TreeList.BeforeCollapse Event

Fires before a node is collapsed.

Namespace: DevExpress.XtraTreeList

Assembly: DevExpress.XtraTreeList.v20.1.dll

NuGet Package: DevExpress.Win.TreeList

Declaration

C#

public event BeforeCollapseEventHandler BeforeCollapse

VB.NET

Public Event BeforeCollapse As BeforeCollapseEventHandler

Event Data

The BeforeCollapse event's data class is BeforeCollapseEventArgs. The following properties provide information specific to this event:

Property	Description
CanCollapse	Gets or sets a value indicating whether a Tree List node can collapse.
Node	Gets the current Tree List node. Inherited from NodeEventArgs.

Remarks

By default, the following actions result in collapsing a parent node:

• An end-user user has clicked the node’s collapse button, double-clicked its corresponding indicator cell or pressed the PLUS or CTRL + LEFT ARROW keys when the node is focused.
• The node has been collapsed in code via the TreeListNode.Expanded property, the TreeList.CollapseToLevel or TreeList.CollapseAll method.

Each time a node is about to be collapsed, the BeforeCollapse event fires. Write a handler for this event to specify whether the node is allowed to be collapsed. You must set the event’s BeforeCollapseEventArgs.CanCollapse parameter to false to prohibit this action.

You can use the event for the following purposes:

• Prohibit the ability to collapse a specific node(s).
• Allow the ability to collapse nodes only when certain conditions are met.

If you want specific actions to be performed each time a node is collapsed, consider using the TreeList.AfterCollapse event.

Example

The following sample code prohibits collapsing root-level nodes. The TreeList.BeforeCollapse event is handled for this purpose.

C#

using DevExpress.XtraTreeList;

private void treeList1_BeforeCollapse(object sender, BeforeCollapseEventArgs e) {
   if (e.Node.Level == 0) e.CanCollapse = false;
}

VB.NET

Imports DevExpress.XtraTreeList

Private Sub TreeList1_BeforeCollapse(ByVal sender As Object, _
ByVal e As BeforeCollapseEventArgs) Handles TreeList1.BeforeCollapse
   If e.Node.Level = 0 Then e.CanCollapse = False
End Sub

Related GitHub Examples

The following code snippet (auto-collected from DevExpress Examples) contains a reference to the BeforeCollapse event.

Note

The algorithm used to collect these code examples remains a work in progress. Accordingly, the links and snippets below may produce inaccurate results. If you encounter an issue with code examples below, please use the feedback form on this page to report the issue.

VB.NET

how-to-implement-the-cells-multi-selection-feature-in-treelist-e2795/VB/WindowsApplication1/Helper/BlockSelector.vb#L34

AddHandler fList.BeforeExpand, AddressOf fList_BeforeExpand
AddHandler fList.BeforeCollapse, AddressOf fList_BeforeCollapse
AddHandler fList.KeyDown, AddressOf fList_KeyDown