WPF ContextMenu MVVM Command CommandParameter Path=PlacementTarget

<DataGrid ItemsSource="{Binding BooksList,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
          SelectionMode="Extended">
    <DataGrid.ContextMenu>
        <ContextMenu>
            <MenuItem Header="Export" Command="{Binding ExportCmd}"
                      CommandParameter="{Binding RelativeSource={RelativeSource Mode=FindAncestor,AncestorType=ContextMenu},Path=PlacementTarget}"/>
        </ContextMenu>
    </DataGrid.ContextMenu>
</DataGrid>


//cs
private void ExportCmdExecuted(object obj)
{
    var dg = obj as DataGrid;
    if (dg != null)
    {
        string jsonStr = JsonConvert.SerializeObject(dg.Items, Formatting.Indented);
        System.IO.File.AppendAllText($"{DateTime.Now.ToString("yyyyMMddHHmmssffff")}Json.json", jsonStr);
        MessageBox.Show("Export Successfully!");
    }

}

Whole code 

//xaml
<Window x:Class="WpfApp204.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:WpfApp204"
        mc:Ignorable="d" WindowState="Maximized"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <DataGrid ItemsSource="{Binding BooksList,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
                  SelectionMode="Extended">
            <DataGrid.ContextMenu>
                <ContextMenu>
                    <MenuItem Header="Export" Command="{Binding ExportCmd}"
                              CommandParameter="{Binding RelativeSource={RelativeSource Mode=FindAncestor,AncestorType=ContextMenu},Path=PlacementTarget}"/>
                </ContextMenu>
            </DataGrid.ContextMenu>
        </DataGrid>
    </Grid>
</Window>



//cs
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
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;
using Newtonsoft.Json;

namespace WpfApp204
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            var vm = new BookVM();
            this.DataContext = vm;
        }
    }

    public class BookVM : INotifyPropertyChanged
    {
        public BookVM()
        {
            InitData();
        }

        private DelCmd exportCmd;
        public DelCmd ExportCmd
        {
            get
            {
                if(exportCmd == null) 
                {
                    exportCmd = new DelCmd(ExportCmdExecuted, ExportCmdCanExecute);
                }
                return exportCmd;
            }
        }

        private bool ExportCmdCanExecute(object obj)
        {
            return BooksList != null && BooksList.Any();
        }

        private void ExportCmdExecuted(object obj)
        {
            var dg = obj as DataGrid;
            if (dg != null)
            {
                string jsonStr = JsonConvert.SerializeObject(dg.Items, Formatting.Indented);
                System.IO.File.AppendAllText($"{DateTime.Now.ToString("yyyyMMddHHmmssffff")}Json.json", jsonStr);
                MessageBox.Show("Export Successfully!");
            }

        }

        private void InitData()
        {
            BooksList = new ObservableCollection<Book>();
            for(int i=0;i<10000000;i++)
            {
                BooksList.Add(new Book()
                {
                    Id = i + 1,
                    Name = $"Name_{i + 1}"
                });
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
        public void OnPropertyChanged(string propName) 
        {
            var handler = PropertyChanged;
            if(handler!=null)
            {
                handler?.Invoke(this,new PropertyChangedEventArgs(propName));
            }
        }

        private ObservableCollection<Book> booksList;
        public ObservableCollection<Book> BooksList
        {
            get
            {
                return booksList;
            }
            set
            {
                if(booksList!=value)
                {
                    booksList = value;
                    OnPropertyChanged(nameof(BooksList));
                }
            }
        }
    }

    public class DelCmd : ICommand
    {
        private readonly Action<Object> execute;
        private readonly Predicate<object> canExecute;

        public DelCmd(Action<object> executeValue, Predicate<object> canExecuteValue)
        {
            execute = executeValue;
            canExecute = canExecuteValue;
        }

        public DelCmd(Action<object> executeValue) : this(executeValue, null)
        {

        }

        public event EventHandler CanExecuteChanged;

        public bool CanExecute(object parameter)
        {
            if (canExecute == null)
            {
                return true;
            }
            return canExecute.Invoke(parameter);
        }

        public void Execute(object parameter)
        {
            execute(parameter);
        }
    }

    public class Book
    {
        public int Id { get; set; }
        public string Name { get; set; }
    }
}

Referenced https://blog.csdn.net/jumtre/article/details/19540371