By default, NavigationService does not store an instance of a content object in navigation history. Instead, NavigationService creates a new instance of the content object each time it is navigated to by using navigation history. This behavior is designed to avoid excessive memory consumption when large numbers and large pieces of content are being navigated to. Consequently, the state of the content is not remembered from one navigation to the next. However, WPF provides the ability to associate a piece of custom state with the navigation history entry for a piece of content.

Custom state that is associated with a navigation history entry must be a class that derives from CustomContentState. You associate a CustomContentState object with a navigation history entry by using one of the following techniques:

• Calling AddBackEntry:

  • NavigationService..::.AddBackEntry

  • NavigationWindow..::.AddBackEntry

  • Frame..::.AddBackEntry.

• Setting NavigatingCancelEventArgs..::.Content when one of the following events are raised:

  • NavigationService..::.Navigating

  • NavigationWindow..::.Navigating

  • Frame..::.Navigating

  • NavigationWindow..::.Navigating

• By implementing IProvideCustomContentState on the class that wants custom state to be associated with it.

Note:
If you call the AddBackEntry method, you must handle the Navigating event or implement IProvideCustomContentState.

When the navigation history entry is navigated to, WPF checks to see if a custom CustomContentState object is associated with it. If so, it calls Replay to allow the custom CustomContentState object to apply the state it remembered from the previous navigation.

A custom CustomContentState class can override JournalEntryName to change the name that appears for the navigation history entry to which the CustomContentState object is associated. The value that JournalEntryName returns is visible from the navigation UI of the various navigators (Internet Explorer 7, NavigationWindow, Frame).

A class that derives from CustomContentState must be serializable, which means it must at least be augmented with SerializableAttribute, and optionally implement ISerializable.

For an example that uses CustomContentState to remember a single set of state across page instances, see Remember a Single Set of State Across Page Instances.

For an example that uses CustomContentState to remember multiple sets of state for a single page instance, see Remember Multiple Sets of State per Page Instance.

Important Note:
When you store information in custom content state, you cannot store any references to the instance of the page for which you are remembering state if don’t want the content to be retained in memory. This prevents WPF from releasing the page instance, and defeats the purpose of the default navigation history behavior. If you must do this, consider using KeepAlive instead.

The following is an example of a CustomContentState implementation that overrides JournalEntryName.

using System;
using System.Windows.Controls;
using System.Windows.Navigation;
[Serializable]
public class MyCustomContentState : CustomContentState
{
    string dateCreated;
    TextBlock dateTextBlock;

    public MyCustomContentState(string dateCreated, TextBlock dateTextBlock)
    {
        this.dateCreated = dateCreated;
        this.dateTextBlock = dateTextBlock;
    }

    public override string JournalEntryName
    {
        get
        {
            return "Journal Entry " + this.dateCreated;
        }
    }

    public override void Replay(NavigationService navigationService, NavigationMode mode)
    {
        this.dateTextBlock.Text = this.dateCreated;
    }
}

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003

.NET Framework

Reference