Develop Reference

SelectedItem binding on ComboBox not showing selected value

I'm trying to build a settings page to allow the user to choice which action to execute on item swipe, like the Outlook app.
To do this I created an enum containing the available actions, and I'm binding it to a ComboBox.
Everything works, the user can choose the action and his choice is saved correctly. The problem is that the ComboBox doesn't show the selected item when I navigate to the page, it shows it only after selection.
This means that if user changes selection then the ComboBox is updated, but the selected item is shown as blank upon navigation.
Here's my code:
(XAML)
<ComboBox x:Uid="LeftActionComboBox"
Grid.Row="0"
HorizontalAlignment="Stretch"
SelectedItem="{Binding LeftSwipeActionType, Mode=TwoWay, Converter={StaticResource StringToSwipeActionTypesConverter}}"
ItemsSource="{Binding LeftSwipeActionType, Converter={StaticResource EnumToStringListConverter}}"/>
(VM Property)
public SwipeActionTypes LeftSwipeActionType
{
get { return _settings.LeftSwipeActionTypeProperty; }
set
{
_settings.LeftSwipeActionTypeProperty = value;
// RaisePropertyChanged causes a StackOverflow, but not using it is not the problem since the ComboBox is empty only before set
}
}
(Converter StringToSwipeActionTypesConverter, localization-ready)
// Returns localized string value for the Enum
public object Convert(object value, Type targetType, object parameter, string language)
{
var enumValue = (SwipeActionTypes) value;
switch (enumValue)
{
case SwipeActionTypes.Copy:
return App.ResourceLoader.GetString("CopySwipeActionName");
case SwipeActionTypes.Delete:
return App.ResourceLoader.GetString("DeleteSwipeActionName");
default:
throw new ArgumentOutOfRangeException();
}
}
// Parses the localized string into the enum value
public object ConvertBack(object value, Type targetType, object parameter, string language)
{
var stringValue = (string) value;
if (stringValue.Equals(App.ResourceLoader.GetString("CopySwipeActionName")))
{
return SwipeActionTypes.Copy;
}
if (stringValue.Equals(App.ResourceLoader.GetString("DeleteSwipeActionName")))
{
return SwipeActionTypes.Delete;
}
return null;
}
(Converter EnumToStringListConverter)
public object Convert(object value, Type targetType, object parameter, string language)
{
var valueType = value.GetType();
return Enum.GetNames(valueType).ToList();
}
public object ConvertBack(object value, Type targetType, object parameter, string language)
{
return value;
}
Any idea on why this is failing?

The reason you are getting a StackOverflow exception is because every time you change LeftSwipeActionType property you are changing the ItemsSource of the ComboBox which changes the SelectedItem which fires INotifyPropertyChanged which changes the ItemsSource and so on and so on.
Once you stop using the same property for ItemsSource and SelectedItem then the correct initial selection will be set.
Rather than use a converter to create your ItemsSource you should just create is in your ViewModel
public MyViewModel(type enumType)
{
SourceForItems = Enum.GetValues(enumType);
}
public IEnumerable SourceForItems { get; private set; }

First of all, here is whats wrong with your approach:
You are binding your ItemsSource to the same property as the SelectedItem, even tough you are using a converter this can cause an infinite update circle - and you don't want that.
Generating the same static list of elements over and over again seems a bit wasteful. Instead of passing an instance of a type, lets just pass the type itself to the converter:
EnumToMembersConverter.cs
public class EnumToMembersConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, string language)
{
return Enum.GetValues((Type)value).ToList();
}
public object ConvertBack(object value, Type targetType, object parameter, string language)
{
return DependencyProperty.UnsetValue;
}
}
XAML
ItemsSource="{Binding Source={x:Type whateverNamespace:SwipeActionTypes}, Converter={StaticResource EnumToMembersConverter}}"
This will give you all Values of SwipeActionTypes, therefore you can bind it directly, without converting back again.
SelectedItem="{Binding LeftSwipeActionType, Mode=TwoWay}"
There is nothing wrong with using a ComboBox for types other than string, so lets make this your base for further steps:
<ComboBox x:Uid="LeftActionComboBox"
Grid.Row="0"
HorizontalAlignment="Stretch"
SelectedItem="{Binding LeftSwipeActionType, Mode=TwoWay}"
ItemsSource="{Binding Source={x:Type whateverNamespace:SwipeActionTypes}, Converter={StaticResource EnumToMembersConverter}}"/>
The reason you wrote all those converts is probably because the ComboBox showed strange values instead of readable strings. No worries, we already have your converter, you just need to invert it (Convert SwipeActionTypes to String) and apply it to a TextBox:
<ComboBox x:Uid="LeftActionComboBox"
Grid.Row="0"
HorizontalAlignment="Stretch"
SelectedItem="{Binding LeftSwipeActionType, Mode=TwoWay}"
ItemsSource="{Binding Source={x:Type whateverNamespace:SwipeActionTypes}, Converter={StaticResource EnumToMembersConverter}}">
<ComboBox.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding Path=., Converter = {StaticResource SwipeActionTypesStringConverter}}"/>
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>
Note, I didn't run this code so you might need to adjust the used namespaces accordingly

