlock2" HorizontalAlignment="Left" Margin="10,225,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Height="23" Width="88"><Run Text="Average:"/><Run Text=" 0"/></TextBlock>


    </Grid>
</Window>