Question

Trying to do something like this…

public class MiniObject
{
    public bool Checked { get; set; }
    public String Caption { get; set; }

    public ICollection<MiniObject> Content { get; set;}
}


<Window.Resources>
    <local:MiniObject x:Key="RootItem" Caption="Item0" Checked="True">
        <local:MiniObject.Content>
            <local:MiniObject Caption="Item1" Checked="True">
            </local:MiniObject>
            <local:MiniObject Caption="Item2" Checked="True">
            </local:MiniObject>
        </local:MiniObject.Content>
    </local:MiniObject>
</Window.Resources>

This of course doesn’t work returning the error:

Object of type 'WorkflowTest.MiniObject' cannot be converted to type 'System.Collections.Generic.ICollection`1[WorkflowTest.MiniObject]'.

Is there a way to do this within WPF? If so do I need to change the shape of my objects at all or can I simply provide a specialized object that only WPF uses like…

<Window.Resources>
    <local:MiniObject x:Key="RootItem" Caption="Item0" Checked="True">
        <local:MiniObject.Content>
            <local:MiniObjectCollection>
                <local:MiniObject Caption="Item1" Checked="True">
                </local:MiniObject>
                <local:MiniObject Caption="Item2" Checked="True">
                </local:MiniObject>
            </local:MiniObjectCollection>
        </local:MiniObject.Content>
    </local:MiniObject>
</Window.Resources>

Answer 1

You’re trying to use XAML’s implicit collection syntax. In order to do this, the property (Content, in this case) must be of a type that implements ICollection. Note: not ICollection, but a type that implements ICollection.

You can’t just use an interface because the XamlReader needs to know what type of object to create. If you haven’t told it the type, how should it decide? By searching through all of the types available to your assembly, finding the ones that implement ICollection<MiniObject>, discarding the ones that don’t have a parameterless constructor, and then choosing one at random? No.

When you define Content as List<MiniObject>, the XamlReader knows what type of object it should create. Since that’s a type that implements ICollection, it can use the implicit collection syntax. So it just creates the object and calls Add to add the child items, and if you stuck a child item in there that isn’t a MiniObject you’ll get a runtime error.

You say that “I need to avoid using an actual implementation” in your Content property. In that case, you cannot use the implicit collection syntax. You will need to do what you do in your second example: explicitly define a type that implements ICollection<MiniObject>, and add a child element in your XAML to create it explicitly.