How to make watermark in wpf datagrid textbox column

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C#

WPF

I need to make the textboxes in each cell have a watermark when the textbox is empty.
I have a datagrid and a grouping method in place for the type of Container class that's added to the datagrid.

I'm not sure how to add a watermark style to a DataGrid.TextBoxColumn.


Here's my code:

MainWindow.xaml:

<Window x:Class="DataGrid.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:System="clr-namespace:System;assembly=mscorlib" 
        xmlns:l="clr-namespace:DataGrid" Title="MainWindow" Height="350" Width="525">
    <Grid>
        <TabControl TabStripPlacement="Left">
            <TabItem Header="Grouping">
                <DataGrid ItemsSource="{Binding GroupedContainers}">
                    <DataGrid.GroupStyle>
                        <GroupStyle>
                            <GroupStyle.HeaderTemplate>
                                <DataTemplate>
                                    <StackPanel>
                                        <TextBlock Text="{Binding Path=Name}" FontWeight="Bold" Padding="3"/>
                                    </StackPanel>
                                </DataTemplate>
                            </GroupStyle.HeaderTemplate>
                            <GroupStyle.ContainerStyle>
                                <Style TargetType="{x:Type GroupItem}">
                                    <Setter Property="Template">
                                        <Setter.Value>
                                            <ControlTemplate TargetType="{x:Type GroupItem}">
                                                <Expander>
                                                    <Expander.Header>
                                                        <StackPanel Orientation="Horizontal">
                                                            <TextBlock Text="{Binding Path=Name}" />
                                                            <TextBlock Text="{Binding Path=ItemCount}" Margin="8,0,4,0"/>
                                                            <TextBlock Text="Items"/>
                                                        </StackPanel>
                                                    </Expander.Header>
                                                    <ItemsPresenter />
                                                </Expander>
                                            </ControlTemplate>
                                        </Setter.Value>
                                    </Setter>
                                </Style>
                            </GroupStyle.ContainerStyle>
                        </GroupStyle>
                    </DataGrid.GroupStyle>
                </DataGrid>
            </TabItem>
        </TabControl>
    </Grid>
</Window>

MainWindow.Xaml.cs:

using System.Collections.Generic;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.ComponentModel;
namespace DataGrid
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            DataContext = new MainWindowViewModel();
        }
    }
    public class MainWindowViewModel
    {
        public ICollectionView Containers { get; private set; }
        public ICollectionView GroupedContainers { get; private set; }
        public MainWindowViewModel()
        {
            var _customers = new List<Container>
                                 {
                                     new Container
                                         {
                                             Type = ContainerType.Class,
                                             Name = "Program",
                                             Access = AccessLevel.Public
                                         }
                                     
                                 };
            Containers = CollectionViewSource.GetDefaultView(_customers);
            GroupedContainers = new ListCollectionView(_customers);
            GroupedContainers.GroupDescriptions.Add(new PropertyGroupDescription("Type"));
        }
    }
    public enum ContainerType
    {
        Namespace, Class, Interface, Struct, Enum, Property, Method, Event, Expression, Variable
    }
    public enum AccessLevel
    {
        Public, Protected, Internal, Private
    }
    public class Container : INotifyPropertyChanged
    {
        private string _Name;
        private ContainerType _type;
        private AccessLevel _access;
        public string Name
        {
            get { return _Name; }
            set
            {
                _Name = value;
                NotifyPropertyChanged("Name");
            }
        }
        public ContainerType Type
        {
            get { return _type; }
            set
            {
                _type = value;
                NotifyPropertyChanged("Type");
            }
        }
        public AccessLevel Access
        {
            get
            {
                return _access;
            }
            set
            {
                _access = value;
                NotifyPropertyChanged("Access");
            }
        }
        #region INotifyPropertyChanged Members
        public event PropertyChangedEventHandler PropertyChanged;
        #endregion
        #region Private Helpers
        private void NotifyPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
        #endregion
    }
    public class TypeTemplateSelector : DataTemplateSelector
    {
        public DataTemplate ClassTemplate { get; set; }
        public DataTemplate EnumTemplate { get; set; }
        public DataTemplate InterfaceTemplate { get; set; }
        public override DataTemplate SelectTemplate(object item, DependencyObject container)
        {
            var customer = item as Container;
            if (customer == null)
                return base.SelectTemplate(item, container);
            if (customer.Type == ContainerType.Class)
                return ClassTemplate;
            else if (customer.Type == ContainerType.Enum)
                return EnumTemplate;
            else if (customer.Type == ContainerType.Interface)
                return InterfaceTemplate;
            return ClassTemplate;
        }
    }
}

Original DataGrid sample can be found here: Link

In summary this datagrid allows users the ability to add a container to the datagrid as a row. Each Container is grouped by type, just like the c# language. It's suppose to be a programming tool, any help appreciated.