File path to file name String converter not working

Using a wpf ListBox I'm trying to display a list of filename without displaying the full path (more convenient for user).
Data comes from an ObservableCollection which is filled using Dialog.
private ObservableCollection<string> _VidFileDisplay = new ObservableCollection<string>(new[] {""});
public ObservableCollection<string> VidFileDisplay
{
get { return _VidFileDisplay; }
set { _VidFileDisplay = value; }
}
In the end I want to select some items and get back the full file path. For this I have a converter :
public class PathToFilenameConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
//return Path.GetFileName(value.ToString());
string result = null;
if (value != null)
{
var path = value.ToString();
if (string.IsNullOrWhiteSpace(path) == false)
result = Path.GetFileName(path);
}
return result;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
return value;
}
}
Which I bind to my listbox itemsource :
<ListBox x:Name="VideoFileList" Margin="0" Grid.Row="1" Grid.RowSpan="5" Template="{DynamicResource BaseListBoxControlStyle}" ItemContainerStyle="{DynamicResource BaseListBoxItemStyle}" ScrollViewer.HorizontalScrollBarVisibility="Disabled" ItemsSource="{Binding Path=DataContext.VidFileDisplay, Converter={StaticResource PathToFileName},ElementName=Ch_Parameters, Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" SelectedItem="{Binding Path=SelectedVidNames,ElementName=Ch_Parameters, Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}">
Without the converter, it is working fine (but of course this is the full path displayed in the listbox). With the converter I have one character per line... displaying this :
System.Collections.ObjectModel.ObservableCollection`1[System.String]
Where am I wrong ?
Thank you

In ItemsSource binding converter applies to the whole list and not to each item in the collection. If you want to apply your converter per item you need to do it ItemTemplate
<ListBox x:Name="VideoFileList" ItemsSource="{Binding Path=DataContext.VidFileDisplay, ElementName=Ch_Parameters}" ...>
<ListBox.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding Path=., Converter={StaticResource PathToFileName}}"/>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>

How to data-bind to a property of a composite object in a collection?

I need to allow selection of several items from this predefined list:
public enum QuarkType{
Up,
Down,
[Description("Magical Being")] Charm,
[Description("Quite Odd")] Strange,
Top,
Bottom
}
So I use CheckComboBox, and use the DescriptionAttribute where I need to use custom description. I feed the CheckComboBox using a MarkupExtension that returns a list of all values of the given enum as IEnumerable<EnumDescriptionPair>, where EnumDescriptionPair is:
public class EnumDescriptionPair{
public object Value { get; set; }
public string Description { get; set; }
}
Now the problem is how to pass the Values of this list to the code-behind list:
public ObservableCollection<QuarkType> SelectedQuarksList { get; set; }
I mean, how to take just the Value out of the EnumDescriptionPair for each item of the selected list ?
This is what I have thus far. It obviously doesn't work (meaning it shows the right strings in the CheckComboBox, and allows selecting several items, but isn't reflected in the SelectedQuarksList mentioned above):
<Window x:Class="MyEditor.MainWindow"
xmlns:loc="clr-namespace:MyEditor"
xmlns:toolKit="clr-namespace:Xceed.Wpf.Toolkit;assembly=Xceed.Wpf.Toolkit"
DataContext="{Binding RelativeSource={RelativeSource Self}}">
<StackPanel>
<toolKit:CheckComboBox x:Name="Ccb" Delimiter=","
ItemsSource="{loc:EnumItemsValueConverter {x:Type loc:QuarkType}}"
DisplayMemberPath="Description"
SelectedItemsOverride="{Binding SelectedQuarksList}" />
<ListBox ItemsSource="{Binding SelectedQuarksList}" />
</StackPanel>
</Window>

To do exactly what your question asks, you could try using a converter on the SelectedQuarksList binding that does a ".Select(q => q.Value)" in the ConvertBack function.
To get the behavior you want, I have done this successfully in the past (example with 2 of your values), this sets up the enum as "Flags" so the value sequence goes 0, 1, 2, 4...:
<StackPanel Orientation="Horizontal">
<Checkbox Content="Up" IsChecked="{Binding Path=SelectedQuarksFlags, Converter={Static Resource HasFlagToBoolConverter}, ConverterParamater={x:Static Quarks.Up}}"
<Checkbox Content="Magical Being" IsChecked="{Binding Path=SelectedQuarksFlags, Converter={Static Resource HasFlagToBoolConverter}, ConverterParamater={x:Static Quarks.Charm}}"
</StackPanel>
The converter looks like:
Quark _lastSeenValue;
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
Quark paramQuark = (Quark)parameter;
Quark currentQuark = (Quark)value;
_lastSeenValue = currentQuark;
return currentQuark.HasFlag(paramQuark);
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
Quark newQuark = _lastSeenValue;
Quark paramQuark = (Quark)parameter;
if ((bool)value)
{
newQuark |= paramQuark;
}
else
{
newQuark &= ~paramQuark;
}
_lastSeenValue = newQuark;
return newQuark;
}
This could be converted to add or remove from a list relatively easily, but I know the code above works.

