Question

I have a user control which has a CheckBox, a Button, and a CommandBinding. If the CheckBox is checked, the Button is enabled. The MainWindow uses the UserControl. When the Button in the main window is pressed, the UserControl is removed from UI, and GC.Collect() is called, but CanExecute method still runs.

I find that if I click the button in main window twice, CanExecute will no longer run. It seems that I don’t call GC.Collect() at the right time.

I want to know what is the good timing to call GC to clean the unused user control, so that CanExecute will not be called.

XAML

<UserControl x:Class="WpfApplication1.UserControl1"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <UserControl.Resources>
        <RoutedUICommand x:Key="okCommand" Text="OK"/>
    </UserControl.Resources>
    <UserControl.CommandBindings>
        <CommandBinding Command="{StaticResource okCommand}" CanExecute="CommandBinding_CanExecute_1"/>
    </UserControl.CommandBindings>
    <StackPanel>
        <CheckBox Name="checkBox" Content="CheckBox"/>
        <Button Command="{StaticResource okCommand}" Content="{Binding Path=Text, Source={StaticResource okCommand}}"/>
    </StackPanel>
</UserControl>

Code behind

public partial class UserControl1 : UserControl
{
    public UserControl1()
    {
        InitializeComponent();
    }

    private void CommandBinding_CanExecute_1(object sender, CanExecuteRoutedEventArgs e)
    {
        e.CanExecute = checkBox.IsChecked.GetValueOrDefault(false);

        System.Media.SystemSounds.Beep.Play();
    }
}

MainWindow

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Loaded="Window_Loaded_1">
    <StackPanel>
        <Border Name="container"/>

        <Button Content="Set Null" Click="Button_Click_1"/>
    </StackPanel>
</Window>

Code behind

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    private void Button_Click_1(object sender, RoutedEventArgs e)
    {
        container.Child = null;

        GC.Collect();
        GC.WaitForPendingFinalizers();
    }

    private void Window_Loaded_1(object sender, RoutedEventArgs e)
    {
        UserControl1 uc = new UserControl1();
        container.Child = uc;
    }
}

Answer 1

I find another solution. That is to call CommandBindings.Clear() in UserControl1 when it unloads.

I believe this is a neat way, since the caller of UserControl1 doesn’t take care of the cleaning job of UserControl1.