What is DataContext for?

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When you write

<Label name="myLabel" Content="{Binding Path=Name}" />

you are binding to myLabel.DataContext.Name, and not myLabel.Name.

The XAML in WPF is just a pretty user interface to display and interact with the actual data, otherwise known as the DataContext. The purpose of other binding sources (RelativeSource, ElementName, etc) is to point to another property that doesn’t exist in the current control’s DataContext

So suppose you have a Window. Without setting the DataContext, the window still displays but there is no data behind it.

Now suppose to set myWindow.DataContext = new ClassA();. Now the data that the window is displaying is ClassA. If ClassA has a property called Name, I could write a label and bind it to Name (such as your example), and whatever value is stored in ClassA.Name would get displayed.

Now, suppose ClassA has a property of ClassB and both classes have a property called Name. Here is a block of XAML which illustrates the purpose of the DataContext, and an example of how a control would refer to a property not in it’s own DataContext

<Window x:Name="myWindow"> <!-- DataContext is set to ClassA -->
    <StackPanel> <!-- DataContext is set to ClassA -->

        <!-- DataContext is set to ClassA, so will display ClassA.Name -->
        <Label Content="{Binding Name}" />

         <!-- DataContext is still ClassA, however we are setting it to ClassA.ClassB -->
        <StackPanel DataContext="{Binding ClassB}">

            <!-- DataContext is set to ClassB, so will display ClassB.Name -->
            <Label Content="{Binding Name}" />

            <!-- DataContext is still ClassB, but we are binding to the Window's DataContext.Name which is ClassA.Name -->
            <Label Content="{Binding ElementName=myWindow, Path=DataContext.Name}" /> 
        </StackPanel>
    </StackPanel>
</Window>

As you can see, the DataContext is based on whatever data is behind the UI object.

Update: I see this question so often from new WPF users that I expanded this answer into a post on my blog: What is this “DataContext” you speak of?

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