绑定到Datagrid的ItemsSource集合属性，而不是单个项目

Question

I'm confused while trying to bind some properties housed inside a collection rather than the properties of the elements. I'm not even sure how to phrase it right... code might explain better: here are the types (not actual code, I've shortened it to the basics):

public class myType
{
    public int P {get;set;}
}
public class myTypeCollection : ObservableCollection<myType>
{
    public int Ptotal {get { return this.Items.Select(i=>i.P).Aggregate((c,t)=>t = t + c); }}
    public int Pmin { get { this.Items.Min(i => i.P); } } //concept
    public int Pmax { get { this.Items.Max(i => i.P); } } //concept
}

They're being used in a templated control, whose XAML looks like this: (adding comments to make it as clear as i'm able to)

<!-- myGridObject = new myTemplatedControl(); -->
<!-- myGridObject.DataContext = new myTypeCollection(); -->
<!-- NOTE: collection obviously is NOT empty in the real code -->
<sdk:DataGrid ItemsSource={Binding DataContext}> 
    <sdk:DataGridTemplateColumn Width="Auto">
        <sdk:DataGridTemplateColumn.HeaderStyle>
            <Style TargetType="sdk:DataGridColumnHeader">
                <Setter Property="ContentTemplate">
                    <Setter.Value>
                        <DataTemplate>

                            <!-- ?????? write out Ptotal in the header of the column ??????? -->
                            <!-- This throws a binding-related ArgumentException -->
                            <TextBlock Text="{Binding ???? Ptotal ?????}" />

    <!-- closing tags cut off -->
    <sdk:DataGridTemplateColumn.CellTemplate>
        <DataTemplate>
            <TextBlock Text="{Binding P}" />
    <!-- closing tags cut off once more-->

{Binding P} works as expected, since P is a property of the items, but how do I access the collection's properties like Ptotal , Pmin , etc. ?

Thanks for taking the time to read this. If any info is missing just point it out and I'll post it.

Answer 1

So you need the collection object as your binding source.

You need these:

RelativeSource MarkupExtension

Binding.RelativeSource Property

Something like this (not tested):

    <TextBlock Text="{Binding RelativeSource={RelativeSource FindAncestor, 
AncestorType={x:Type myTemplatedControl}}, Path=DataContext.Ptotal}" />

If the DataGrid is inside your custom myTemplatedControl . It not totally clear for me what is myGridObject exactly. The main idea is:

As the MSDN documentation says: Binding.RelativeSource Gets or sets the binding source by specifying its location relative to the position of the binding target.

If you stuck with the x:Type extension, here is a link about it, so you can use it with your custom control:

X:Type

Another approach is if you name your container element (where your collection is the datacontext), then you can set that element as the binding source:

<TextBlock Text="{Binding ElementName=yourElementName, Path=DataContext.Ptotal}" />

Answer 2

I think the issue is that the DataGrid is bound to the collection, and each row is bound to an individual item, not the collection. You need to gain access a level up the chain (back to the collection itself).

If you're running Silverlight 4+ you can use a relativesource. For instance:

<TextBlock Text="{Binding Mode=OneWay, RelativeSource={RelativeSource FindAncestor, AncestorType=sdk:DataGrid, AncestorLevel=1}, Path=DataContext.Count}"

Otherwise perhaps create a static access to the context to access it via the binding Source

Answer 3

Turns out the customer changed his mind about the grid headers, he doesn't want total displayed in the header anymore.

By the way, I must have tried 20 different approached including patching in various types of converters but I haven't been able to accomplish this not-as-simple-as-it-looks-apparently task.

Thanks again for the nonetheless interesting suggestions.