Must have non-null value for 'Binding' problem

Question

I have made a custom Control in WPF which is a star rating. When there are 5 stars, these should be golden.

public partial class RatingControl
{
   private const int Max = 5;

   public static readonly DependencyProperty RatingProperty = DependencyProperty.Register(nameof(Rating),
       typeof(int),
       typeof(RatingControl),
       new FrameworkPropertyMetadata(0, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
           RatingChanged));

   public int Rating
   {
      get => (int)GetValue(RatingProperty);
      set
      {
         if (value < 0)
         {
            SetValue(RatingProperty, 0);
         }
         else if (value > Max)
         {
            SetValue(RatingProperty, Max);
         }
         else
         {
            SetValue(RatingProperty, value);
         }
      }
   }

   public RatingControl()
   {
      InitializeComponent();
   }

   private static void RatingChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
   {
      var item = sender as RatingControl;
      var newVal = (int)e.NewValue;
      var children = ((Grid)(item.Content)).Children;

      ToggleButton button;

      for (var i = 0; i < newVal; i++)
      {
         button = children[i] as ToggleButton;
         if (button != null)
            button.IsChecked = true;
      }

      for (var i = newVal; i < children.Count; i++)
      {
         button = children[i] as ToggleButton;
         if (button != null)
            button.IsChecked = false;
      }
   }

   private void ClickEventHandler(object sender, RoutedEventArgs args)
   {
      var button = sender as ToggleButton;

      if (button == null)
      {
         return;
      }

      var newValue = int.Parse(button.Tag.ToString());
      Rating = newValue;
   }
}

If I run this, I get this exception:

InvalidOperationException: Must have non-null value for 'Binding'.

Do I bind something wrong inside my condition? Because if I remove the Rating-Binding, it works, but I don't see my mistake here.

thatguy · Accepted Answer

There are multiple issues in your Condition bindings:

The first binding in the MultiDataTrigger uses a Condition that sets Property, which is wrong.

For a MultiDataTrigger, each condition in the collection must set both the Binding and Value properties. For more information, see Binding.

Set the Binding property using a RelativeSource binding to Self instead.
The second binding to Rating is faulty, since it would only ever set any star to be golden and opaque if the rating is exactly five stars. So no stars are selected for any other rating.

In order resolve these issues, you can use a simple Trigger on the IsChecked property.

If it is really your requirement to only make the stars golden and opaque if the rating is exactly 5, then you could correct the MultiDataTrigger as stated above:

Another note on value coercion. As @Clemens stated in the comments, setting a property in XAML e.g. like below will bypass your setter and call SetValue directly. Your setter should only call SetValue as the behavior will differ otherwise when setting properties in XAML or through the property.

Instead of the checks in the setter, you can specify a value coercion callback in the dependency property declaration.

public static readonly DependencyProperty RatingProperty = DependencyProperty.Register(
   nameof(Rating), typeof(int), typeof(RatingControl), new FrameworkPropertyMetadata(
      0, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, RatingChanged, CoerceRating));

Then create a CoerceRating method that contains your checks and returns the corresponding values.

private static object CoerceRating(DependencyObject d, object baseValue)
{
   var value = (int)baseValue;

   if (value < 0)
   {
      return 0;
   }

   if (value > Max)
   {
      return Max;
   }

   return value;
}

Finally, remove all checks from the setter of Rating.

public int Rating
{
   get => (int)GetValue(RatingProperty);
   set => SetValue(RatingProperty, value);
}

The value coercion callback will automatically be called, when the property is set through SetValue and therefore ensure that the Rating value is within the valid interval.

Must have non-null value for 'Binding' problem

Answers (1)

Related Questions

Must have non-null value for &#39;Binding&#39; problem

Answers (1)

Related Questions

Must have non-null value for 'Binding' problem