What is DataContext for?

问题:

As a continuation of the question Linking DataContext with another property in WPF.

At the very end of the research I was very surprised to find out that when one writes something like that:

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

The DataContext against which the Content property is binded is of the Label control itself! The fact that it still works is due to the default inheritance of the DataContext value from the nearest parent.

But if you have this label wrapped in a custom control, and you don\'t want to bind your data to the DataContext property of that control, you would more likely love to have:

<Controls:SearchSettings Settings=\"{Binding Path=Settings}\" />

And here you are. Now you need to set Settings as the DataContext for the SearchSettings control, for Label inside to bind against, but you can\'t, because that will trigger re-binding of Settings property.

I can\'t see the point in mixing binding properties using different sources: DataContext, by ElementName, etc. So why would I ever use DataContext?

回答1:

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?

回答2:

From CodeProject by kishore Gaddam:

DataContext is one of the most fundamental concepts in Data Binding. The Binding object needs to get its data from somewhere, and there are a few ways to specify the source of the data like using Source property directly in the Binding, inheriting a DataContext from the nearest element when traversing up in the tree, setting the ElementName and RelativeSource properties in the Binding object.

Detailed example on CodeProject: http://www.codeproject.com/Articles/321899/DataContext-in-WPF

回答3:

In that particular case, you could do:

<Controls:SearchSettings DataContext=\"{Binding Path=Settings}\" Settings=\"{Binding}\" />

Assuming you want everything that may be content of the SearchSettings to use Settings as it\'s data context. Basically, the DataContext affects the element itself an any descendants that don\'t explicitly override it.

回答4:

In most cases you do want to bind to the DataContext, in some templates on ItemsControls it is the only way to bind to the currently templated item for example. Further bindings to the DataContext are nice to write and read as they are concise.

In your example you can still set the DataContext, you only need to modify the binding on the Settings respectively:

<Controls:SearchSettings DataContext=\"{Binding Settings}\" Settings=\"{Binding}\"/>