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TreeNode.PopulateOnDemand Property

Gets or sets a value that indicates whether the node is populated dynamically.

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

public bool PopulateOnDemand { get; set; }

Property Value

Type: System.Boolean
true to populate the node dynamically; otherwise, false. The default is false.

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 a node's PopulateOnDemand property is set to true, that node is populated at run time through a postback event when the node is expanded. To populate a node dynamically, an event-handling method that populates the node must be defined for the TreeNodePopulate event.

Supported browsers can also take advantage of client-side node population. When enabled, this allows the TreeView control to dynamically populate a node 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.

The value of this property is stored in view state.

The following code example demonstrates how to use the PopulateOnDemand property to indicate that a node is populated dynamically.

<%@ 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();
        newNode.Text = row["CategoryName"].ToString(); 
        newNode.Value = 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 PopulateNodesFromClient Example</title>
</head>
<body>
    <form id="form1" runat="server">

      <h3>TreeView PopulateNodesFromClient Example</h3>

      <asp:TreeView id="LinksTreeView"
        Font-Names= "Arial"
        ForeColor="Blue"
        EnableClientScript="true"
        PopulateNodesFromClient="true"  
        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>

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

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

.NET Framework

Supported in: 3.5, 3.0, 2.0

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