Question

i want to Display a treeview bound to a Model
It works but:

<TreeView ItemsSource="{Binding Items}">
            <TreeView.ItemContainerStyle>
                <Style TargetType="{x:Type TreeViewItem}" >
                    <Style.Triggers>
                        <Trigger Property="HasItems" Value="true">
                            <Setter Property="Focusable" Value="False"/>
                        </Trigger>
                    </Style.Triggers>
                </Style>
            </TreeView.ItemContainerStyle>
                <TreeView.ItemTemplate>
                <HierarchicalDataTemplate ItemsSource="{Binding SubNodes}">
                    <Grid>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="20"/>
                            <ColumnDefinition/>
                        </Grid.ColumnDefinitions>
                        <CheckBox Grid.Column="0"/>
                        <TextBlock Text="{Binding Text}" Grid.Column="1"/>
                    </Grid>
                </HierarchicalDataTemplate>
            </TreeView.ItemTemplate>
        </TreeView>

I want to have three Templates : one for the rootitem , a second for the subnodes and a third for subnodes but alternative to the other template.

Answer 1

Its relatively simple.

Define three different types to which the tree is bound:

// defines root nodes in the tree
public sealed class RootNode : ITreeNode // or some other interface or base type
{
    public IEnumerable<SubNode> SubNodes {get;set;}
}

//  defines all middle nodes
public class SubNode : ITreeNode
{
   public IEnumerable<SubNode> Children {get;set;}
}

// defines leafs
public sealed class LeafNode : SubNode { }

Construct them and add them to your ViewModel

public sealed class ViewModel 
{
    // or use an OC<T> or whatever your design needs
    public IEnumerable<RootNode> Roots {get;set;}

Then use HierarchicalDataTemplate.DataType to designate the template for each:

<TreeView
  ItemsSource="{Binding Roots}">
  <TreeView.Resources>
      <HierarchicalDataTemplate 
          xmlns:t="clr-namespace:NamespaceForNodeClasses"
          DataType="{x:Type t:RootNode}
          ItemsSource="{Binding SubNodes}">
          <TextBlock Text="I'm a root node!"/>
      </HierarchicalDataTemplate>
      <HierarchicalDataTemplate 
          xmlns:t="clr-namespace:NamespaceForNodeClasses"
          DataType="{x:Type t:SubNode}
          ItemsSource="{Binding Children}">
          <TextBlock Text="I'm a regular tree node!"/>
      </HierarchicalDataTemplate>
      <HierarchicalDataTemplate 
          xmlns:t="clr-namespace:NamespaceForNodeClasses"
          DataType="{x:Type t:LeafNode}>
          <TextBlock Text="I'm a leaf!"/>
      </HierarchicalDataTemplate>
  </TreeView.Resources>
</TreeView>