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TreeView.TreeNodePopulate Event

Occurs when a node with its PopulateOnDemand property set to true is expanded in the TreeView control.

Namespace:  System.Web.UI.WebControls
Assembly:  System.Web (in System.Web.dll)

public event TreeNodeEventHandler TreeNodePopulate
<asp:TreeView OnTreeNodePopulate="TreeNodeEventHandler" />

Sometimes, it is not practical to statically predefine the tree structure due to data size or custom content that depends on user input. Because of this, the TreeView control supports dynamic node population. When the PopulateOnDemand property for a node is set to true, that node gets populated at run time when the node is expanded.

To populate a node dynamically, first set the PopulateOnDemand property for the node to true. Next, define an event-handling method for the TreeNodePopulate event that populates the node programmatically. A typical event-handling method retrieves node data from a data source, places the data into a node structure, and then adds the node structure to the ChildNodes collection of the node being populated. A node structure is created by adding TreeNode objects to the ChildNodes collection of a parent node.

NoteNote

When the PopulateOnDemand property for a node is set to true, the node must be populated dynamically. You cannot declaratively nest another node below it; otherwise, an error will occur on the page.

Supported browsers (Microsoft Internet Explorer 4.0-compatible browsers and later) can also take advantage of client-side node population. When enabled, this allows the TreeView control to populate a node dynamically on the client when that node is expanded, which prevents the need to post back to the server. For more information on client-side node population, see PopulateNodesFromClient.

For more information about handling events, see Consuming Events.

The following code example demonstrates how to use the TreeNodePopulate event to populate the nodes in the TreeView control dynamically on the server. Notice that the EnableClientScript property is set to false to prevent expanding-node events from being handled on the client.


<%@ Page Language="C#" %>
<%@ Import Namespace="System.Data" %>
<%@ Import Namespace="System.Data.SqlClient" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script runat="server">

  void PopulateNode(Object sender, TreeNodeEventArgs e)
  {

    // Call the appropriate method to populate a node at a particular level.
    switch(e.Node.Depth)
    {
      case 0:
        // Populate the first-level nodes.
        PopulateCategories(e.Node);
        break;
      case 1:
        // Populate the second-level nodes.
        PopulateProducts(e.Node);
        break;
      default:
        // Do nothing.
        break;
    }

  }

  void PopulateCategories(TreeNode node)
  {

    // Query for the product categories. These are the values
    // for the second-level nodes.
    DataSet ResultSet = RunQuery("Select CategoryID, CategoryName From Categories");

    // Create the second-level nodes.
    if(ResultSet.Tables.Count > 0)
    {

      // Iterate through and create a new node for each row in the query results.
      // Notice that the query results are stored in the table of the DataSet.
      foreach (DataRow row in ResultSet.Tables[0].Rows)
      {

        // Create the new node. Notice that the CategoryId is stored in the Value property 
        // of the node. This will make querying for items in a specific category easier when
        // the third-level nodes are created. 
        TreeNode NewNode = new TreeNode(row["CategoryName"].ToString(), row["CategoryID"].ToString());

        // Set the PopulateOnDemand property to true so that the child nodes can be 
        // dynamically populated.
        NewNode.PopulateOnDemand = true;

        // Set additional properties for the node.
        NewNode.SelectAction = TreeNodeSelectAction.Expand;

        // Add the new node to the ChildNodes collection of the parent node.
        node.ChildNodes.Add(NewNode);

      }

    }

  }

  void PopulateProducts(TreeNode node)
  {

    // Query for the products of the current category. These are the values
    // for the third-level nodes.
    DataSet ResultSet = RunQuery("Select ProductName From Products Where CategoryID=" + node.Value);

    // Create the third-level nodes.
    if(ResultSet.Tables.Count > 0)
    {

      // Iterate through and create a new node for each row in the query results.
      // Notice that the query results are stored in the table of the DataSet.
      foreach (DataRow row in ResultSet.Tables[0].Rows)
      {

        // Create the new node.
        TreeNode NewNode = new TreeNode(row["ProductName"].ToString());

        // Set the PopulateOnDemand property to false because these are leaf nodes and
        // do not need to be populated.
        NewNode.PopulateOnDemand = false;

        // Set additional properties for the node.
        NewNode.SelectAction = TreeNodeSelectAction.None;

        // Add the new node to the ChildNodes collection of the parent node.
        node.ChildNodes.Add(NewNode);

      }

    }

  }

  DataSet RunQuery(String QueryString)
  {

    // Declare the connection string. This example uses Microsoft SQL Server and connects to the
    // Northwind sample database.
    String ConnectionString = "server=localhost;database=NorthWind;Integrated Security=SSPI"; 

    SqlConnection DBConnection = new SqlConnection(ConnectionString);
    SqlDataAdapter DBAdapter;
    DataSet ResultsDataSet = new DataSet();

    try
    {

      // Run the query and create a DataSet.
      DBAdapter = new SqlDataAdapter(QueryString, DBConnection);
      DBAdapter.Fill(ResultsDataSet);

      // Close the database connection.
      DBConnection.Close();

    }
    catch(Exception ex)
    {

      // Close the database connection if it is still open.
      if(DBConnection.State == ConnectionState.Open)
      {
        DBConnection.Close();
      }

      Message.Text = "Unable to connect to the database.";

    }

    return ResultsDataSet;

  }

</script>

<html xmlns="http://www.w3.org/1999/xhtml" >
  <head runat="server">
    <title>TreeView TreeNodePopulate Example</title>
</head>
<body>
    <form id="form1" runat="server">

      <h3>TreeView TreeNodePopulate Example</h3>

      <asp:TreeView id="LinksTreeView"
        Font-Names= "Arial"
        ForeColor="Blue"
        EnableClientScript="false" 
        OnTreeNodePopulate="PopulateNode"
        runat="server">

        <Nodes>

          <asp:TreeNode Text="Inventory" 
            SelectAction="Expand"  
            PopulateOnDemand="true"/>

        </Nodes>

      </asp:TreeView>

      <br /><br />

      <asp:Label id="Message" runat="server"/>

    </form>
  </body>
</html>

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0

Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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