Display a Tree List using the ITreeNode Interface

• Feb 25, 2022
• 7 minutes to read

To display data in a tree-like structure, the ITreeNode interface should be implemented in the appropriate business classes. The objects that support this interface are displayed via the ASPxTreeList and TreeList controls from the ASPxTreeList and XtraTreeList libraries. This is provided by the TreeList Editors module. To learn more about the ITreeNode interface and the List Editor supplied for it with the TreeList Editors module, refer to the TreeList Editors Module Overview topic. This topic demonstrates how to implement the ITreeNode interface.

Tip

A complete sample project is available in the DevExpress Code Examples database at How to Use Tree List Editors to Display List Views.

We will implement a tree structure like this:

For this purpose, we will implement the ProjectGroup, Project and ProjectArea classes. All of them will be inherited from an abstract class that will implement the ITreeNode interface. To display a tree of these objects, an item that corresponds to that abstract class will be added to the navigation control. When clicking this item, all the objects of the types derived from the abstract class will be organized in a tree via the TreeListEditor. This editor will be provided by the TreeList Editors module, which we will add to the application.

Important

The example below is for XPO. If you use Entity Framework, you can find a similar Category entity source at C:\Program Files (x86)\DevExpress 18.2\Components\Sources\DevExpress.Persistent\DevExpress.Persistent.BaseImpl.EF\HCategory.cs.

• Implement an abstract Category class as shown in the following code:

  C#

  using System.ComponentModel;
  using DevExpress.Persistent.Base;
  using DevExpress.Persistent.Base.General;
  using DevExpress.Persistent.BaseImpl;
  using DevExpress.Xpo;
  //...
  [NavigationItem]
  public abstract class Category : BaseObject, ITreeNode {
     private string name;
     protected abstract ITreeNode Parent {
        get;
     }
     protected abstract IBindingList Children {
        get;
     }
     public Category(Session session) : base(session) {}
     public string Name {
        get {
           return name;
        }
        set{
           SetPropertyValue("Name", ref name, value);
        }
     }
     #region ITreeNode
     IBindingList ITreeNode.Children{
        get{
           return Children;
        }
     }
     string ITreeNode.Name{
        get {
           return Name;
        }
     }
     ITreeNode ITreeNode.Parent{
        get{
           return Parent;
        }
     }
     #endregion
  }
  

  VB.NET

  Imports System.ComponentModel
  Imports DevExpress.Persistent.Base
  Imports DevExpress.Persistent.Base.General
  Imports DevExpress.Persistent.BaseImpl
  Imports DevExpress.Xpo
  '...
  <NavigationItem> _
  Public MustInherit Class Category
        Inherits BaseObject
        Implements ITreeNode
     Private fName As String
     Protected MustOverride ReadOnly Property Parent() As ITreeNode
     Protected MustOverride ReadOnly Property Children() As IBindingList
     Public Sub New(ByVal session As Session)
        MyBase.New(session)
     End Sub
     Public Property Name() As String
        Get
           Return fName
        End Get
        Set
           SetPropertyValue("Name", fName, Value)
        End Set
     End Property
     #Region "ITreeNode"
     Private ReadOnly Property ICategorizedItem_Children() As IBindingList _
           Implements ITreeNode.Children
        Get
           Return Children
        End Get
     End Property
     Private ReadOnly Property ICategorizedItem_Name() As String Implements ITreeNode.Name
        Get
           Return Name
        End Get
     End Property
     Private ReadOnly Property ICategorizedItem_Parent() As ITreeNode Implements ITreeNode.Parent
        Get
           Return Parent
        End Get
     End Property
     #End Region
  End Class
  

  The NavigationItemAttribute is applied so that the corresponding navigation item is added to the navigation control.

  The Parent and Children properties will be overridden in the descendant classes, to return objects of the required types.

• Implement the ProjectGroup, Project and ProjectArea classes by inheriting them from the Category class. The following code demonstrates how to do this:

  C#

  using System.ComponentModel;
  using DevExpress.Persistent.Base.General;
  using DevExpress.Xpo;
  //...
  public class ProjectGroup : Category{
     protected override ITreeNode Parent {
        get {
           return null;
        }
     }
     protected override IBindingList Children {
        get {
           return Projects;
        }
     }
     public ProjectGroup(Session session) : base(session){}
     public ProjectGroup(Session session, string name) : base(session) {
        this.Name = name;
     }
     [Association("ProjectGroup-Projects"), Aggregated]
     public XPCollection<Project> Projects {
        get {
           return GetCollection<Project>("Projects");
        }
     }
  }
  
