Question

Experts,

In XAML I would like to create a many-to-many relationship between entities.

Basically I would like for multiple “Manager” objects to be able to manage multiple “Items”. The following XAML should describe what I’m looking for:

<Grid>
    <Grid.Resources>
        <cc:Manager x:Key="Manager1"/>
        <cc:Manager x:Key="Manager2"/>
    </Grid.Resources>
    <cc:Item>
        <cc.Manager.ManagedBy>
            <StaticResource ResourceKey="Manager1" />
        </cc.Manager.ManagedBy>
    </cc:Item>
    <cc:Item>
        <cc.Manager.ManagedBy>
            <StaticResource ResourceKey="Manager1" />
            <StaticResource ResourceKey="Manager2" /> <!-- ERROR HERE -->
        </cc.Manager.ManagedBy>
    </cc:Item>
    <cc:Item>
        <cc.Manager.ManagedBy>
            <StaticResource ResourceKey="Manager2" />
        </cc.Manager.ManagedBy>
    </cc:Item>
</Grid>

The attached property (Manager.ManagedBy) is of type ManagedByCollection…

ManagedByCollection : List<ManageBy>

With this I get the following error message:

The object 'Object' already has a child and cannot add 'StaticResourceExtension'. 'Object' can accept only one child. Line NN Position NN.

So, I wen’t back to MSDN and realized there’s a ContentPropertyAttribute to tell the XAML compiler what property is the default one when nothing else is specified. The LinearGradientBrush, for example, uses that attribute to enable us to write just …

<LinearGradientBrush ... >
   <GradientStop ... />
   <GradientStop ... />
   <GradientStop ... />
</LinearGradientBrush>

… instead of …

<LinearGradientBrush ... >
   <GradientStopCollection>
       <GradientStop ... />
       <GradientStop ... />
       <GradientStop ... />
   </GradientStopCollection>
</LinearGradientBrush>

So, I was thinking I just needed to specify the indexer of ManagedByCollection as the class’ ContentProperty:

[ContentProperty("Item")
ManagerCollection : List<Manager>

Unfortunately, this does not resolve the issue. Currently the following works…

<cc.Manager.ManagedBy>
    <ManagerCollection>
        <StaticResource ResourceKey="Manager1" />
        <StaticResource ResourceKey="Manager2" />
    <cc:ManagerCollection>
</cc.Manager.ManagedBy>

… but, again, I would prefer the more readble syntax:

<cc.Manager.ManagedBy>
    <StaticResource ResourceKey="Manager1" />
    <StaticResource ResourceKey="Manager2" />
</cc.Manager.ManagedBy>

Any help or hints would be appreciated.

Answer 1

You can initialize the collection explicitly in the constructor of Item:

public Item()
{
    Manager.SetManagedBy(this, new ManagedByCollection());
}