WPF XAML：DataContext作为XAML元素的属性还是属性之间的区别？

Question

I'm currently getting started with XAML and I have a question regarding how to define the DataContext of an element.

I've created a View that includes a Page with the following markup:

<Page x:Class="View.MainView"
  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"
  xmlns:ViewModel="clr-namespace:ViewModel"
  mc:Ignorable="d"
  Title="MainView">
  ...
  </Page>

When I want to give this Page a DataContext to be used by child elements, this works:

<Page x:Class="View.MainView"
  ...
  mc:Ignorable="d"
  Title="MainView">
  <Page.DataContext>
    <ViewModel:MainViewModel />
  </Page.DataContext>
  ...
  </Page>

And this doesn't:

<Page x:Class="View.MainView"
  ...
  mc:Ignorable="d"
  Title="MainView" DataContext="ViewModel:MainViewModel">
  ...
  </Page>

For me, it looks like the Page element expects the DataSource to be defined as a XAML property and not as an attribute. However, the IntelliSense in Visual Studio offers me a DataContext attribute for the Page, so I guess I'm just using a wrong syntax here. Can you point that out to me?

Thanks!

Answer 1

You can use the attribute to specify the DataContext , but you should consider how does your viewmodel get instantiated.

Using a property in this way

<Page.DataContext>
     <ViewModel:MainViewModel />
</Page.DataContext>

you tell WPF to instantiate the MainViewModel and to assign the created object to the DataContext property of the Page .

With an attribute, you just specify a string in that case:

DataContext="ViewModel:MainViewModel"

But you want WPF to create an instance for you.

So you can use eg a Binding or a StaticResource / DynamicResource to assign a created instance to the DataContext property:

DataContext="{Binding ViewModel}"

or

<Page DataContext="{StaticResource ViewModel}">
    <Page.Resources>
        <ViewModel:MainViewModel x:Key = "ViewModel"/>
    </Page.Resources>
</Page>