How to pass a static value to IValueConverter in XAML

I would like to use static texts fetched from a web service in my WP7 app. Each text has a Name (the indetifier) and a Content property.
For example a text could look like this:
Name = "M43";
Content = "This is the text to be shown";
I would then like to pass the Name (i.e. the identifier) of the text to an IValueConverter, which would then look up the the Name and return the text.
I figured the converter to look something like this:
public class StaticTextConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if (value != null)
{
return App.StaticTexts.Items.SingleOrDefault(t => t.Name.Equals(value)).Content;
}
return null;
}
}
Then in the XAML:
<phone:PhoneApplicationPage.Resources>
<Helpers:StaticTextConverter x:Name="StaticTextConverter" />
</phone:PhoneApplicationPage.Resources>
...
<TextBlock Text="{Binding 'M43', Converter={StaticResource StaticTextConverter}}"/>
However, this does not seem to work and I am not sure that I pass in the value to the converter correctly.
Does anyone have some suggestions?

I finally found the answer. The answer was a mix between that of #Shawn Kendrot and another question I asked here: IValueConverter not getting invoked in some scenarios
To summarize the solution for using the IValueConverter I have to bind my control in the following manor:
<phone:PhoneApplicationPage.Resources>
<Helpers:StaticTextConverter x:Name="TextConverter" />
</phone:PhoneApplicationPage.Resources>
<TextBlock Text="{Binding Converter={StaticResource TextConverter}, ConverterParameter=M62}" />
Since the ID of the text is passed in with the converter parameter, the converter looks almost the same:
public class StaticTextConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if (parameter != null && parameter is string)
{
return App.StaticTexts.Items.SingleOrDefault(t => t.Name.Equals(parameter)).Content;
}
return null;
}
}
However, as it turns out, the binding and thus the converter is not invoked if it does not have a DataContext. To solve this, the DataContext property of the control just has to be set to something arbitrary:
<TextBlock DataContext="arbitrary"
Text="{Binding Converter={StaticResource TextConverter}, ConverterParameter=M62}" />
And then everything works as intended!

The problem lies in your binding. It will check the DataContext, and on this object, it will try to evaluate the properties M62 and ValueboxConsent on that object.
You might want to add static keys somewhere in your application where you can bind to:
<TextBlock Text="{Binding Source="{x:Static M62.ValueboxConsent}", Converter={StaticResource StaticTextConverter}}" />
Where M62 is a static class where your keys are located.. like so:
public static class M62
{
public static string ValueboxConsent
{
get { return "myValueBoxConsentKey"; }
}
}

If you want to use a value converter, you'll need to pass the string to the parameter of value converter
Xaml:
<TextBlock Text="{Binding Converter={StaticResource StaticTextConverter}, ConverterParameter=M43}"/>
Converter:
public class StaticTextConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if (parameter != null)
{
return App.StaticTexts.Items.SingleOrDefault(t => t.Name.Equals(parameter)).Content;
}
return null;
}
}

xmlns:prop="clr-namespace:MyProj.Properties;assembly=namespace:MyProj"
<TextBlock Text="{Binding Source={x:Static prop:Resources.MyString}, Converter={StaticResource StringToUpperCaseConverter}}" />

Where to set the Converter for items of a collection in XAML

I just made my first converter to convert from int to string. I have a combobox fill with integers(years) but if the value is 0 I want the combobox to show 'All'.
This is my converter:
public class IntToString : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if (value != null)
{
int intY = (int)value;
if (intY == 0)
{
String strY = "All";
return strY;
}
else
{
return intY.ToString();
}
}
return String.Empty;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
}
}
In XAML where should I set the converter ? I tried in the ItemsSource of the combobox:
ItemsSource="{Binding YearsCollection, Converter={StaticResource intToStringYearConverter}}"
But I always get InvalidcastException on this line:
int intY = (int)value;

The problem is that you are trying to convert the entire collection rather than just one item from the collection.
You would want to do something like this:
<ListBox ItemsSource="{Binding YearsCollection}">
<ListBox.ItemTemplate>
<DataTemplate>
<Border DataContext="{Binding Converter={StaticResource intToStringYearConverter}">
...
</Border>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>

You can't use the converter like this, converter in ItemsSource is supposed to convert whole collection, not individual items. The collection object can't be cast to integer, so you get the exception.
You have to use DataTemplate and apply the converter on individual items.
Or - if all you need is cast to int - you could use ItemStringFormat.
Also, for setting the default message when the source is null, you can use TargetNullValue property of a Binding.