problem with treeview loading

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WPF

treeview

MVVM

Hey all! This is a simple MVVM treeview project which displays (is supposed to display!) a treeView of Tasks and a textbox showing the Task contents (TaskName for instance). The problem is that the tree items are not shown at all! These are the classes used:

public class TaskTreeViewModel
{
    TaskViewModel vmRooT;
    List<TaskViewModel> vmFirstGeneration;
    public TaskTreeViewModel(Task rootTask)
    {
        vmRooT = new TaskViewModel(rootTask);
        vmFirstGeneration = new List<TaskViewModel>(new TaskViewModel[] { vmRooT });

    }
}

class TaskViewModel : INotifyPropertyChanged
{
    private Task vmTask;    
    private List<TaskViewModel> vmSubTasks;
    TaskViewModel vmParent;
    //Task status
    bool vmIsExpanded;
    bool vmIsSelected;

    #region properties

    public string Name
    {
        get
        {return vmTask.TaskName;}
    }
    public List<TaskViewModel> SubTasks
    {
        get
        {return vmSubTasks;}
    }
    public TaskViewModel Parent
    {
        get
        {return vmParent;}
    }
    #endregion

    #region ctors

    private TaskViewModel(Task task, TaskViewModel parent)
    {
        vmTask = task; 
        vmParent = parent; 
        ///<summary>
        ///recursively walks down the Task tree, wrapping each Task object in a TaskViewModel
        ///</summary>
        vmSubTasks = new List<TaskViewModel>( 
                (from t in vmTask.SubTasks
                 select new TaskViewModel(t, this)).ToList<TaskViewModel>());          
    }
    public TaskViewModel(Task task)
        : this(task, null)
    { }

    #endregion

    #region Presentation Members

    /// <summary>
    /// Gets/sets whether the TreeViewItem 
    /// associated with this object is expanded.
    /// </summary>
    public bool IsExpanded
    {
        get { return vmIsExpanded; }
        set
        {
            if (value != vmIsExpanded)
            {
                vmIsExpanded = value;
                this.OnPropertyChanged("IsExpanded");
            }

            // Expand all the way up to the root.
            if (vmIsExpanded && vmParent != null)
                vmParent.vmIsExpanded = true;   //fantastic! (executes the setter again)
        }
    }
    /// <summary>
    /// Gets/sets whether the TreeViewItem 
    /// associated with this object is selected.
    /// </summary>
    public bool IsSelected
    {
        get { return vmIsSelected; }
        set
        {
            if (value != vmIsSelected)
            {
                vmIsSelected = value;
                this.OnPropertyChanged("IsSelected");
            }
        }
    }

    #endregion

    #region INotifyPropertyChanged Members

    public event PropertyChangedEventHandler PropertyChanged;
    protected virtual void OnPropertyChanged(string propertyName)
    {
        if (this.PropertyChanged != null)
            this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }

    #endregion 
}

public class Task
{
    //auto-implemented properites
    public string TaskName { get; set; }
    public string Description { get; set; }
    public List<Task> SubTasks { get; set; }
    #region ctors
    public Task()
    {
        TaskName = string.Empty;
        Description = string.Empty;
        SubTasks = new List<Task>();    
    }
    public Task(string strTaskName)
        : this()
    {
        TaskName = strTaskName;
    }
    public Task(string strTaskName, string strDescription)
        : this(strTaskName)
    {
        Description = strDescription;       
    }
    public Task(string strTaskName, List<Task> subs)
        : this(strTaskName)
    {
        SubTasks = subs;
    }
    public Task(string strTaskName, string strDescription, List<Task> subs)
        : this(strTaskName, strDescription)
    {
        SubTasks = subs;
    }
    public Task(string strTaskName, string strDescription, params string[] strSubTasksNames)
        : this(strTaskName, strDescription)
    {
        SubTasks = new List<Task>();
        foreach (var strSub in strSubTasksNames)
        {
            SubTasks.Add(new Task(strSub));
        }
    }
    #endregion ctors
}

And these are the XAML file and its code-behind:

<Window x:Class="Task_it__with_MVVM_.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Task it!" Height="570" Width="850" WindowStartupLocation="CenterScreen">
    
    <Grid FlowDirection="RightToLeft" Margin="10">
        <Grid.ColumnDefinitions>
            <ColumnDefinition></ColumnDefinition>
            <ColumnDefinition></ColumnDefinition>
        </Grid.ColumnDefinitions>
            <TreeView Grid.Column="0" Margin="10" MinHeight="280" ItemsSource="{Binding vmFirstGeneration}">
                <TreeView.ItemContainerStyle>
                    <Style TargetType="{x:Type TreeViewItem}">
                        <Setter Property="IsExpanded" Value="{Binding IsExpanded, Mode=TwoWay}" />
                        <Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay}" />
                        <Setter Property="FontWeight" Value="Normal" />
                        <Style.Triggers>
                            <Trigger Property="IsSelected" Value="True">
                                <Setter Property="FontWeight" Value="Bold" />
                            </Trigger>
                        </Style.Triggers>
                    </Style>
                </TreeView.ItemContainerStyle>
                <TreeView.ItemTemplate>
                    <HierarchicalDataTemplate ItemsSource="{Binding SubTasks}">
                        <TextBlock Text="{Binding Name}" FontSize="14"/>
                    </HierarchicalDataTemplate>
                </TreeView.ItemTemplate>
                
            </TreeView>
        <GroupBox Grid.Column="1" Header="مشخصات" Margin="10">
            <StackPanel Margin="10">
                <StackPanel Margin="1">
                    <Label FontSize="14" Margin="1">نام</Label>
                    <TextBox Name="txtName"  Text="{Binding Name}" FontSize="16" Margin="1" Width="auto">
                    </TextBox>
                </StackPanel>
            </StackPanel>
        </GroupBox>
    </Grid>

</Window>

code-behind:

public partial class MainWindow : Window
{
    //RootTask which is the fixed permanent Task at the root
    public Task RootTask { get; set; }
    TaskTreeViewModel vmTaskTree { get; set; }
    //ctor
    public MainWindow()
    {
        InitializeComponent();
        PrepareTree_Data();
    }
    public void PrepareTree_Data()
    {
        // Get raw task tree data
        RootTask = SaveLoad.LoadData();//This is method just initializes the RootTask
        // Create UI-friendly wrappers around the
        // raw data objects (i.e. the view-model).
        vmTaskTree = new TaskTreeViewModel(RootTask);
        // Let the UI bind to the view-model.
        base.DataContext = vmTaskTree;
    }
}