Binding To Single List Item

Kevin Marois

I have a list of booleans, and I want to bind checkboxes to this list.\ Code Behind

private List _OnOffValues;
public List OnOffValues
{
get { return _OnOffValues; }
set
{
if (_OnOffValues != value)
{
_OnOffValues = value;
RaisePropertyChanged("OnOffValues");
}
}
}
public MyControl()
{
InitializeComponent();
this.DataContext = this;
OnOffValues = new List();
for (int x = 0; x < 24; x++)
{
OnOffValues.Add(true);
}
}

XAML

The output window shows

System.Windows.Data Error: 17 : Cannot get 'Item[]' value (type 'Boolean') from 'OnOffValues' (type 'List`1'). BindingExpression:Path=OnOffValues[0]; DataItem='MyControl' (Name='control'); target element is 'CheckBox' (Name=''); target property is 'IsChecked' (type 'Nullable`1') ArgumentOutOfRangeException:'System.ArgumentOutOfRangeException: Specified argument was out of the range of valid values.
Parameter name: index'

The code in the contructor runs and the list is populated, but the output message appears BEFORE that. What am I doing wrong here??

If it's not broken, fix it until it is. Everything makes sense in someone's mind. Ya can't fix stupid.

Meshack Musundi

Your binding assumes that OnOffValues is a dependency property of the control.

"As beings of finite lifespan, our contributions to the sum of human knowledge is one of the greatest endeavors we can undertake and one of the defining characteristics of humanity itself"

Pete OHanlon

If you check the timing of what's happening, the bind is happening as the control is being created - it happens in the InitializeComponent call in the user control. What you could do here is supply a FallbackValue so that the binding is satisfied until the point that your ViewModel is created.

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Pete OHanlon

There's nothing there that says it's expecting a DP.

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Meshack Musundi

Look at his binding, IsChecked="{Binding OnOffValues[0], ElementName=control}" and the property, OnOffValues, is in the code behind of his user control. If he changes the property to a dependency property he won't get the error.

"As beings of finite lifespan, our contributions to the sum of human knowledge is one of the greatest endeavors we can undertake and one of the defining characteristics of humanity itself"

Meshack Musundi

Something like this, Code behind

public partial class SomeControl : UserControl
{
public SomeControl()
{
InitializeComponent();
for (int i = 0; i < 20; i++)
{
OnOffValues.Add(true);
}
}

public List<bool> OnOffValues
{
    get { return (List<bool>)GetValue(OnOffValuesProperty); }
    set { SetValue(OnOffValuesProperty, value); }
}

public static readonly DependencyProperty OnOffValuesProperty =
    DependencyProperty.Register("OnOffValues", typeof(List<bool>),
        typeof(SomeControl), new PropertyMetadata(new List<bool>()));

}

XAML

"As beings of finite lifespan, our contributions to the sum of human knowledge is one of the greatest endeavors we can undertake and one of the defining characteristics of humanity itself"

Pete OHanlon

He's set the UserControl as its own DataContext. He doesn't need to create a DP if he's set the UC to implement INotifyPropertyChanged (which the property change notification suggests).

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Pete OHanlon

As explained below, it looks like he's using INPC so this is completely irrelevant.

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Meshack Musundi

Quite true, I had taken note of that. I only supposed that the more apt approach would be to use a dependency property.

"As beings of finite lifespan, our contributions to the sum of human knowledge is one of the greatest endeavors we can undertake and one of the defining characteristics of humanity itself"

Pete OHanlon

Not when he already set the DataContext. The issue was purely a timing issue and could be fixed with a FallbackValue.

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Lost User

Changing / setting the DataContext invokes the binding mechanism.

public MyControl()
{
OnOffValues = new List();
for (int x = 0; x < 24; x++)
{
OnOffValues.Add(true);
}

InitializeComponent();
this.DataContext = this;

}

And there is no "ElementName" in play here.

"(I) am amazed to see myself here rather than there ... now rather than then". ― Blaise Pascal