How do I implement the CollectionLengthToVisibility converter?
I want to implement a converter so that some XAML elements show / disappear only if ObservableCollection
there are elements in there.
I have referenced How to access a shared property without knowing the private generic type , but cannot get it to work with my implementation. It builds and deploys OK (for emulator and Windows Phone 7 device) but doesn't work. Additionally, Blend will throw an exception and will no longer render the page,
NullReferenceException: Object reference not set to instance object.
That's what I have so far,
// Sets the vsibility depending on whether the collection is empty or not depending if parameter is "VisibleOnEmpty" or "CollapsedOnEmpty"
public class CollectionLengthToVisibility : System.Windows.Data.IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo cultureInfo)
{
// From /questions/349808/how-to-access-generic-property-without-knowing-the-closed-generic-type
var p = value.GetType().GetProperty("Length");
int? length = p.GetValue(value, new object[] { }) as int?;
string s = (string)parameter;
if ( ((length == 0) && (s == "VisibleOnEmpty"))
|| ((length != 0) && (s == "CollapsedOnEmpty")) )
{
return Visibility.Visible;
}
else
{
return Visibility.Collapsed;
}
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo cultureInfo)
{
return null;
}
}
This is how I referenced the Blend / XAML converter
<TextBlock Visibility="{Binding QuickProfiles, ConverterParameter=CollapsedOnEmpty, Converter={StaticResource CollectionLengthToVisibility}}">Some Text</TextBlock>
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I would use an extension method Enumerable.Any()
. It will work for anyone IEnumerable<T>
and will let you know which collection you mean. Since you don't know T
, you can simply use.Cast<object>()
public class CollectionLengthToVisibility : System.Windows.Data.IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo cultureInfo)
{
var collection = value as System.Collections.IEnumerable;
if (collection == null)
throw new ArgumentException("value");
if (collection.Cast<object>().Any())
return Visibility.Visible;
else
return Visibility.Collapsed;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo cultureInfo)
{
throw new NotImplementedException();
}
}
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