<Window x:Class="wpftest3.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>
    <Grid.RowDefinitions>
      <RowDefinition />
      <RowDefinition />
    </Grid.RowDefinitions>
    <DataGrid
      ItemsSource="{Binding Data}" />
    <Button
      Grid.Row="1"
      VerticalAlignment="Center"
      HorizontalAlignment="Center"
      Padding="5"
      Content="Test"
      Click="ButtonClick" />
  </Grid>
</Window>

using System;
using System.Collections.Generic;
using System.Linq;

namespace wpftest3
{
  public partial class MainWindow
  {
    public MainWindow()
    {
      InitializeComponent();
      Data = new List<Foo>{new Foo{Bar= "Zot"}};
      DataContext = this;
    }

    public List<Foo> Data { get;  }

    private void ButtonClick(object sender, System.Windows.RoutedEventArgs e)
    {
      Console.WriteLine(@"{0} {1}", Data.Count, Data.LastOrDefault()?.Bar);
    }
  }

  public class Foo
  {
    public string Bar { get; set; }
  }
}