XAML绑定到代码隐藏类属性集合为空（WPF）

Question

My codebehind defines a simple class with properties and a constructor as so:

public class Question
{
    public string[] Answers
    {
        get; set;
    }
    public int CorrectAnswerIndex
    {
        get; set;
    }
    public Question(string[] answers, int correctIndex)
    {
        this.Answers = answers;
        this.CorrectAnswerIndex = correctIndex;
    }
}

There then exists a public object of that type that gets initialised in the window's constructor like so:

 CurrentQuestion = new Question(
     new string[] { "First", "Second", "Third", "Fourth" }, 2
 );

I then have the following XAML in an attempt to print out all of the possible answers from said question.

<Grid Margin="150,150,150,150" DataContext="local:CurrentQuestion">
        <ListBox ItemsSource="{Binding Answers}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel Orientation="Horizontal">
                        <TextBlock Text="{Binding}" />
                    </StackPanel>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
</Grid>

The local namespace is defined previously as the CLR namespace.

However, my list emerges entirely empty. There are no binding errors at runtime.

What's going on here? It seems a simple example that just won't run. I feel I've missed something "obvious."

Answer 1

This will look at ListBox.DataContext for a property named Answers , and try to use that for the ItemsSource .

<ListBox ItemsSource="{Binding Answers}">

ListBox.DataContext will be inherited from the parent Grid . Unfortunately, the grid's DataContext is a string, and strings don't have a property called Answers . So the Binding can't do anything and gives you null .

<Grid Margin="150,150,150,150" DataContext="local:CurrentQuestion">
    <ListBox ItemsSource="{Binding Answers}">

XAML implicit conversions are a Do-What-I-Mean thing, thus a source of much confusion. There are times when you can put local:CurrentQuestion in an attribute value and have it be taken as a data type -- but this is not one of those times. And a data type isn't what you meant to provide anyway. You wanted a property by that name. But local: is a namespace, a literal CLR namespace like System.Windows.Controls , not a reference to an object.

Here's how the XAML in a UserControl can bind to a property of the UserControl . If it's a Window , change UserControl to Window .

<Grid Margin="150,150,150,150">
    <ListBox 
        ItemsSource="{Binding CurrentQuestion.Answers, RelativeSource={RelativeSource AncestorType=UserControl}}">

I'm just guessing that CurrentQuestion is a property of the UserControl . Let me know if it's somewhere else.

You're also probably going to run into problems when you update CurrentQuestion , unless it's a dependency property. If it's a plain old CLR property like this, the UI won't be notified when its value changes:

public Question CurrentQuestion { get; set; }