Access DataContext instance in code when set in XAML

Question

I usually instantiate my ViewModel in codebehind and set it as the DataContext for the Window or UserControl.

For this simple ViewModel:

public class ViewModel
{
    public ObservableCollection<string> items { get; set; }

    public ViewModel()
    {
        items = new ObservableCollection<string>();
        items.Add("FirstItem");
        items.Add("SecondItem");
        items.Add("ThirdItem");
    }
}

i am adding my local namespace and the following syntax will set the things right:

<Window.DataContext>
    <local:ViewModel/>
</Window.DataContext>

Fine, i understand that a ViewModel object is instantiated and set as DataContext for the window, it works as expected but is there any way to access the instantiated object from code?

If i create the object in the codebehind with

ViewModel vm = new VewModel();
this.DataContext = vm;

i am able to access vm instance but with the XAML approach, how can it be reached?

I've created this small example in order to find out if there is a simple answer available.

Answer 1

When you set the DataContext of any element, then all of the children of said element will also have the same DataContext.

Picture the scene:

<Window.DataContext>
    <local:ViewModel/>
</Window.DataContext>

Here you have given the Window a DataContext. Now, all child elements within your window effectively have the same DataContext.

<TextBox Text="{Binding MyProperty}" />

To get hold of the DataContext in code-behind, you can simply reference the DataContext of the element.

ViewModel vm = (ViewModel)this.DataContext;

The code above references the DataContext of the Window.

If you need to be more specific, and get the DataContext of a specific element, then you can simply reference the element by name.

ViewModel vm = (ViewModel)elementName.DataContext;

All that aside, you should never set the DataContext in code-behind. The MVVM design pattern likes to keep things separated, and if you start setting the DataContext of a UserControl for example, then things get screwed up pretty quickly.

Setting the DataContext in XAML is the right way to go.