Exemplo de código assíncrono com async e await no C#

MainWindow.xaml

<Window x:Class="WpfApplication1.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">
    <Grid>
        <Button Content="Button" HorizontalAlignment="Left" Margin="155,60,0,0" VerticalAlignment="Top" Width="75" Click="Button_Click"/>
        <Label x:Name="lbl" Content="Label" HorizontalAlignment="Left" Margin="156,108,0,0" VerticalAlignment="Top"/>
        <TextBox x:Name="txt" HorizontalAlignment="Left" Height="23" Margin="156,31,0,0" TextWrapping="Wrap" Text="TextBox" VerticalAlignment="Top" Width="120"/>

    </Grid>
</Window>

MainWindow.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
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();
        }

        private async void Button_Click(object sender, RoutedEventArgs e)
        {
            string texto = null;
            try
            {
                texto = await ObtemTextoAsync();
            }
            catch (Exception ex)
            {
                texto = ex.Message;
            }
            if (texto != null)
            {
                lbl.Content = texto;
            }
        }

        private async Task<string> ObtemTextoAsync()
        {
            await Task.Delay(1000);
            throw new Exception("erro");
            return txt.Text;
        }
    }
}