[Browsable(false)]
public object CustomStyleId { get; set; }

<Browsable(False)>
Public Property CustomStyleId As Object

Imports System
Imports System.Collections.Generic
Imports System.Linq
Imports System.Text
Imports System.Windows

Namespace DXDiagram.ContentItem
    Public Class ButtonItemModel
        Public Sub ShowMessage()
            MessageBox.Show("This command has been created by our POCO machanism based on the ShowMessage method")
        End Sub
    End Class
End Namespace

<Window x:Class="DXDiagram.ContentItem.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:dxdiag="http://schemas.devexpress.com/winfx/2008/xaml/diagram" 
        mc:Ignorable="d"
        Title="MainWindow" Height="700" Width="1200">
    <Window.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="DiagramResources.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Window.Resources>
    <Grid>
        <dxdiag:DiagramDesignerControl Name="diagramControl" SelectedStencils="BasicShapes" />
    </Grid>
</Window>

Imports System
Imports System.Collections.Generic
Imports System.Linq
Imports System.Text
Imports System.Windows
Imports System.Windows.Controls
Imports System.Windows.Data
Imports System.Windows.Documents
Imports System.Windows.Input
Imports System.Windows.Media
Imports System.Windows.Media.Imaging
Imports System.Windows.Navigation
Imports System.Windows.Shapes
Imports DevExpress.Diagram.Core
Imports DevExpress.Xpf.Core
Imports DevExpress.Xpf.Diagram
Imports DevExpress.Mvvm.POCO

Namespace DXDiagram.ContentItem

    Partial Public Class MainWindow
        Inherits Window

        Public Sub New()
            InitializeComponent()
            RegisterContentItemTools()
            LoadDiagram()
        End Sub

        Private Sub RegisterContentItemTools()
            Dim stencil As New DiagramStencil("CustomTools", "Content Item Tools")
            stencil.RegisterTool(New FactoryItemTool("Text", Function() "Text", Function(diagram) New DiagramContentItem() With {.CustomStyleId = "formattedTextContentItem"}, New Size(230, 110), True))
            stencil.RegisterTool(New FactoryItemTool("Logo", Function() "Logo", Function(diagram) New DiagramContentItem() With {.CustomStyleId = "devExpressLogoContentItem"}, New Size(230, 80), True))
                stencil.RegisterTool(New FactoryItemTool("Action", Function() "Button", Function(diagram) New DiagramContentItem() With {.CustomStyleId = "buttonContentItem"}, New Size(230, 80), True))
                DiagramToolboxRegistrator.RegisterStencil(stencil)
        End Sub

        Private Sub LoadDiagram()
            diagramControl.DocumentSource = "DiagramData.xml"
            diagramControl.SelectedStencils = New StencilCollection(New String() { "CustomTools" })
        End Sub
    End Class
End Namespace

<ResourceDictionary 
                    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:dxdiag="http://schemas.devexpress.com/winfx/2008/xaml/diagram"
                    xmlns:dxmvvm="http://schemas.devexpress.com/winfx/2008/xaml/mvvm"
                    xmlns:local="clr-namespace:DXDiagram.ContentItem"
                    xmlns:sys="clr-namespace:System;assembly=mscorlib">

    <Style x:Key="formattedTextContentItem" TargetType="dxdiag:DiagramContentItem">
        <Setter Property="Background" Value="#FF35779C"/>
        <Setter Property="ContentTemplate">
            <Setter.Value>
                <DataTemplate>
                    <TextBlock TextWrapping="Wrap" TextAlignment="Center" VerticalAlignment="Center" Margin="10">
                             DiagramContentItem allows you to use custom content in diagram items. For example, you can use it for displaying 
                        <Run FontSize="14" Foreground="#FFF3F3A8" FontWeight="Bold">formatted</Run>
                        text.
                    </TextBlock>
                </DataTemplate>
            </Setter.Value>
        </Setter>
    </Style>

    <Style x:Key="devExpressLogoContentItem" TargetType="dxdiag:DiagramContentItem">
        <Setter Property="Background" Value="#FFEC7D32"/>
        <Setter Property="ContentTemplate">
            <Setter.Value>
                <DataTemplate>
                    <StackPanel HorizontalAlignment="Center" Margin="10" VerticalAlignment="Center">
                        <TextBlock Text="Item with several images" VerticalAlignment="Center" FontSize="20"/>
                        <StackPanel Orientation="Horizontal" HorizontalAlignment="Center">
                            <Image Source="Images\DevExpressLogo.png" MaxHeight="32" Margin="5,0,0,0"/>
                            <Image Source="Images\Globe.png" MaxHeight="32" />
                            <Image Source="Images\Apply.png" MaxHeight="32" />
                        </StackPanel>
                    </StackPanel>
                </DataTemplate>
            </Setter.Value>
        </Setter>
    </Style>

    <Style x:Key="buttonContentItem" TargetType="dxdiag:DiagramContentItem">
        <Setter Property="Background" Value="Gray"/>
        <Setter Property="ContentTemplate">
            <Setter.Value>
                <DataTemplate>
                    <StackPanel HorizontalAlignment="Center" VerticalAlignment="Center" Margin="10" DataContext="{dxmvvm:ViewModelSource Type={x:Type local:ButtonItemModel}}">
                        <TextBlock Text="Item with a command" VerticalAlignment="Center" FontSize="20"/>
                        <Button Content="Show additional info" Command="{Binding ShowMessageCommand}" MaxHeight="40" Margin="5,0,0,0" Cursor="Hand"/>
                    </StackPanel>
                </DataTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>

using System;
using System.Collections.Generic;
using System.Linq;
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;
using DevExpress.Diagram.Core;
using DevExpress.Xpf.Core;
using DevExpress.Xpf.Diagram;
using DevExpress.Mvvm.POCO;

namespace DXDiagram.ContentItem {

    public partial class MainWindow : Window {
        public MainWindow() {
            InitializeComponent();
            RegisterContentItemTools();
            LoadDiagram();
        }

        void RegisterContentItemTools() {
            DiagramStencil stencil = new DiagramStencil("CustomTools", "Content Item Tools");
            stencil.RegisterTool(new FactoryItemTool(
                "Text",
                () => "Text",
                diagram => new DiagramContentItem() {
                    CustomStyleId = "formattedTextContentItem"
                },
                new Size(230, 110), true));
            stencil.RegisterTool(new FactoryItemTool(
                "Logo",
                () => "Logo",
                diagram => new DiagramContentItem() {
                    CustomStyleId = "devExpressLogoContentItem",
                },
                new Size(230, 80), true));
                stencil.RegisterTool(new FactoryItemTool(
                "Action",
                () => "Button",
                diagram => new DiagramContentItem()
                {
                    CustomStyleId = "buttonContentItem",
                },
                new Size(230, 80), true));
                DiagramToolboxRegistrator.RegisterStencil(stencil);
            }

        void LoadDiagram() {
            diagramControl.DocumentSource = "DiagramData.xml";
            diagramControl.SelectedStencils = new StencilCollection(new string[] { "CustomTools" });
        }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;

namespace DXDiagram.ContentItem
{
    public class ButtonItemModel
    {
        public void ShowMessage()
        {
            MessageBox.Show("This command has been created by our POCO machanism based on the ShowMessage method");
        }
    }
}