<DataGrid.ContextMenu>
     <ContextMenu>
         <MenuItem Header="{Binding FirstMIHeader}"
                   FontSize="50"
                   Width="350"
                   Height="150"
                   Command="{Binding FirstCmd}"
                   CommandParameter="{Binding RelativeSource={RelativeSource AncestorType=ContextMenu},
             Path=PlacementTarget.SelectedItems}"/>
     </ContextMenu>
 </DataGrid.ContextMenu>

private DelCmd firstCmd;
public DelCmd FirstCmd
{
    get
    {
        if (firstCmd == null)
        {
            firstCmd = new DelCmd(FirstCmdExecuted);
        }
        return firstCmd;
    }
}

private void FirstCmdExecuted(object? obj)
{
    _ = Task.Run(async() =>
    {
        var items = ((System.Collections.IList)obj).Cast<Book>()?.ToList();
        if (items != null && items.Any())
        {
            string jsonStr = JsonConvert.SerializeObject(items, Formatting.Indented);
            string jsonFile = string.Empty;

            await Application.Current?.Dispatcher?.InvokeAsync(() =>
            {
                SaveFileDialog dlg = new SaveFileDialog();
                dlg.Filter = $"Json Files|*.json";
                dlg.FileName = $"Json_{DateTime.Now.ToString("yyyyMMddHHmmssffff")}.json";
                if (dlg.ShowDialog() == true)
                {
                    jsonFile = dlg.FileName;
                }
            });

            using(StreamWriter jsonWriter=new StreamWriter(jsonFile,false,Encoding.UTF8))
            {
                jsonWriter.WriteLine(jsonStr);
            }

            await Application.Current?.Dispatcher?.InvokeAsync(() =>
            {
                MessageBox.Show($"Saved {items.Count} items in {jsonFile}");
            },System.Windows.Threading.DispatcherPriority.Background);
        }
    });            
}

private string firstMIHeader="Save In Json";
public string FirstMIHeader
{
    get
    {
        return firstMIHeader;
    }
    set
    {
        if (value != firstMIHeader)
        {
            firstMIHeader = value;
            OnPropertyChanged();
        }
    }
}

Install-Package Newtonsoft.json

<Window x:Class="WpfApp10.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:WpfApp10"
        mc:Ignorable="d"
        Title="{Binding MainTitle}" WindowState="Maximized">
    <Window.DataContext>
        <local:MainVM/>
    </Window.DataContext>
    <Grid>
        <DataGrid ItemsSource="{Binding BooksCollection}"
                  VirtualizingPanel.IsVirtualizing="True"
                  VirtualizingPanel.VirtualizationMode="Recycling"
                  VirtualizingPanel.CacheLength="5,5"
                  VirtualizingPanel.CacheLengthUnit="Item"
                  ScrollViewer.IsDeferredScrollingEnabled="True"
                  ScrollViewer.CanContentScroll="True"
                  UseLayoutRounding="True"
                  SnapsToDevicePixels="True"
                  CanUserAddRows="False"
                  AutoGenerateColumns="True"
                  EnableRowVirtualization="True"
                  EnableColumnVirtualization="True">
            <DataGrid.Resources>
                <Style TargetType="DataGridRow">
                    <Setter Property="FontSize" Value="30"/>
                    <Style.Triggers>
                        <Trigger Property="IsMouseOver" Value="True">
                            <Setter Property="FontSize" Value="40"/>
                            <Setter Property="Foreground" Value="Red"/>
                        </Trigger>
                    </Style.Triggers>
                </Style>
            </DataGrid.Resources>
            <DataGrid.ContextMenu>
                <ContextMenu>
                    <MenuItem Header="{Binding FirstMIHeader}"
                              FontSize="50"
                              Width="350"
                              Height="150"
                              Command="{Binding FirstCmd}"
                              CommandParameter="{Binding RelativeSource={RelativeSource AncestorType=ContextMenu},
                        Path=PlacementTarget.SelectedItems}"/>
                </ContextMenu>
            </DataGrid.ContextMenu>
        </DataGrid>
    </Grid>
</Window>


using Microsoft.Win32;
using Newtonsoft.Json;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.IO;
using System.Net.Http;
using System.Runtime.CompilerServices;
using System.Text;
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 WpfApp10
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
    }

    public class MainVM : INotifyPropertyChanged
    {
        string originUrl = "http://localhost:51537/BookService.svc/getbookslist?cnt=";
        static readonly HttpClient client = new HttpClient();
        private readonly object isLoadingLock = new object();
        private readonly SemaphoreSlim semaphoreThrottle = new SemaphoreSlim(10, 10);

        public MainVM()
        {
            if (DesignerProperties.GetIsInDesignMode(new DependencyObject()))
            {
                return;
            }
            _ = Task.Run(async () =>
            {
                while (true)
                {
                    await InitBooksCollectionAsync();
                    await Task.Delay(20000);
                }
            });

        }
        private async Task InitBooksCollectionAsync(int cnt = 1000000)
        {
            if (!await semaphoreThrottle.WaitAsync(20000))
            {
                return;
            }
            if (IsLoading)
            {
                return;
            }

            IsLoading = true;

            try
            {
                await Application.Current?.Dispatcher?.InvokeAsync(() =>
                {
                    MainTitle = $"{DateTime.Now},loading...";
                }, System.Windows.Threading.DispatcherPriority.Background);

                string requestUrl = $"{originUrl}{cnt}";
                await Task.Run(async () =>
                {
                    string jsonStr = await client.GetStringAsync(requestUrl);
                    if (string.IsNullOrWhiteSpace(jsonStr))
                    {
                        return;
                    }

