How to bind inverse boolean properties in WPF?

user_32772 0
Viewed 245608

What I have is an object that has an IsReadOnly property. If this property is true, I would like to set the IsEnabled property on a Button, ( for example ), to false.

I would like to believe that I can do it as easily as IsEnabled="{Binding Path=!IsReadOnly}" but that doesn't fly with WPF.

Am I relegated to having to go through all of the style settings? Just seems too wordy for something as simple as setting one bool to the inverse of another bool.

<Button.Style>
    <Style TargetType="{x:Type Button}">
        <Style.Triggers>
            <DataTrigger Binding="{Binding Path=IsReadOnly}" Value="True">
                <Setter Property="IsEnabled" Value="False" />
            </DataTrigger>
            <DataTrigger Binding="{Binding Path=IsReadOnly}" Value="False">
                <Setter Property="IsEnabled" Value="True" />
            </DataTrigger>
        </Style.Triggers>
    </Style>
</Button.Style>
.net-3.5 styles wpf
13 Answers

You can use a ValueConverter that inverts a bool property for you.

XAML:

IsEnabled="{Binding Path=IsReadOnly, Converter={StaticResource InverseBooleanConverter}}"

Converter:

[ValueConversion(typeof(bool), typeof(bool))]
    public class InverseBooleanConverter: IValueConverter
    {
        #region IValueConverter Members

        public object Convert(object value, Type targetType, object parameter,
            System.Globalization.CultureInfo culture)
        {
            if (targetType != typeof(bool))
                throw new InvalidOperationException("The target must be a boolean");

            return !(bool)value;
        }

        public object ConvertBack(object value, Type targetType, object parameter,
            System.Globalization.CultureInfo culture)
        {
            throw new NotSupportedException();
        }

        #endregion
    }
user_799100

Have you considered an IsNotReadOnly property? If the object being bound is a ViewModel in a MVVM domain, then the additional property makes perfect sense. If it's a direct Entity model, you might consider composition and presenting a specialized ViewModel of your entity to the form.

user_764560

.Net Core Solution

Handles null situation and does not throw an exception, but returns true if no value is presented; otherwise takes the inputted Boolean and reverses it.

public class BooleanToReverseConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
     => !(bool?) value ?? true;

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
     => !(value as bool?);
}

Xaml

IsEnabled="{Binding IsSuccess Converter={StaticResource BooleanToReverseConverter}}"

App.Xaml I like to put all my converter statics in the app.xaml file so I don't have to redeclare them throughout the windows/pages/controls of the project. 

<Application.Resources>
    <converters:BooleanToReverseConverter x:Key="BooleanToReverseConverter"/>
    <local:FauxVM x:Key="VM" />
</Application.Resources>

To be clear converters: is the namespace to the actual class implementation (xmlns:converters="clr-namespace:ProvingGround.Converters").

user_2857950

I had an inversion problem, but a neat solution.

Motivation was that the XAML designer would show an empty control e.g. when there was no datacontext / no MyValues (itemssource).

Initial code: hide control when MyValues is empty. Improved code: show control when MyValues is NOT null or empty.

Ofcourse the problem is how to express '1 or more items', which is the opposite of 0 items.

<ListBox ItemsSource={Binding MyValues}">
  <ListBox.Style x:Uid="F404D7B2-B7D3-11E7-A5A7-97680265A416">
    <Style TargetType="{x:Type ListBox}">
      <Style.Triggers>
        <DataTrigger Binding="{Binding MyValues.Count}">
          <Setter Property="Visibility" Value="Collapsed"/>
        </DataTrigger>
      </Style.Triggers>
    </Style>
  </ListBox.Style>
</ListBox>

I solved it by adding:

<DataTrigger Binding="{Binding MyValues.Count, FallbackValue=0, TargetNullValue=0}">

Ergo setting the default for the binding. Ofcourse this doesn't work for all kinds of inverse problems, but helped me out with clean code.

user_6818950

Following @Paul's answer, I wrote the following in the ViewModel: 

public bool ShowAtView { get; set; }
public bool InvShowAtView { get { return !ShowAtView; } }

I hope having a snippet here will help someone, probably newbie as I am.
And if there's a mistake, please let me know!

BTW, I also agree with @heltonbiker comment - it's definitely the correct approach only if you don't have to use it more than 3 times...

user_89527110

I did something very similar. I created my property behind the scenes that enabled the selection of a combobox ONLY if it had finished searching for data. When my window first appears, it launches an async loaded command but I do not want the user to click on the combobox while it is still loading data (would be empty, then would be populated). So by default the property is false so I return the inverse in the getter. Then when I'm searching I set the property to true and back to false when complete.

private bool _isSearching;
public bool IsSearching
{
    get { return !_isSearching; }
    set
    {
        if(_isSearching != value)
        {
            _isSearching = value;
            OnPropertyChanged("IsSearching");
        }
    }
}

public CityViewModel()
{
    LoadedCommand = new DelegateCommandAsync(LoadCity, LoadCanExecute);
}

private async Task LoadCity(object pArg)
{
    IsSearching = true;

    //**Do your searching task here**

    IsSearching = false;
}

private bool LoadCanExecute(object pArg)
{
    return IsSearching;
}

Then for the combobox I can bind it directly to the IsSearching:

<ComboBox ItemsSource="{Binding Cities}" IsEnabled="{Binding IsSearching}" DisplayMemberPath="City" />
user_65014260

I use a similar approach like @Ofaim

private bool jobSaved = true;
private bool JobSaved    
{ 
    get => jobSaved; 
    set
    {
        if (value == jobSaved) return;
        jobSaved = value;

        OnPropertyChanged();
        OnPropertyChanged("EnableSaveButton");
    }
}

public bool EnableSaveButton => !jobSaved;
user_58836320
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