Dependency property not getting set for datagrid in control template

Question

I have some requirement to customize datagrid, so i create my own dataGrid extending WPF datagrid. Small relevant code posted below -

public class ExtendedDataGrid : DataGrid
{
    public ExtendedDataGrid()
    {
        this.SelectionMode = DataGridSelectionMode.Extended;
    }
}

I create its instance in a window and set SelectionMode to Single which works perfectly fine and property gets set to Single for the dataGrid. So far all good.

But in case i place my DataGrid in ControlTemplate, SelectionMode never gets set to Single. SelectionMode is just for example no DP is getting set via XAML if i explicitly set that value in constructor of DataGrid.

Small sample replicating the problem is here -

 <Grid>
    <Grid.Resources>
        <ControlTemplate x:Key="MyTemplate">
            <local:ExtendedDataGrid ItemsSource="{Binding Collection,
                                                RelativeSource={RelativeSource   
                                                  Mode=FindAncestor, 
                                                  AncestorType=Window}}"
                                    SelectionMode="Single">
                <local:ExtendedDataGrid.Columns>
                    <DataGridTextColumn Binding="{Binding}"/>
                </local:ExtendedDataGrid.Columns>
            </local:ExtendedDataGrid>
        </ControlTemplate>
    </Grid.Resources>
    <ContentControl Template="{StaticResource MyTemplate}"/>
    <local:ExtendedDataGrid ItemsSource="{Binding Collection,
                                          RelativeSource={RelativeSource 
                                          Mode=FindAncestor,
                                          AncestorType=Window}}"
                            Grid.Row="1" SelectionMode="Single">
        <local:ExtendedDataGrid.Columns>
            <DataGridTextColumn Binding="{Binding}"/>
        </local:ExtendedDataGrid.Columns>
    </local:ExtendedDataGrid>
</Grid>

For second DataGrid its working fine but not working for DataGrid placed inside ControlTemplate. Why this weird behaviour? Is it some bug in DataGrid code?

Note - It will work fine if i comment the line from DataGrid constructor where i am explicitly setting SelectionMode to Extended. I know that's default value and after removing that it will work fine for both cases (also there are many ways to set default value) but i want to know why it works in one case and not in other.

Answer 1

This is good question and to answer it requires the insight of how WPF engine is creating these two instances of DataGrid.

For the first instance of DataGrid that is the direct child of your Window, that instance is created when InitializeComponents() from the constructor of Window is called. I won't go deeply inside how InitializeComponents work, but just a jist is it calls the method System.Windows.Application.LoadComponent() and LoadComponent() loads the XAML file that is located at the passed in URI, and converts it to an instance of the object that is specified by the root element of the XAML file. While doing so, it first calls the default constructor of element to be created and then again set the DependancyProperties mentioned in the attribute.

Now, the second instance that you have placed inside the ControlTemplate. That instance will be created when ControlTemplate will be applied on the element. If you are not applying the Template, the instance will never get created. While applying the Template, ControlTemplate.LoadContent() is called to create the root element of the ControlTemplate. Now LoadContent() takes the different coarse for creating the UIElements defined in the controlTemplate. It does call the default constructor for each elements, but when it comes to setting the DependancyProperties, it runs number of checks to decide what would be the property value. In the nutshell, it checks if any value is already set for the particular DependancyProperty on the element instance (i.e the value is not default value, it is the local value entry for the instance in the ValueDictionary of the DependancyObject) it does not consider the value specified in the xaml. Hence, in this case when LoadComponents call the default constructor for the DataGrid, we set the SelectionModeProperty value. While loading the content, ControlTemplate checks it and return the same value and ignores the value specified in the xaml.

And this is true for all the controls not only DataGrid.

Answer 2

I'm sorry, but I can't answer your question as to why it doesn't work when using the ControlTemplate, but I can give you a better method for setting a default value for an inherited property in an extended class which could fix your problem.

It is possible to provide new meta data with a default value for an inherited DependencyProperty using the DependencyProperty.OverrideMetadata method. You could set your own default value for the SelectionMode property using the static constructor like this:

static ExtendedDataGrid()
{
    SelectionModeProperty.OverrideMetadata(typeof(ExtendedDataGrid), 
        new FrameworkPropertyMetadata(2));
}

---

UPDATE >>>

The code will compile if you replace the SelectionMode enumeration with the integer that represents the value that you want. I merely used the SelectionMode.Extended value (now replaced with its integer value - 2) because that's what you used in your example.

