Question

I have an control that is subclassed from ItemsControl, called WorkSheet:

public sealed class WorkSheet : ItemsControl {

Its elements are forced to be WorkTiles:

    /// <inheritdoc />
    protected override bool IsItemItsOwnContainerOverride(object item) {
        return (item is WorkTile);
    }

    /// <inheritdoc />
    protected override DependencyObject GetContainerForItemOverride() {
        return new WorkTile();
    }

So far – so good. I want the WorkSheet to use a Canvas for presenting the WorkItems, position being determined by overriing ArrangeOverride, which is called and positions properly determined. The exact psosition is being determined in overrides. I am pretty lost in the styling, though. I simply can not get the items to appear. In The Generic.xaml, I have defined the styles. They work, but not as they should:

<Style TargetType="{x:Type local:WorkSheet}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate>
                <Border>
                    <ItemsPresenter />
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
    <Setter Property="ItemsPanel">
        <Setter.Value>
            <ItemsPanelTemplate>
                <Canvas />
            </ItemsPanelTemplate>
        </Setter.Value>
    </Setter>
    <Setter Property="ItemTemplate">
        <Setter.Value>
            <DataTemplate>
                <ContentPresenter />
            </DataTemplate>
        </Setter.Value>
    </Setter>
</Style>

Here,pretty much, in the presentation tree, the ItemsPresenter is the lowest element. The subelements of ItemsPresenter never appear. I also tried putting a Canvas into the ControlTemplate with IsItemsHost="true" – again, no items. Anyone any idea what I am doing wrong here?

Again, as explanation – I put a breakpoint into a button handler on the form and use the Visualizer to see the visual tree. The hierarchy I get is:

WorkSheet -> Border -> ItemsPresenter … and nothing below.

Obviously this means the ControlTemplate is used, but the ItemsPanel is never invoked.

Answer 1

Place a TargetType in your ControlTemplate as well:

<ControlTemplate TargetType="{x:Type local:WorkSheet}">

Does not work 😉

Update 2:

I replicated your things in a side project and you have two problems:

First problem is the ItemTemplate setter in your Style which will trigger a StackOverflow exception (how ironic ;)). Remove the ContentPresenter, remove the whole template or use keys.

Second problem is the GetContainerForItemOverride method. Removing this method will give me stuff on screen!

Here is my code:

public sealed class WorkSheet : ItemsControl
{
    /// <inheritdoc />
    protected override bool IsItemItsOwnContainerOverride(object item)
    {
        return (item is WorkTile);
    }

}

And the xaml of Window with the Style:

<Window x:Class="WpfApplication8.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:WpfApplication8="clr-namespace:WpfApplication8"
    Title="Window1" Height="300" Width="300">

    <Window.Resources>
        <Style TargetType="{x:Type WpfApplication8:WorkSheet}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate >
                        <Border>
                            <ItemsPresenter />
                        </Border>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
            <Setter Property="ItemsPanel">
                <Setter.Value>
                    <ItemsPanelTemplate>
                        <Canvas />
                    </ItemsPanelTemplate>
                </Setter.Value>
            </Setter>
        </Style>

    </Window.Resources>

    <Grid>
        <WpfApplication8:WorkSheet x:Name="sheet" />
    </Grid>
</Window>