WPF Style setting based on data type?

Question

Here is the problem. I bind TreeView to several different types of objects. Each object is a node, and SOME objects have the IsNodeExpanded property, and of course, some others are not. Here is my style:

<Style TargetType="{x:Type TreeViewItem}">
    <Setter Property="IsExpanded" Value="{Binding IsNodeExpanded, Mode=TwoWay}" />
</Style>

Now the problem is to bind elements that DO NOT have this property, we get this error in the output:

System.Windows.Data Error: 39 : BindingExpression path error: 'IsNodeExpanded' property not found on 'object' ''CompensationChannel' (HashCode=56992474)'. BindingExpression:Path=IsNodeExpanded; DataItem='CompensationChannel' (HashCode=56992474); target element is 'TreeViewItem' (Name=''); target property is 'IsExpanded' (type 'Boolean')

Of course, we get many times. So I'm trying to come up with a way to switch the TreeViewItem style based on the DataType that it holds. Any idea on how to do this?

Some information: I cannot do this manually for each element, because I do not create them in XAML, they are dynamically created from the data source.

EDIT: I found this answer , but it did not work for me.

Answer 1

Try using a property TreeView.ItemContainerStyleSelectorwith a custom class StyleSelectorthat changes the style depending on if the associated object has this property or not.

public class TreeItemStyleSelector : StyleSelector
{
    public Style HasExpandedItemStyle { get; set; }
    public Style NoExpandedItemStyle { get; set; }

    public override Style SelectStyle(object item, DependencyObject container)
    {
        // Choose your test
        bool hasExpandedProperty = item.GetType().GetProperty("IsExpanded") != null;

        return hasExpandedProperty
                   ? HasExpandedItemStyle
                   : NoExpandedItemStyle;
    }
}

In XAML resources:

<Style x:Key="IsExpandedStyle" TargetType="{x:Type TreeViewItem}">
    <Setter Property="IsExpanded" Value="{Binding IsNodeExpanded, Mode=TwoWay}" />
</Style>

<Style x:Key="NoExpandedStyle" TargetType="{x:Type TreeViewItem}">
</Style>

<x:TreeViewItemStyleSelector x:Key="TreeViewItemStyleSelector"
                             HasExpandedItemStyle="{StaticResource IsExpandedStyle}"
                             NoExpandedItemStyle="{StaticResource NoExpandedStyle}" />

In XAML:

<TreeView ItemsSource="{Binding ...}"
          ItemContainerStyleSelector="{StaticResource TreeItemStyleSelector}">

Answer 2

UPDATE

<TreeView.Resources>

    ... following is only for one type of data
    <HierarchicalDataTemplate 
      DataType="{x:Type local:RegionViewModel}" 
      ItemsSource="{Binding Children}"
      >

      ... define your style
      <HierarchicalDataTemplate.ItemContainerStyle>
           <Style TargetType="{x:Type TreeViewItem}" 
                  ... following line is necessary
                  BasedOn="{StaticResource {x:Type TreeViewItem}}">
                ..... your binding stuff....
           </Style>
      </HierarchicalDataTemplate.ItemContainerStyle>

      <StackPanel Orientation="Horizontal">
        <Image Width="16" Height="16" 
           Margin="3,0" Source="Images\Region.png" />
        <TextBlock Text="{Binding RegionName}" />
      </StackPanel>
    </HierarchicalDataTemplate>
...
</TreeView.Resources>

ALTERNATIVE WAY

Instead of switching styles, you should use the HierarchicalDataTemplate and DataTemplate to create the TreeViewItem style, they will work in the same way if you do not want to change some properties of the inherited structure.

You can define different “DataTemplate” and “HeirarchicalDataTemplate” based on different types of objects associated with the template of the TreeView elements.

, Selector .. , , WPF .

, TreeView DataBinding

, ,

<TreeView.Resources>
    <HierarchicalDataTemplate 
      DataType="{x:Type local:RegionViewModel}" 
      ItemsSource="{Binding Children}"
      >
      <StackPanel Orientation="Horizontal">
        <Image Width="16" Height="16" 
           Margin="3,0" Source="Images\Region.png" />
        <TextBlock Text="{Binding RegionName}" />
      </StackPanel>
    </HierarchicalDataTemplate>

    <HierarchicalDataTemplate 
      DataType="{x:Type local:StateViewModel}" 
      ItemsSource="{Binding Children}"
      >
      <StackPanel Orientation="Horizontal">
        <Image Width="16" Height="16" 
          Margin="3,0" Source="Images\State.png" />
        <TextBlock Text="{Binding StateName}" />
      </StackPanel>
    </HierarchicalDataTemplate>

    <DataTemplate DataType="{x:Type local:CityViewModel}">
      <StackPanel Orientation="Horizontal">
        <Image Width="16" Height="16" 
           Margin="3,0" Source="Images\City.png" />
        <TextBlock Text="{Binding CityName}" />
      </StackPanel>
    </DataTemplate>
  </TreeView.Resources>

Answer 3

FallbackValue ? , ...

<Style TargetType="{x:Type TreeViewItem}">
    <Setter Property="IsExpanded" Value="{Binding IsNodeExpanded, Mode=TwoWay, FallbackValue=False}" />
</Style>