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TreeView Constructor
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TreeView Constructor

Initializes a new instance of the TreeView class.

Namespace:  System.Windows.Forms
Assembly:  System.Windows.Forms (in System.Windows.Forms.dll)

public TreeView()

The following code example demonstrates how to change the collapse state of a TreeView so that all the checked nodes are visible. First, all the nodes are expanded, and a handler is added for the BeforeCollapse event. Next, all the nodes are collapsed. The BeforeCollapse event handler determines whether a given node has child nodes that are checked. If a node has checked children, the collapse is canceled for that node. In order to allow standard node collapse when the minus sign next to a node is clicked, the BeforeCollapse event handler is then removed.

This behavior can also be implemented by handling the BeforeExpand event, as illustrated in the example for that topic.

using System;
using System.Drawing;
using System.Windows.Forms;

public class Form1 : Form
{
    private TreeView treeView1;
    private Button showCheckedNodesButton;
    private TreeViewCancelEventHandler checkForCheckedChildren;

    public Form1()
    {
        treeView1 = new TreeView();
        showCheckedNodesButton = new Button();
        checkForCheckedChildren = 
            new TreeViewCancelEventHandler(CheckForCheckedChildrenHandler);

        this.SuspendLayout();

        // Initialize treeView1.
        treeView1.Location = new Point(0, 25);
        treeView1.Size = new Size(292, 248);
        treeView1.Anchor = AnchorStyles.Top | AnchorStyles.Left | 
            AnchorStyles.Bottom | AnchorStyles.Right;
        treeView1.CheckBoxes = true;

        // Add nodes to treeView1.
        TreeNode node;
        for (int x = 0; x < 3; ++x)
        {
            // Add a root node.
            node = treeView1.Nodes.Add(String.Format("Node{0}", x*4));
            for (int y = 1; y < 4; ++y)
            {
                // Add a node as a child of the previously added node.
                node = node.Nodes.Add(String.Format("Node{0}", x*4 + y));
            }
        }

        // Set the checked state of one of the nodes to 
        // demonstrate the showCheckedNodesButton button behavior.
        treeView1.Nodes[1].Nodes[0].Nodes[0].Checked = true;

        // Initialize showCheckedNodesButton.
        showCheckedNodesButton.Size = new Size(144, 24);
        showCheckedNodesButton.Text = "Show Checked Nodes";
        showCheckedNodesButton.Click += 
            new EventHandler(showCheckedNodesButton_Click);

        // Initialize the form. 
        this.ClientSize = new Size(292, 273);
        this.Controls.AddRange(new Control[] 
            { showCheckedNodesButton, treeView1 } );

        this.ResumeLayout(false);
    }

    [STAThreadAttribute()]
    static void Main() 
    {
        Application.Run(new Form1());
    }

    private void showCheckedNodesButton_Click(object sender, EventArgs e)
    {
        // Disable redrawing of treeView1 to prevent flickering  
        // while changes are made.
        treeView1.BeginUpdate();

        // Collapse all nodes of treeView1.
        treeView1.ExpandAll();

        // Add the checkForCheckedChildren event handler to the BeforeExpand event.
        treeView1.BeforeCollapse += checkForCheckedChildren;

        // Expand all nodes of treeView1. Nodes without checked children are  
        // prevented from expanding by the checkForCheckedChildren event handler.
        treeView1.CollapseAll();

        // Remove the checkForCheckedChildren event handler from the BeforeExpand  
        // event so manual node expansion will work correctly.
        treeView1.BeforeCollapse -= checkForCheckedChildren;

        // Enable redrawing of treeView1.
        treeView1.EndUpdate();
    }

    // Prevent collapse of a node that has checked child nodes. 
    private void CheckForCheckedChildrenHandler(object sender, 
        TreeViewCancelEventArgs e)
    {
        if (HasCheckedChildNodes(e.Node)) e.Cancel = true;
    }

    // Returns a value indicating whether the specified  
    // TreeNode has checked child nodes. 
    private bool HasCheckedChildNodes(TreeNode node)
    {
        if (node.Nodes.Count == 0) return false;
        foreach (TreeNode childNode in node.Nodes)
        {
            if (childNode.Checked) return true;
            // Recursively check the children of the current child node. 
            if (HasCheckedChildNodes(childNode)) return true;
        }
        return false;
    }

}

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1

.NET Framework Client Profile

Supported in: 4, 3.5 SP1
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