<Window x:Class="WpfDataGrids.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:WpfDataGrids"
          mc:Ignorable="d"
          Title="MainWindow" Height="450" Width="800">
     <Grid>

          <!-- data DataGrid -->
          <DataGrid
               AutoGenerateColumns="False"
               Margin="10"
               IsReadOnly="True"
               ItemsSource="{Binding Customers}"
               FontFamily="Gisha" FontSize="20">

               <DataGrid.Columns>
                    <DataGridTextColumn Header="Customer Name" Binding="{Binding CustomerName}" Width="270" />
                    <DataGridTextColumn Header="Country" Binding="{Binding CustomerCountry}" Width="150"/>
                    <DataGridTextColumn Header="Project Name" Binding="{Binding ProjectName}" Width="150"/>
               </DataGrid.Columns>

               <!-- row detail -->
               <DataGrid.RowDetailsTemplate >
                    <DataTemplate>
                         <StackPanel Orientation="Horizontal">

                              <!-- photo -->
                              <Image Source="{Binding CustomerPhoto}" VerticalAlignment="Top"></Image>

                              <!-- tasks -->
                              <TextBlock FontSize="16" FontWeight="Bold">Tasks:</TextBlock>

                              <!-- sub DataGrid -->
                              <DataGrid
                                   AutoGenerateColumns="False"
                                   Margin="10"
                                   IsReadOnly="True"
                                   ItemsSource="{Binding Tasks}"
                                   FontFamily="Gisha" FontSize="20">
                                  
                                   <DataGrid.Columns>
                                        <DataGridTextColumn Header="Task Name" Binding="{Binding TaskName}" Width="150" />
                                        <DataGridTextColumn Header="Status" Binding="{Binding Status}" Width="150"/>
                                        <DataGridTextColumn Header="Estimation (days)" Binding="{Binding Estimation}" Width="150"/>
                                   </DataGrid.Columns>
                                  
                              </DataGrid>

                         </StackPanel>
                    </DataTemplate>
               </DataGrid.RowDetailsTemplate>
          </DataGrid>

     </Grid>
</Window>

using System;
using System.Collections.ObjectModel;
using System.Windows;
using System.Windows.Input;

namespace WpfDataGrids
{
     public class Task
     {
          public string TaskName { get; set; }
          public string Status { get; set; }
          public int Estimation { get; set; }
     }

     public class Customer
     {
          public string CustomerName { get; set; }
          public string CustomerCountry { get; set; }
          public string CustomerPhoto { get; set; }
          public string ProjectName { get; set; }
          public Task[] Tasks { get; set; }
     }

     public partial class MainWindow : Window
     {
          public ObservableCollection<Customer> Customers { get; set; }

          public MainWindow()
          {
               InitializeComponent();

               // for binding
               this.DataContext = this;

               // data for datagrid
               Customers = new ObservableCollection<Customer>()
               {
                         new Customer() {
                              CustomerName="Daniel", CustomerCountry="Germany", CustomerPhoto="D:/Daniel.jpg", ProjectName = "AirLines",
                              Tasks = new Task[] {
                                   new Task(){TaskName = "Create login page", Status = "Finished", Estimation = 3 },
                                   new Task(){TaskName = "Update seats page", Status = "In Progrsss", Estimation = 2 },
                                   new Task(){TaskName = "Implement algorithm", Status = "Not Started", Estimation = 4 },
                                   new Task(){TaskName = "Writing tests", Status = "Not Started", Estimation = 3 },
                              } },

                         new Customer() {
                              CustomerName="Johanes", CustomerCountry="USA", CustomerPhoto="D:/Johanes.jpg", ProjectName = "Book shop",
                              Tasks = new Task[] {
                                   new Task(){TaskName = "Adding db", Status = "In Progrsss", Estimation = 3 },
                                   new Task(){TaskName = "Query records", Status = "Not Started", Estimation = 3 },
                              } },

                         new Customer() {
                              CustomerName="Rebeca", CustomerCountry="Poland", CustomerPhoto="D:/Rebeca.jpg", ProjectName = "Food site",
                              Tasks = new Task[] {
                                   new Task(){TaskName = "Create dishes list", Status = "Finished", Estimation = 5 },
                                   new Task(){TaskName = "Add tests", Status = "In Progrsss", Estimation = 2 },
                                   new Task(){TaskName = "Check resolutions", Status = "Not Started", Estimation = 1 },
                              } }
               };
          }

     }
}

<Window x:Class="WpfDataGrids.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:WpfDataGrids"
          xmlns:l="clr-namespace:WpfDataGrids"
          mc:Ignorable="d"
          Title="MainWindow" Height="450" Width="800">
     <Window.Resources>
          <l:BoolToVisibilityConverter x:Key="boolToVisibility" />
     </Window.Resources>

     <Grid>

          <!-- data DataGrid -->
          <DataGrid
               AutoGenerateColumns="False"
               Margin="10"
               IsReadOnly="True"
               RowDetailsVisibilityMode="Visible"
               ItemsSource="{Binding Customers}"
               SelectedItem="{Binding SelectedCustomer}"
               FontFamily="Gisha" FontSize="20">

               <!-- LeftClick will Expand (by settting flag IsVisible = true) / Collapse (by settting flag IsVisible = false) -->
               <DataGrid.InputBindings>
                    <MouseBinding
                         MouseAction="LeftClick"
                         Command="{Binding ClickMouseOnCustomer}"/>
               </DataGrid.InputBindings>

