Why do I get a "Specified Cast is not Valid" when using a custom view with a binding but not when I set manually?

Question

I'm building a custom view using an ObservableCollection as a BindableProperty (my goal was to update the format of the view depending its contents, in this case a list of colors). I think I have the property set up properly in the custom view. If I set the bindable property in C#, it works. If I try to use a {Binding varname} inside XAML, it throws a "Specified Cast is not Valid" error. I'm trying to figure out where my issue is.

Below in public MainPage() you can see the commented out line where I manually set the property. If I uncomment this and take "Color={Binding Test}" out of MainPage.xaml, everything works. If I run this as is, I get the exception.

Here is all my code:

MainPage.xaml.cs

public partial class MainPage : ContentPage
{
    public ObservableCollection<Color> Test { get; set; }
    public MainPage()
    {
        Test = new ObservableCollection<Color>();

        Test.Add(Color.Blue);
        Test.Add(Color.Green);
        Test.Add(Color.Red);
        Test.Add(Color.Purple);
        
        this.BindingContext = this;
        InitializeComponent();

        //myView.Colors = Test;
    }

    

    private void OnClicked(object sender, EventArgs e)
    {
        myView.Colors.Add(Color.Yellow);
    }
}

MainPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         xmlns:local="clr-namespace:ColorBoxes"
         x:Class="ColorBoxes.MainPage">

    <StackLayout HorizontalOptions="CenterAndExpand">
        <local:ColorBoxView  x:Name="myView" Colors="{Binding Test}" WidthRequest="400" HeightRequest="600"/>
        <Button Text="Add Color" Clicked="OnClicked"/>
    </StackLayout>

</ContentPage>

ColorBoxView.xaml.cs

public partial class ColorBoxView : ContentView
{
    public ColorBoxView()
    {
       InitializeComponent();
       
    }
    
    public static BindableProperty ColorList = BindableProperty.Create(
        propertyName: "Colors",
        returnType: typeof(ObservableCollection<Color>),
        declaringType: typeof(ColorBoxView),
        defaultValue: new ObservableCollection<Color>(),
        defaultBindingMode: BindingMode.OneWay,
        propertyChanged: HandleColorListPropertyChanged
    );

    public ObservableCollection<Color> Colors
    {
        get { return (ObservableCollection<Color>) base.GetValue(ColorList); }
        set
        {
            if (value != this.Colors)
            {
                base.SetValue(ColorList, value);
                Colors.CollectionChanged += Items_CollectionChanged;
            }
        }
    }
   
    
    void Items_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
    {
        buildView(Colors);
    }

    private void buildView(ObservableCollection<Color> newColors)
    {
        /* This code is never reached */
    }
    private static void HandleColorListPropertyChanged(BindableObject bindable, object oldValue, object newValue)
    {
        ObservableCollection<Color> newColors = (ObservableCollection<Color>) newValue;
        ColorBoxView thisView = (ColorBoxView) bindable;

        thisView.buildView(newColors);
        
    }

ColorBoxView.xaml

<?xml version="1.0" encoding="utf-8"?>

<ContentView xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         x:Class="ColorBoxes.ColorBoxView" >

    <Grid  x:Name="BoxGrid"  HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand" 
           RowSpacing="0" ColumnSpacing="0" Padding="0">
    </Grid>

</ContentView>

Stack Trace:

System.InvalidCastException: Specified cast is not valid. 
   at ColorBoxes.MainPage.InitializeComponent () [0x00023] in /Users/docmani/RiderProjects/ColorBoxes/ColorBoxes/ColorBoxes/obj/Debug/netstandard2.0/MainPage.xaml.g.cs:26
   at ColorBoxes.MainPage..ctor () [0x00060] in /Users/docmani/RiderProjects/ColorBoxes/ColorBoxes/ColorBoxes/MainPage.xaml.cs:25
   at ColorBoxes.App..ctor () [0x0000f] in /Users/docmani/RiderProjects/ColorBoxes/ColorBoxes/ColorBoxes/App.xaml.cs:15
   at ColorBoxes.iOS.AppDelegate.FinishedLaunching (UIKit.UIApplication app, Foundation.NSDictionary options) [0x00007] in /Users/docmani/RiderProjects/ColorBoxes/ColorBoxes/ColorBoxes.iOS/AppDelegate.cs:25
   at at (wrapper managed-to-native) UIKit.UIApplication.UIApplicationMain(int,string[],intptr,intptr)
   at UIKit.UIApplication.Main (System.String[] args, System.IntPtr principal, System.IntPtr delegate) [0x00005] in /Library/Frameworks/Xamarin.iOS.framework/Versions/13.18.2.1/src/Xamarin.iOS/UIKit/UIApplication.cs:86
   at UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) [0x0000e] in /Library/Frameworks/Xamarin.iOS.framework/Versions/13.18.2.1/src/Xamarin.iOS/UIKit/UIApplication.cs:65
   at ColorBoxes.iOS.Application.Main (System.String[] args) [0x00001] in /Users/docmani/RiderProjects/ColorBoxes/ColorBoxes/ColorBoxes.iOS/Main.cs:16

Additional Info - I tried upgrading Xamarin.Forms in NuGet since it had been a while. Now it won't even compile with the binding. It gives me:

 MainPage.xaml(9, 46): [XFC0009] No property, BindableProperty, or event found for "Colors", or mismatching type between value and property.

But again, if I manually set the value in C# it still works. I'm definitely missing something in my XAML but I don't know what it is.

Leon Lu - MSFT Leon Lu - MSFT · Accepted Answer · 2020-08-17T03:16:17

You can change the BindableProperty like following ColorListProperty, then make a test in the xaml.

public partial class ColorBoxView : ContentView
    {
        public ColorBoxView ()
        {
            InitializeComponent ();
        }
 
        public static readonly BindableProperty ColorListProperty = BindableProperty.Create(
            nameof(ColorList), 
            typeof(ObservableCollection<object>), 
            typeof(ColorBoxView), 
            new ObservableCollection<object>(), 
            BindingMode.OneWay, 
            propertyChanged: ContractListPropertyChangedDelegate);
        public ObservableCollection<object> ColorList
        {
            get =>
            (ObservableCollection<object>)GetValue(ColorListProperty);
            set => SetValue(ColorListProperty, value);
        }
        private static void ContractListPropertyChangedDelegate(BindableObject bindable, object oldValue, object newValue)
        {
            // throw new NotImplementedException();
         //   ColorBoxView colorBoxView= bindable as ColorBoxView;


        }

 <local:ColorBoxView  x:Name="myView" ColorList="{Binding Test}"  WidthRequest="400" HeightRequest="600"/>

If you want to CollectionChanged event to be executed, when you click the Button, you can add the ColorList.CollectionChanged += ColorList_CollectionChanged in ColorBoxView's constructor.

public ColorBoxView ()
        {
            InitializeComponent ();
            ColorList.CollectionChanged += ColorList_CollectionChanged;

        }

        private void ColorList_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
        {
           // throw new NotImplementedException();
        }

Your set method will be executed, when you set a new ObservableCollection, If you just add the item to existed ObservableCollection, use CollectionChanged in the ColorBoxView maybe better.

Why do I get a "Specified Cast is not Valid" when using a custom view with a binding but not when I set manually?

2 Answers