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Tip/Trick
Posted 2 May 2018

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WPF Behavior to Set Control Value to DefaultValue on Visibility Changed

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2 May 2018CPOL1 min read
The tip provides a behavior that will set a control to a default value when the Visibility changes

Introduction

I was doing work where a lot of the functionality in an WPF MVVM project was within the View. Part of the View would change visibility, and when these parts again became visible, they needed to be back to a default value. With this behavior, this functionality can be implemented with a behavior attached to the control.

I had previously done a couple of tips that also reset values on visibility changed. They were behaviors that attacked the control containing a number of ToggleButton/RadioButton/CheckBox controls. On working on these behaviors, I realized that it may be desirable to be able to handle more types of controls, and to be able to set not just bool Type values, but more general values.

Using the Code

Here is the code for the behaviour:

C#
public class DefaultValueOnVisibilityBehavior
{
    private static void OnDefaultValuePropertyChanged(DependencyObject d,
        DependencyPropertyChangedEventArgs e)
    {
        var element = (FrameworkElement)d;
        element.IsVisibleChanged -= ElementOnIsVisibleChanged;

        element.IsVisibleChanged += ElementOnIsVisibleChanged;
    }

    public static object GetDefaultValue(DependencyObject obj)
    {
        return obj.GetValue(DefaultValueProperty);
    }

    public static void SetDefaultValue(DependencyObject obj, object value)
    {
        obj.SetValue(DefaultValueProperty, value);
    }

    public static readonly DependencyProperty DefaultValueProperty =
        DependencyProperty.RegisterAttached("DefaultValue",
            typeof(object), typeof(DefaultValueOnVisibilityBehavior),
            new PropertyMetadata(OnDefaultValuePropertyChanged));

    private static void ElementOnIsVisibleChanged(object sender,
        DependencyPropertyChangedEventArgs dependencyPropertyChangedEventArgs)
    {
        FrameworkElement frameworkElement = (FrameworkElement)sender;
        var defaultValue = GetDefaultValue(frameworkElement);
        var defaultValueString = defaultValue.ToString();
        var dependencyObject = (UIElement)sender;

        if (sender is ToggleButton toggleBusson)
            toggleBusson.IsChecked = defaultValueString.ToLower() == "true"
                ? true : defaultValueString.ToLower() == "false"
                ? false : (bool?)null;
        else if (sender is TextBox textBox)
            textBox.Text = defaultValueString;
        else if (sender is ContentControl contentControl)
            contentControl.Content = defaultValue;
    }
}

There is a single DependencyProperty, DefaultValue. This is the value that a DependencyProperty on the control is set to. The changing of this DependencyProperty will cause the OnDefaultValuePropertyChanged event handler to be assigned to the VisibilityChanged event. When the Visibility changes, then the value of the DefaultValue DependencyProperty will be assigned to a DependencyProperty of the control based on the control Type. The following are the DependencyProperty definitions that are affected:

  • CheckBox, ToggleButton, RadioButton: IsChecked DependencyProperty.
  • TextBox: Text DependencyProperty.
  • ContentControl (other than above): Content DependencyProperty.

More can be defined, such as for ListBox, and could add the functionality to select the DependencyProperty that is affected, but I did not need this capability.

Using the Behavior

Here are some simple examples of how to use this behavior:

XML
<RadioButton local:DefaultValueOnVisibilityBehavior.DefaultValue="True"
             Content="RadioButton Two (Default true)" />
<ToggleButton local:DefaultValueOnVisibilityBehavior.DefaultValue="True"
              Content="ToggleButton Two (Default true)"/>
<TextBox Grid.Row="2"
              Grid.Column="1" Margin="5"
              local:DefaultValueOnVisibilityBehavior.DefaultValue
    ="{Binding ElementName=ComboBox,
                               Path=SelectedItem.Content}" />
<RadioButton Grid.Row="3" Margin="5"
             Grid.Column="0"
             Content="RadioButton Four" />
<TextBox Grid.Row="3"
              Grid.Column="1" Margin="5"
          local:DefaultValueOnVisibilityBehavior.DefaultValue="This is the default Value" />

Image 1Image 2

History

  • 05/02/2018: Initial version

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Clifford Nelson
Software Developer (Senior) Clifford Nelson Consulting
United States United States
Has been working as a C# developer on contract for the last several years, including 3 years at Microsoft. Previously worked with Visual Basic and Microsoft Access VBA, and have developed code for Word, Excel and Outlook. Started working with WPF in 2007 when part of the Microsoft WPF team. For the last eight years has been working primarily as a senior WPF/C# and Silverlight/C# developer. Currently working as WPF developer with BioNano Genomics in San Diego, CA redesigning their UI for their camera system. he can be reached at qck1@hotmail.com.

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