Setting the DataContext in "MainView" filters down to all "ChildView's"

Question

I would like to know if this is a standard feature of .NET: when setting the DataContext in the ParentView, it filters down to all child views.

Say you have ParentView, ChildView1 and ChildView2:

<UserControl x:Class="DXWPFApplication1.ParentView"
             xmlns:view="clr-namespace:DXWPFApplication1"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <Grid>
        <view:ChildView1  x:Name="childView1"/>
    </Grid>
</UserControl>

<UserControl x:Class="DXWPFApplication1.ChildView1"
             xmlns:view="clr-namespace:DXWPFApplication1"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <Grid>
        <view:ChildView2  x:Name="childView2"/>
    </Grid>
</UserControl>

Code behind of ParentView:

public ParentView() 
        {
            InitializeComponent();

            DataContext = "ViewModel"; //BreakPoint here

            //
            //When the first DataContext is set, all the DataContext's below are set as well
            //

            childView1.DataContext = DataContext;
            childView1.childView2.DataContext = DataContext;
        }

NOTE: Breakpoint when setting first DataContext

Why are all the DataContexts set when I have only set the ParentView's DataContext?

What can I do to prevent this from happening?

Answer 1

This is standard behaviour, and normally desired. To prevent it, set DataContext to {x:Null} in your markup

Answer 2

A component in the visual tree inherts the data context from its parent. Your child view resides in the visual tree of the parent view so it will get the parent context assigned. You need to explicitly set it to something different if you want to change it (either inside the child view constructor or in the xaml, for example <view:ChildView2 DataContext="{x:Null}" x:Name="childView2"/>).

Why was that done: Because it is almost always what you want.