Develop Reference

Bind to the Visibility Property of a Label to a Custom Property

I have a function in my UI where I want to be able to collapse/make visible a text message depending on the value of a custom property in my window object.
Using online references, I have come up with this code-behind to register the property:
public bool ValidInterval
{
get { return pValidInterval; }
}
private bool pValidInterval = true;
public static readonly DependencyProperty ValidIntervalProperty = DependencyProperty.Register("ValidInterval", typeof(bool), typeof(Settings), new UIPropertyMetadata(true));
And this corresponding XAML for the label:
<Label Name="DynamicWarning" Content="Time interval must be a valid positive integer.">
<Label.Style>
<Style TargetType="Label">
<Setter Property="Visibility" Value="Collapsed" />
<Style.Triggers>
<DataTrigger Binding="{Binding ValidInterval}" Value="true">
<Setter Property="Visibility" Value="Collapsed" />
</DataTrigger>
<DataTrigger Binding="{Binding ValidInterval}" Value="false">
<Setter Property="Visibility" Value="Visible" />
</DataTrigger>
</Style.Triggers>
</Style>
</Label.Style>
</Label>
Unfortunately, this does not work. I can get it to set the parameter depending on the initial value of the property, but doesn't update the visibility dynamically like I want. I have been looking at this for an hour and what I have seems consistent with examples I am finding online for similar operations. Am I doing something wrong, or can you not update the visibility on the fly? If the latter, how do I achieve an equivalent effect?

ΩmegaMan's answer is correct. But I wanted to clarify, you don't need a backing field with a dependency property, the static dependency property IS the backing field.
public bool ValidInterval
{
get { return (bool)GetValue(ValidIntervalProperty); }
set { SetValue(ValidIntervalProperty, value); }
}
However, if you aren't in need of a dependency object specifically, then you may just want to use INotifyPropertyChanged as ΩmegaMan has it right. Dependency properties are typically used when you need to bind another property to them, such as when making your own custom control. For example Visibility itself is the dependency property in your example and ValidInterval just needs to be a normal property that invokes the NotifyPropertyChanged event.

You need for the holder class of the properties which are bound to the page, to adhere to INotifyPropertyChanged Interface and implement it.
That process informs the bound controls on the page that something has changed, and when it has changed, then the control is "notified" of the change; then it reads afresh the property it is bound to.
For WPF/Xaml they specify the seperation of data concerns for the views to business logic, is done by implementing the Model-View-ViewModel or MVVM pattern.
The link provided is dry, and there are other resources which can describe on the net, but it simply says put all your business logic that is bound from the View to a separate View Model Class; which is instantiated on your View.
I provide a basic example, any version of .Net can be used, on my blog:
MVVM Example for Easier Binding

The type "SettingsViewModel" does not include any accessible constructors. Is there a way to get rid of this error?

I'm making a simple application with WPF and binding one of the windows to a view model. The view model needs a few references at run-time to get the data it needs, so my view model has a constructor with parameters. I also have a parameter-less constructor to use during design to test the look of the window.
The problem I'm running in to is that whenever a constructor with parameters exists visual studio displays the error:
'The type "SettingsViewModel" does not include any accessible constructors.'
If I comment out the run-time constructor then no errors occur and everything works as expected.
Additionally, I've tried making the run-time constructor private and exposing it through a factory method, which had no effect. I've tried commenting out code-behind code referencing the second constructor. I've also tried cleaning the solution, restarting visual studio, then rebuilding.
So far the only thing I have found to work is commenting out the run-time constructor and any references to it. Also just to be clear I am not using the SettingsViewModel instance for anything yet, at this point I'm just trying to define it as a resource.
SettingsWindow.xaml:
<Window.Resources>
<local:SettingsViewModel x:Key="DevelopTimeModel" />
<Style TargetType="{x:Type ListBoxItem}">
<Setter Property="BorderBrush" Value="Gray"></Setter>
<Setter Property="BorderThickness" Value="1"></Setter>
<Setter Property="Padding" Value="2 3"></Setter>
<Style.Triggers>
<Trigger Property="ItemsControl.AlternationIndex" Value="0">
<Setter Property="Background" Value="#00000000"></Setter>
</Trigger>
<Trigger Property="ItemsControl.AlternationIndex" Value="1">
<Setter Property="Background" Value="LightGray"></Setter>
</Trigger>
</Style.Triggers>
</Style>
</Window.Resources>
SettingsWindow.xaml.cs
public SettingsWindow(Autodesk.Revit.UI.UIApplication app)
{
InitializeComponent();
SettingsViewModel viewModel = SettingsViewModel.GetRuntimeViewModel(app, this);
this.DataContext = viewModel;
MainListBox.SelectionChanged += (o, e) => viewModel.RefreshCommands();
}
SettingViewModel.cs
public SettingsViewModel()
{
//creation of design-time data here
}
public static SettingsViewModel GetRuntimeViewModel(UIApplication app, SettingsWindow settingsWindow)
{
return null;// new SettingsViewModel(app, settingsWindow);
}
private SettingsViewModel(UIApplication app, SettingsWindow settingsWindow)
{
ContextRouter = new RevitContextRouter(app, this.ToString());
members.PropertyChanged += (o, e) => NotifyPropertyChanged("Members." + e.PropertyName);
Commands = new SettingsCommands(this);
LoadSettings();
SettingsWindowRef = settingsWindow;
settingsWindow.Closed += (s,e) => SaveSettings() ;
}
Any help getting rid of this error message and similar ones when they come up would be appreciated.