I suggested this alternative way of setting that default value because you said that your problem would go away if you commented out the line in the constructor where you set the default value of the SelectionMode property to SelectionMode.Extended. I thought that if you used this to replace that line then your problem might go away.

Answer 3

Finally after bit of digging code in PresentationFramework assembly using reflector, i was able to find out the exact RCA for this issue. As mentioned by nit, this behaviour is valid for all DP's and for all Controls not just DataGrid.

This is all to do with Dependency Property Value Precedence that which value is given precedence over other while setting DP's. (Enum is BaseValueSourceInternal which store this precedence order of DP's in WindowsBase.dll assembly)

DependencyObject class contains method UpdateEffectiveValue which is responsible for setting final actual value on any DP by calling SetValue method on DataGrid instance. UpdateEffectiveValue method contains lot of logic before actually calling SetValue method on DP.

Interesting check which is stopping from setting it via ControlTemplate is this one (It checks if new value precedence order is higher than old value precedence order only in that case value will be set on DP otherwise return without setting DP) -

if ((newEntry.BaseValueSourceInternal != BaseValueSourceInternal.Unknown)
    && (newEntry.BaseValueSourceInternal < oldEntry.BaseValueSourceInternal))
{
    return (UpdateResult) 0;
}

---

In first case where dataGrid is direct child of window, DP property is set by these steps -

1. WPF engine read BAML(compiled XAML) from top to bottom and once it encounter DataGrid, creates an instance of it.
2. From constructor when we set selection mode DP, SetValueCommon method of DependencyObject class gets called. It pass on old value and new value to method UpdateEffectiveValue.
3. Now, old value's BaseValueSourceInternal is Unkown and new value's BaseValueSourceInternal is set as Local by SetValueCommon method. Hence, it gets passed from the if check mentioned above and DP gets set.
4. Now, after creating a DataGrid instance, all attributes associated with DataGrid are read from BAML one by one and SetValueCommon method is called on every DP encountered.
5. Since, SetValueCommon method's set new value with BaseValueSourceInternal.Local and old value is already BaseValueSourceInternal.Local. So precedence order is same, that's why Single value gets set on DP.

---

In second case where DataGrid is placed inside ControlTemplate -

1. DataGrid is not created when WPF engine is reading BAML since it is contained inside a ContentControl. It will be created only when ContentControl is rendered on GUI. Framework's ApplyTemplate gets called which calls LoadContent method to load the template.
2. LoadContent internally calls much more methods which at last create an instance of DataGrid and set DP like in previous case which set current value set with BaseValueSourceInternal.Local.
3. Now, once dataGrid instance is created ApplyTemplatedParentValue method gets called which tries to set all the DP's found on it by calling method UpdateEffectiveValue.
4. Current value set on DP is set with BaseValueSourceInternal.Local but new value which it tries to set is set as BaseValueSourceInternal.ParentTemplate.
5. So, finally when it goes to method UpdateEffectiveValue, the if condition mentioned above fails since ParentTemplate precedence order is lower than Local. Hence, SetValue never gets called on DP with new value which is a reason than when we comment the line from constructor it's working fine since old value's BaseValueSourceInternal is Unknown and new value's BaseValueSourceInternal is ParentTemplate.

---

As mentioned in the DP's precedence order that property set via animation holds more preference than Local value. So, ideally if we set the property in ControlTemplate via animation, it should work fine. I tried it out and it works completely fine -

<ControlTemplate x:Key="MyTemplate">
    <local:ExtendedDataGrid ItemsSource="{Binding Collection,
                                         RelativeSource={RelativeSource
                                       Mode=FindAncestor, AncestorType=Window}}">
         <local:ExtendedDataGrid.Columns>
             <DataGridTextColumn Binding="{Binding}"/>
         </local:ExtendedDataGrid.Columns>
         <local:ExtendedDataGrid.Triggers>
             <EventTrigger RoutedEvent="FrameworkElement.Loaded">
                 <BeginStoryboard>
                     <Storyboard>
                        <ObjectAnimationUsingKeyFrames 
                             Storyboard.TargetProperty="SelectionMode">
                            <DiscreteObjectKeyFrame KeyTime="00:00:00"
                              Value="{x:Static DataGridSelectionMode.Single}"/>
                        </ObjectAnimationUsingKeyFrames>
                     </Storyboard>
                 </BeginStoryboard>
             </EventTrigger>
          </local:ExtendedDataGrid.Triggers>
    </local:ExtendedDataGrid>
</ControlTemplate>