               <DataGrid.Columns>
                    <DataGridTextColumn Header="Customer Name" Binding="{Binding CustomerName}" Width="270" />
                    <DataGridTextColumn Header="Country" Binding="{Binding CustomerCountry}" Width="150"/>
                    <DataGridTextColumn Header="Project Name" Binding="{Binding ProjectName}" Width="150"/>
               </DataGrid.Columns>

               <!-- row detail -->
               <DataGrid.RowDetailsTemplate >
                    <DataTemplate>
                         <StackPanel Orientation="Horizontal"
                              Visibility="{Binding IsVisible, Converter={StaticResource boolToVisibility}}">

                              <!-- photo -->
                              <Image Source="{Binding CustomerPhoto}" VerticalAlignment="Top"></Image>

                              <!-- tasks -->
                              <TextBlock FontSize="16" FontWeight="Bold">Tasks:</TextBlock>

                              <!-- sub DataGrid -->
                              <DataGrid
                                   AutoGenerateColumns="False"
                                   Margin="10"
                                   IsReadOnly="True"
                                   ItemsSource="{Binding Tasks}"
                                   FontFamily="Gisha" FontSize="20">

                                   <DataGrid.Columns>
                                        <DataGridTextColumn Header="Task Name" Binding="{Binding TaskName}" Width="150" />
                                        <DataGridTextColumn Header="Status" Binding="{Binding Status}" Width="150"/>
                                        <DataGridTextColumn Header="Estimation (days)" Binding="{Binding Estimation}" Width="150"/>
                                   </DataGrid.Columns>

                              </DataGrid>

                         </StackPanel>
                    </DataTemplate>
               </DataGrid.RowDetailsTemplate>
          </DataGrid>

     </Grid>
</Window>

using System;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Globalization;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Data;
using System.Windows.Input;

namespace WpfDataGrids
{
     public class BoolToVisibilityConverter : IValueConverter
     {
          public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
          {
               var bVal = (bool)value;
               return bVal ? Visibility.Visible : Visibility.Collapsed;
          }

          public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
          {
               throw new NotImplementedException();
          }
     }

     public class Task
     {
          public string TaskName { get; set; }
          public string Status { get; set; }
          public int Estimation { get; set; }
     }

     public class Customer : INotifyPropertyChanged
     {
          public string CustomerName { get; set; }
          public string CustomerCountry { get; set; }
          public string CustomerPhoto { get; set; }
          public string ProjectName { get; set; }
          public Task[] Tasks { get; set; }

          private bool _isVisible;
          public bool IsVisible { get => _isVisible; set => Set(ref _isVisible, value); }

          public event PropertyChangedEventHandler PropertyChanged;
          protected void OnPropertyChanged([CallerMemberName] string name = null)
          {
               PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
          }

          protected virtual bool Set<T>(ref T field, T value, [CallerMemberName] string PropertyName = null)
          {
               if (Equals(field, value)) return false;
               field = value;
               OnPropertyChanged(PropertyName);
               return true;
          }
     }

     public class MyClickCommand : ICommand
     {
          public event EventHandler CanExecuteChanged;
          private readonly Action<object> _callback;
          public MyClickCommand(Action<object> callback)
          {
               _callback = callback;
          }

          public virtual void Execute(object parameter)
          {
               _callback(parameter);
          }

          public virtual bool CanExecute(object parameter) { return true; }

     }

     public partial class MainWindow : Window
     {
          private Customer _selectedCustomer;
          public Customer SelectedCustomer
          {
               get
               {
                    return _selectedCustomer;
               }
               set
               {
                    _selectedCustomer = value;
                    if (_selectedCustomer!=null)
                         _selectedCustomer.IsVisible = !_selectedCustomer.IsVisible;
               }
          }

          public ObservableCollection<Customer> Customers { get; set; }
          // button handler
          public ICommand ClickMouseOnCustomer { get; }

          public MainWindow()
          {
               InitializeComponent();

               // for binding
               this.DataContext = this;

               ClickMouseOnCustomer = new MyClickCommand((param) => {
                    var customer = _selectedCustomer;
                    if (customer != null)
                    {
                         customer.IsVisible =!customer.IsVisible;
                    }
               });

               // data for grid
               Customers = new ObservableCollection<Customer>()
               {
                         new Customer() {
                              CustomerName="Daniel", CustomerCountry="Germany", CustomerPhoto="D:/Daniel.jpg", ProjectName = "AirLines",
                              Tasks = new Task[] {
                                   new Task(){TaskName = "Create login page", Status = "Finished", Estimation = 3 },
                                   new Task(){TaskName = "Update seats page", Status = "In Progrsss", Estimation = 2 },
                                   new Task(){TaskName = "Implement algorithm", Status = "Not Started", Estimation = 4 },
                                   new Task(){TaskName = "Writing tests", Status = "Not Started", Estimation = 3 },
                              } },

                         new Customer() {
                              CustomerName="Johanes", CustomerCountry="USA", CustomerPhoto="D:/Johanes.jpg", ProjectName = "Book shop",
                              Tasks = new Task[] {
                                   new Task(){TaskName = "Adding db", Status = "In Progrsss", Estimation = 3 },
                                   new Task(){TaskName = "Query records", Status = "Not Started", Estimation = 3 },
                              } },

                         new Customer() {
                              CustomerName="Rebeca", CustomerCountry="Poland", CustomerPhoto="D:/Rebeca.jpg", ProjectName = "Food site",
                              Tasks = new Task[] {
                                   new Task(){TaskName = "Create dishes list", Status = "Finished", Estimation = 5 },
                                   new Task(){TaskName = "Add tests", Status = "In Progrsss", Estimation = 2 },
                                   new Task(){TaskName = "Check resolutions", Status = "Not Started", Estimation = 1 },
                              } }
               };
          }

     }
}