对Window中的UserControl中的DataContext感到困惑

Question

I am confused about DataContext in an UserControl:

I have created an UserControl with Textboxes etc...

In a wpf-window I have included this UserControl and the binding works as I want and expect.

But If I write the following in the UserControl's constructor:

public partial class VehicleFields : UserControl
{
    VehicleAddEdit vm;
    public VehicleFields()
    {
        InitializeComponent();
        vm = this.DataContext as VehicleAddEdit;
    }
 }

the vm and the DataContext is always null. How can I get the window's datacontext in my UserControl ?

Window-XAML:

<Window x:Class="KraftSolution.Windows.Vehicle.VehicleAdd"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:UCs="clr-namespace:Test.UserControls"
        xmlns:local="..."
        >
    <Window.Resources>
            <local:VehicleAddWindow x:Key="VehicleAddWindow"/>
        </ResourceDictionary>
    </Window.Resources>
    <Grid Name="MainGrid"
          Style="{StaticResource DataManipulationGrid}"
          DataContext="{StaticResource VehicleAddWindow}">
     <UCs:VehicleFields/>
    </Grid>

Thanks in advance

Answer 1

During the execution of the constructor the DataContext is not yet set. Move the assignement to a DataContextChanged event handler:

public VehicleFields()
{
    InitializeComponent();

    DataContextChanged += (s, e) => vm = DataContext as VehicleAddEdit;
}

Answer 2

You can do this:

XAML:

<UserControl ...  DataContextChanged="VehicleFields_OnDataContextChanged">

Code begin:

  private void VehicleFields_OnDataContextChanged(object sender, DependencyPropertyChangedEventArgs e)
  {
        vm = DataContext as VehicleAddEdit;
  }