窗口DataContext或ElementName

Question

I am currently facing a little problem with specifing or not specifing the datacontext of a window, and why there is a difference between various methods. Hope you can help me out.

Lets start with some code to show my problem. This is the code behind for my TestWindow.xaml.cs, nothing really special about it just a simple string property

public partial class TestWindow : Window
{
    private string _helloWorld = "Hello World!";

    public string HelloWorld
    {
        get { return _helloWorld; }
        set { _helloWorld = value; }
    }

    public TestWindow()
    {
        InitializeComponent();
    }
}

This code will be the same for all 3 following XAML layouts, so no changes behind the code only in XAML.

1.) Databinding with given ElementName

<Window x:Class="Ktsw.Conx.ConxClient.TestWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="TestWindow" Height="300" Width="300"
        Name="TestWin">
    <Grid>
        <TextBlock Text="{Binding HelloWorld, ElementName=TestWin}"></TextBlock>
    </Grid>
</Window>

2.) Databinding with specifing DataContext on Window

<Window x:Class="Ktsw.Conx.ConxClient.TestWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="TestWindow" Height="300" Width="300"
        DataContext="{Binding RelativeSource={RelativeSource Self}}">
    <Grid>
        <TextBlock Text="{Binding HelloWorld}"></TextBlock>
    </Grid>
</Window>

3.) Neither ElementName nor specifing DataContext

<Window x:Class="Ktsw.Conx.ConxClient.TestWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="TestWindow" Height="300" Width="300">
    <Grid>
        <TextBlock Text="{Binding HelloWorld}"></TextBlock>
    </Grid>
</Window>

The first two methods work just fine, but why fails the 3rd?

In the first method I am not specifing the DataContext and it works automatically, in the second method I am not specifing the ElementName and it works, but without declaring one of those it fails. Why would it fail getting both automatically, but work fine with getting each individually?

Answer 1

Read the Remarks for the Binding.Source property . The binding source is the object on which to look up the property specified by Path (HelloWorld). ElementName , RelativeSource , and Source set the binding source. If they are null, the source is the DataContext of the element that you are binding (TextBlock). If an element's DataContext is not explicitly set, it's inherited from a containing element.

In the first case, any DataContext is irrelevant to the TextBlock binding, because the source is being set explicitly through ElementName.

In the second case, a DataContext is necessary to specify a source because it is not being set explicitly through ElementName, RelativeSource, or Source. TextBlock's DataContext is not set, so Window's is inherited.

In the third case, there is no explicit source and no DataContext to inherit, so the source is null and binding fails. There is no default source.

Answer 2

It is working in 1st and 2nd case because TextBlock is able to find HelloWorld property

In 1st case you are specifying ElemtnName so TextBlock will refer to Window and search for property HelloWorld in Window class.

In 2nd case you are specifying DataContext of Window so Textblock will inherit that and will look for HelloWorld property in DataContext ie again Window class

In 3rd case you aren't specifying anything so TextBlock is unable to find HelloWorld property

In 1st case I am not specifing a DataContext so why should it know what to get?

When you are setting ElementName , TextBlock will try to find HelloWorld from that element

In 2nd case I am not specifing what it should get?

You are setting the DataContext of it's parent that is Window so it automatically inherits that and you are telling it to extract HelloWorld property from DataContext

Answer 3

{Binding} by default always refers to the DataContext .

In your first example you simply deactivate the default behavior by using elementName, this uses the element as source then.

In your second example you explicitly change the DataContext to point at your Window.

So in the last example the {Binding HelloWorld} tries to convert null into and Object with a HelloWorld property which fails.

Answer 4

The third fails because the DataContext is not inferred. If you wanted the third to work, you'd do something like this:

public TestWindow()
{
    InitializeComponent();

    this.DataContext = this;
}

It doesn't know anything about the path HelloWorld because it doesn't have a DataContext .