                    List<Book>? bks = JsonConvert.DeserializeObject<List<Book>>(jsonStr);
                    if (bks != null && bks.Any())
                    {
                        await Application.Current?.Dispatcher?.InvokeAsync(() =>
                        {
                            BooksCollection = new ObservableCollection<Book>(bks);
                            MainTitle = $"{DateTime.Now},FirstID:{BooksCollection.FirstOrDefault()?.Id},LastID:{BooksCollection.LastOrDefault()?.Id}";
                        }, System.Windows.Threading.DispatcherPriority.Background);
                    }
                });
            }
            catch (Exception ex)
            {
                MessageBox.Show($"{DateTime.Now},exception message {ex?.Message}");
            }
            finally
            {
                IsLoading = false;
                semaphoreThrottle.Release();
            }
        }

        private DelCmd firstCmd;
        public DelCmd FirstCmd
        {
            get
            {
                if (firstCmd == null)
                {
                    firstCmd = new DelCmd(FirstCmdExecuted);
                }
                return firstCmd;
            }
        }

        private void FirstCmdExecuted(object? obj)
        {
            _ = Task.Run(async() =>
            {
                var items = ((System.Collections.IList)obj).Cast<Book>()?.ToList();
                if (items != null && items.Any())
                {
                    string jsonStr = JsonConvert.SerializeObject(items, Formatting.Indented);
                    string jsonFile = string.Empty;

                    await Application.Current?.Dispatcher?.InvokeAsync(() =>
                    {
                        SaveFileDialog dlg = new SaveFileDialog();
                        dlg.Filter = $"Json Files|*.json";
                        dlg.FileName = $"Json_{DateTime.Now.ToString("yyyyMMddHHmmssffff")}.json";
                        if (dlg.ShowDialog() == true)
                        {
                            jsonFile = dlg.FileName;
                        }
                    });

                    using(StreamWriter jsonWriter=new StreamWriter(jsonFile,false,Encoding.UTF8))
                    {
                        jsonWriter.WriteLine(jsonStr);
                    }

                    await Application.Current?.Dispatcher?.InvokeAsync(() =>
                    {
                        MessageBox.Show($"Saved {items.Count} items in {jsonFile}");
                    },System.Windows.Threading.DispatcherPriority.Background);
                }
            });            
        }

        private string firstMIHeader="Save In Json";
        public string FirstMIHeader
        {
            get
            {
                return firstMIHeader;
            }
            set
            {
                if (value != firstMIHeader)
                {
                    firstMIHeader = value;
                    OnPropertyChanged();
                }
            }
        }

        private bool isLoading = false;
        public bool IsLoading
        {
            get
            {
                lock (isLoadingLock)
                {
                    return isLoading;
                }
            }
            set
            {
                lock (isLoadingLock)
                {
                    if (value != isLoading)
                    {
                        isLoading = value;
                        OnPropertyChanged();
                    }
                }
            }
        }

        private string mainTitle = $"{DateTime.Now}";
        public string MainTitle
        {
            get
            {
                return mainTitle;
            }
            set
            {
                if (value != mainTitle)
                {
                    mainTitle = value;
                    OnPropertyChanged();
                }
            }
        }

        private ObservableCollection<Book> booksCollection;
        public ObservableCollection<Book> BooksCollection
        {
            get
            {
                return booksCollection;
            }
            set
            {
                if (value != booksCollection)
                {
                    booksCollection = value;
                    OnPropertyChanged();
                }
            }
        }

        public event PropertyChangedEventHandler? PropertyChanged;
        private void OnPropertyChanged([CallerMemberName] string propName = "")
        {
            var handler = Volatile.Read(ref PropertyChanged);
            if (handler == null)
            {
                return;
            }
            handler?.Invoke(this, new PropertyChangedEventArgs(propName));
        }
    }

    public class DelCmd : ICommand
    {
        private readonly Action<object?> execute;
        private readonly Func<object?, bool>? canExecute;
        public DelCmd(Action<object?> executeValue, Func<object?, bool>? canExecuteValue = null)
        {
            execute = executeValue;
            canExecute = canExecuteValue;
        }

        public event EventHandler? CanExecuteChanged;

        public bool CanExecute(object? parameter)
        {
            return canExecute == null ? true : canExecute(parameter);
        }

        public void Execute(object? parameter)
        {
            if (!CanExecute(parameter))
            {
                return;
            }
            execute?.Invoke(parameter);
        }

        public void RaiseCanExecuteChanged(object? parameter)
        {
            var handler = Volatile.Read(ref CanExecuteChanged);
            if (handler == null)
            {
                return;
            }

            if (Application.Current?.Dispatcher?.CheckAccess() == true)
            {
                handler?.Invoke(parameter, EventArgs.Empty);
            }
            else
            {
                Application.Current?.Dispatcher?.Invoke(() =>
                {
                    handler?.Invoke(parameter, EventArgs.Empty);
                });
            }
        }
    }

    public class Book
    {
        public long Id { get; set; }
        public string Name { get; set; }
        public string ISBN { get; set; }
        public string Abstract { get; set; }
        public string Author { get; set; }
        public string Comment { get; set; }
        public string Content { get; set; }
        public string Summary { get; set; }
        public string Title { get; set; }
        public string Topic { get; set; }
    }

}