Accessing Validation Errors in a UserControl

I created a WPF (.Net 4) UserControl containing some ComboBoxes and a TextBox. In XAML, some ValidationRules are bound to the TextBox. If the TextBox contains invalid data, a red frame is shown, and the tooltip is set to the error description. Works well.
Next, I placed two instances of that UserControl on a form, and added a button. In XAML, the button is connected to a RelayCommand of the ViewModel. Now I want the button to be enabled only when both of the UserControls contain valid data only.
Not a problem, I thought, let me use a strategy which works elsewhere. I added a trigger:
<Button Content="_OK" ... Command="{Binding Path=OKCommand}">
<Button.Style>
<Style TargetType="{x:Type Button}">
<Setter Property="IsEnabled" Value="false" />
<Style.Triggers>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding ElementName=cascadingComboFrom, Path=(Validation.HasError)}" Value="false" />
<Condition Binding="{Binding ElementName=cascadingComboTo, Path=(Validation.HasError)}" Value="false" />
</MultiDataTrigger.Conditions>
<Setter Property="IsEnabled" Value="true" />
</MultiDataTrigger>
</Style.Triggers>
</Style>
</Button.Style>
</Button>
But there is a catch: Validation.HasError is always false for the UserControls - the Validation failed for an element inside the UserControl, not for the UserControl proper.
I know that I can register the Click event of the button, check the validity there using the method shown in Detecting WPF Validation Errors, and prevent the execution of the RelayCommand by setting the Handled property of the RoutedEventArgs to true. But that looks more like WTF than WPF.
What do you suggest? How can I retrieve the Validation Errors of the UserControl's children? Or how can I get them in the RelayCommand's CanExecute method? Or some other tricks?

You can set a property on the command binding called ValidatesOnDataErrors.
Implementation would look something like this:
<Button Content="_OK" Command="{Binding, Path=OKCommand, ValidatesOnDataErrors=True}"/>
You can read more about it here.

The Button.IsEnabled property is already hard wired to the CanExecute method of your RelayCommand, so all you need to do is to set that return value to false when the form fields are invalid:
private bool CanExecute(object commandParameter)
{
return areFormFieldsValid;
}
Now, how you set the bool areFormFieldsValid variable to true or false is up to you... there are several ways of doing that. Personally, I prefer to use the IDataErrorInfo interface, which has a handy Error property that you can check. There are many online tutorial on how to implement this, so I won't repeat that here... however, the end result is something like this:
private bool CanExecute(object commandParameter)
{
return string.IsNullOrEmpty(yourDataObject.Error);
}

Clear a dependencyProperty value from the xaml

I have a control which has a default value for a property. When the control first gets its dataContext set, it assigns this property automatically.
In the xaml now, I want it to be possible to UNset this property. I've tried setting it to x:Null of just the empty string, but then I get an error because there's no converter for the property. How do I simply unassign this property from the xaml in the rare cases where I want the feature disabled?
code where it is originally set:
void OmniBox_DataContextChanged(object sender, DependencyPropertyChangedEventArgs e)
{
if( e.NewValue is BindingObjectBaseExtended )
{
BindingObjectBaseExtended value = (BindingObjectBaseExtended)e.NewValue;
this.SetBinding(OmniBox.ContextValidationMessagesProperty, new Binding() { Source = value, Path = new PropertyPath("ValidationMessages") });
}
}
xaml where I want to unset the property.
<Style TargetType="ui:OmniBox">
<Setter Property="ContextValidationMessages" Value="" />
</Style>
Note that if I do not set up the binding automatically when the data context changes, then by default there are no validation messages and I have to do the following in the xaml to set them up:
<Style TargetType="ui:OmniBox">
<Setter Property="ContextValidationMessages" Value="ValidationMessages" />
</Style>
What I'm trying to do is make the above binding the default for my custom OmniBox control, and allow the user to unset it or change it to something else.

