Getting NullReferenceException in a ValueConverter - xamarin.forms

Question

I have the following IValueConverter, StringCaseConverter.cs:
internal sealed class StringCaseConverter : IValueConverter
{
public bool IsUpperCase { private get; set; }
public object Convert(object value, Type targetType, object parameter, CultureInfo language)
{
var stringValue = value.ToString();
return this.IsUpperCase ? stringValue.ToUpperInvariant() : stringValue.ToLowerInvariant();
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo language)
{
return value;
}
}
I registered it in App.xaml:
<Application.Resources>
<ResourceDictionary>
<converters:StringCaseConverter x:Key="StringToLowerCaseConverter" IsUpperCase="False"/>
<converters:StringCaseConverter x:Key="StringToUpperCaseConverter" IsUpperCase="True"/>
</ResourceDictionary>
</Application.Resources>
And used it in MainPage.xaml:
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage
x:Class="App1.MainPage"
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml">
<StackLayout Orientation="Horizontal" HorizontalOptions="Center">
<CollectionView HorizontalOptions="Center">
<CollectionView.ItemsSource>
<x:Array Type="{x:Type x:String}">
<x:String>A</x:String>
<x:String>B</x:String>
<x:String>C</x:String>
<x:String>D</x:String>
<x:String>E</x:String>
<x:String>F</x:String>
<x:String>G</x:String>
<x:String>H</x:String>
</x:Array>
</CollectionView.ItemsSource>
<CollectionView.ItemsLayout>
<LinearItemsLayout Orientation="Horizontal" ItemSpacing="20"/>
</CollectionView.ItemsLayout>
<CollectionView.ItemTemplate>
<DataTemplate>
<StackLayout Orientation="Horizontal" HorizontalOptions="End" VerticalOptions="Center">
<Label Text="{Binding ., Converter={StaticResource StringToLowerCaseConverter}}" HorizontalOptions="Center"/>
</StackLayout>
</DataTemplate>
</CollectionView.ItemTemplate>
</CollectionView>
</StackLayout>
</ContentPage>
When I run the application, I get a NullReferenceExecption in StringCaseConverter.cs (var stringValue = value.ToString();) because value parameter of Convert() method is null.
What am I missing?

Answer 1

You just need to make a null value judgment.
public object Convert(object value, Type targetType, object parameter, CultureInfo language)
{
if (string.IsNullOrEmpty((string)value))
{
return null;
}
var stringValue = value.ToString();
return this.IsUpperCase ? stringValue.ToUpperInvariant() : stringValue.ToLowerInvariant();
}