  public class Project : Category {
     private ProjectGroup projectGroup;
     protected override ITreeNode Parent {
        get {
           return projectGroup;
        }
     }
     protected override IBindingList Children {
        get {
           return ProjectAreas;
        }
     }
     public Project(Session session) : base(session) {}
     public Project(Session session, string name) : base(session) {
        this.Name = name;
     }
     [Association("ProjectGroup-Projects")]
     public ProjectGroup ProjectGroup {
        get {
           return projectGroup;
        }
        set {
           SetPropertyValue("ProjectGroup", ref projectGroup, value);
        }
     }
     [Association("Project-ProjectAreas"), Aggregated]
     public XPCollection<ProjectArea> ProjectAreas {
        get {
           return GetCollection<ProjectArea>("ProjectAreas");
        }
     }
  }
  
  public class ProjectArea : Category {
     private Project project;
     protected override ITreeNode Parent {
        get {
           return project;
        }
     }
     protected override IBindingList Children {
        get {
           return new BindingList<object>();
        }
     }
     public ProjectArea(Session session) : base(session) {}
     public ProjectArea(Session session, string name) : base(session) {
        this.Name = name;
     }
     [Association("Project-ProjectAreas")]
     public Project Project {
        get {
           return project;
        }
        set {
           SetPropertyValue("Project", ref project, value);
        }
     }
  }
  

  VB.NET

  Imports System.ComponentModel
  Imports DevExpress.Persistent.Base.General
  Imports DevExpress.Xpo
  '...
  Public Class ProjectGroup
        Inherits Category
     Protected Overrides ReadOnly Property Parent() As ITreeNode
        Get
           Return Nothing
        End Get
     End Property
     Protected Overrides ReadOnly Property Children() As IBindingList
        Get
           Return Projects
        End Get
     End Property
     Public Sub New(ByVal session As Session)
        MyBase.New(session)
     End Sub
     Public Sub New(ByVal session As Session, ByVal name As String)
        MyBase.New(session)
        Me.Name = name
     End Sub
     <Association("ProjectGroup-Projects"), Aggregated> _
     Public ReadOnly Property Projects() As XPCollection(Of Project)
        Get
           Return GetCollection(Of Project)("Projects")
        End Get
     End Property
  End Class
  
  Public Class Project
        Inherits Category
     Private fProjectGroup As ProjectGroup
     Protected Overrides ReadOnly Property Parent() As ITreeNode
        Get
           Return fProjectGroup
        End Get
     End Property
     Protected Overrides ReadOnly Property Children() As IBindingList
        Get
           Return ProjectAreas
        End Get
     End Property
     Public Sub New(ByVal session As Session)
        MyBase.New(session)
     End Sub
     Public Sub New(ByVal session As Session, ByVal name As String)
        MyBase.New(session)
        Me.Name = name
     End Sub
     <Association("ProjectGroup-Projects")> _
     Public Property ProjectGroup() As ProjectGroup
        Get
           Return fProjectGroup
        End Get
        Set
           SetPropertyValue("ProjectGroup", fProjectGroup, Value)
        End Set
     End Property
     <Association("Project-ProjectAreas"), Aggregated> _
     Public ReadOnly Property ProjectAreas() As XPCollection(Of ProjectArea)
        Get
           Return GetCollection(Of ProjectArea)("ProjectAreas")
        End Get
     End Property
  End Class
  
  Public Class ProjectArea
        Inherits Category
     Private fProject As Project
     Protected Overrides ReadOnly Property Parent() As ITreeNode
        Get
           Return fProject
        End Get
     End Property
     Protected Overrides ReadOnly Property Children() As IBindingList
        Get
           Return New BindingList(Of Object)()
        End Get
     End Property
     Public Sub New(ByVal session As Session)
        MyBase.New(session)
     End Sub
     Public Sub New(ByVal session As Session, ByVal name As String)
           MyBase.New(session)
           Me.Name = name
     End Sub
     <Association("Project-ProjectAreas")> _
     Public Property Project() As Project
        Get
           Return fProject
        End Get
        Set
           SetPropertyValue("Project", fProject, Value)
        End Set
     End Property
  End Class
  

  Note

  There are two important requirements the business classes must satisfy:

  • The parent and child classes that correspond to tree list nodes should inherit the same base class that implements the ITreeNode interface. In the code above, the ProjectGroup, Project and ProjectArea classes are inherited from the Category class.
  • It is not possible to have more than one root class in a tree list. In the code above, the ProjectGroup is the root class.
• Add the TreeList Editors module to the Windows Forms application project. For this purpose, invoke the Application Designer and drag the module from the Toolbox to the Modules panel.

• Run the Windows Forms application. Click the Category item in the navigation control. Then, create ProjectGroup, Project and ProjectArea objects via the New Action. You will see that these objects will be displayed as a tree by the Category List View:

  

Tree nodes can have associated images. These images are displayed by the Tree List Editors within the nodes. For details, refer to the Node Images in a Tree List interface.

You can also display a collection of items for each category node to the right of the tree list. For details, refer to the Categorized List topic.

If you do not need to organize the strong structure as demonstrated above, but wish to use the ready-to use HCategory class implementing the ITreeNode interface, refer to the Display a Tree List using the HCategory Class topic.