DependencyProperty.UnsetValue cannot be used in XAML.
http://msdn.microsoft.com/en-us/library/system.windows.dependencyproperty.unsetvalue(v=vs.90).ASPX

Personally, I would create a separate dependency property, such as bool AutoBindValidation and make it default to true. If it is false, don't do anything when the DataContext changes. This is a little more self-documenting. Depending on what exactly you're trying to do, you might not want to publicly expose ContextValidationMessages at all.
If you really want to do it the way you posted, I'm not sure why setting it to {x:Null} would cause an error (unless the property type is not nullable). But this approach would have problems because DataContextChanged is going to occur after the XAML is parsed. So the user can set it to {x:Null}, but then the DataContext will change and your code will set up the default binding and trample the user's value. You could set up the binding in the control's contstructor, but then if the DataContext does not have a ValidationMessages property, your control will be spitting out binding errors.

This may be impossible, my best bet was this:
<Setter Property="ContextValidationMessages"
Value="{x:Static DependencyProperty.UnsetValue}" />
But that throws "Cannot unset setter value". So you better inverse your logic or keep the property unset another way.

I don't think there is any supported way to do this in the xaml itself. In your code you are setting a local value on the ContextValidationMessagesProperty. The Style setters you included would have a lower dependency property precedence and even if they were evaluated they would set a value based on the specified Value - not clear it. Maybe instead of setting the binding in code you could have a Setter in your default style for OmniBox that sets that property - e.g.
<Setter Property="ContextValidationMessages" Value="{Binding ValidationMessages}" />
If you have to conditionally set the Binding then you could create a custom IValueConverter that checks for the specified type (passed as the parameter). e.g.
public class IsAssignableFromConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
Type typeParameter = parameter as Type;
if (typeParameter == null)
return DependencyProperty.UnsetValue;
return value != null && typeParameter.IsAssignableFrom(value.GetType());
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
return DependencyProperty.UnsetValue;
}
}
Then you might use it like this:
<local:IsAssignableFromConverter x:Key="isAssignableConverter" />
<Style TargetType="ui:OmniBox">
<Style.Triggers>
<DataTrigger Binding="{Binding Converter={StaticResource isAssignableConverter}, ConverterParameter={x:Type ui:BindingObjectBaseExtended}}" Value="True">
<Setter Property="ContextValidationMessages" Value="{Binding ValidationMessages}" />
</DataTrigger>
</Style.Triggers>
</Style>
In the case where you don't want this property to be applied you might set the Style for that instance of the OmniBox to a new style and make sure to set the OverridesDefaultStyle property to true.
I suppose another option is to create another dependency property that will call ClearValue on the ContextValidationMessages property but this seems like it could be a maintenance issue.

For certain cases you can 'reset' to the default value of the parent control by using a RelativeSource. For instance I'm using a DataGrid and this worked for me to reset back to the 'default'.
This is a textblock inside a datagrid cell.
<TextBlock Text="{Binding ServiceName}">
<TextBlock.Style>
<Style>
<Style.Triggers>
<!-- Change text color to purple for FedEx -->
<Trigger Property="TextBlock.Text" Value="FedEx">
<Setter Property="TextBlock.Foreground" Value="Purple"/>
</Trigger>
<!-- Reset if the cell is selected, since purple on blue is illegible -->
<DataTrigger Binding="{Binding IsSelected, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type DataGridCell}}}" Value="True">
<Setter Property="TextBlock.Foreground" Value="{Binding Foreground, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type DataGridCell}}}"/>
</DataTrigger>
</Style.Triggers>
</Style>
</TextBlock.Style>
</TextBlock>
This seems clever enough to inherit the correct color even when the window is inactive.

Set other property of selected objects in ListBox

For example, i have a ListBox with my Employees, via binding.
I want to color all the Employees that earn more than 10,000$.
Thanks

The most straightforward way is to implement a property in the Employee that returns a string value, and then test the string value in a style's data trigger. For instance:
public string SalaryRange
{
get
{
if (salary <= LowRange)
{
return "Low";
}
if (salary <= MiddleRange)
{
return "Middle";
}
return "High";
}
}
Then, in XAML, implement a style that uses a data trigger, e.g.:
<Style TargetType = "ListBoxItem">
<Style.Triggers>
<DataTrigger Binding="{SalaryRange}" Value="Low">
<Setter Property="Foreground" Value="AliceBlue"/>
</DataTrigger>
<DataTrigger Binding="{SalaryRange}" Value="Middle">
<Setter Property="Foreground" Value="Fucschia"/>
</DataTrigger>
<DataTrigger Binding="{SalaryRange}" Value="Middle">
<Setter Property="Foreground" Value="Goldenrod"/>
</DataTrigger>
</Style.Triggers>
</Style>
A lot of people learn about value converters and get all enamored with the idea of using them for this kind of thing. But if you're using MVVM (which you should be), the above is really much cleaner: it's clear what the property does, it's clear what the style does, it's easy to test, and there's no third piece of code hiding in a separate file that's doing all of the translation.