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WPF Interview Questions Answers

Question 1 : What is Windows Presentation Foundation, WPF?

Answer 1 : The Windows Presentation Foundation is the unified presentation subsystem for Windows. It consists of a display engine and a managed-code framework. The Windows Presentation Foundation unifies how Windows creates, displays, and manipulates documents, media, and user interface, which enables developers and designers to create visually-stunning, differentiated user experiences that improve customer connection. When it ships, scheduled for 2006, the Windows Presentation Foundation will be available on Windows XP, Windows Server 2003 and all future releases of the Windows operating system. When delivered, the Windows Presentation Foundation will become Microsoft’s strategic user interface technology.

Question 2 : Can you explain the overall architecture of WPF?

Answer 2 : Above figure shows the overall architecture of WPF. It has three major sections presentation core, presentation framework and milcore. In the same diagram we have shown how other section like direct and operating system interact with the system. So let’s go section by section to understand how every section works. User32:- It decides which goes where on the screen. DirectX: - As said previously WPF uses directX internally. DirectX talks with drivers and renders the content. Milcore: - Mil stands for media integration library. This section is a unmanaged code because it acts like a bridge between WPF managed and DirectX / User32 unmanaged API. Presentation core ;- This is a low level API exposed by WPF providing features for 2D , 3D , geometry etc. Presentation framework:- This section has high level features like application controls , layouts . Content etc which helps you to build up your application.

Question 3 : Does that mean WPF has replaced DirectX?

Answer 3 : No, WPF does not replace DirectX. DirectX will still be still needed to make cutting edge games. The video performance of directX is still many times higher than WPF API. So when it comes to game development the preference will be always DirectX and not WPF. WPF is not a optimum solution to make games, oh yes you can make a TIC TAC TOE game but not high action animation games. One point to remember WPF is a replacement for windows form and not directX.

Question 4 : So is XAML meant only for WPF?

Answer 4 : No,XAML is not meant only for WPF.XAML is a XML-based language and it had various variants. WPF XAML is used to describe WPF content, such as WPF objects, controls and documents. In WPF XAML we also have XPS XAML which defines an XML representation of electronic documents. Silverlight XAML is a subset of WPF XAML meant for Silverlight applications. Silverlight is a cross-platform browser plug-in which helps us to create rich web content with 2-dimensional graphics, animation, and audio and video. WWF XAML helps us to describe Windows Workflow Foundation content. WWF engine then uses this XAML and invokes workflow accordingly.

Question 5 : What is XAML?

Answer 5 : XAML (pronounced as Zammel) is a declarative XML-based language by which you can define object and properties in XML. XAML document is loaded by a XAML parser. XAML parser instantiates objects and set their properties. XAML describes objects, properties and there relation in between them. Using XAML, you can create any kind of objects that means graphical or non-graphical. WPF parses the XAML document and instantiates the objects and creates the relation as defined by XAML. So XAML is a XML document which defines objects and properties and WPF loads this document in actual memory.

Question 6 : Are XAML file compiled or built on runtime?

Answer 6 : XAML files are usually compiled rather than parsing on runtime. But it also supports parsing during runtime. When we build a XAML based project, you will see it creates g.cs extension in obi\Debug folder. Therefore, for every XAMl file you will find a g.cs file. For instance, a Shiv.XAML will have Shiv.g.cs file in obi\Debug folder. In short, in runtime you actually do not see the XAML file. But if you want to do runtime, parsing of XAML file it also allows that to be done.

Question 7 : So can we define WPF in a precise way?

Answer 7 : Windows Presentation Framework is the new presentation API. WPF is a two and three dimensional graphics engine. It has the following capabilities:- • Has all equivalent common user controls like buttons, check boxes sliders etc. • Fixed and flow format documents  • Has all of the capabilities of HTML and Flash  • 2D and 3D vector graphics  • Animation  • Multimedia  • Data binding

Question 8 : Can explain the different elements involved in WPF application practically?

Answer 8 : In order to understand the different elements of WPF, we will do a small ‘hello world’ sample and in that process we will understand the different elements of WPF. Note :- For this sample we have VS 2008 express edition.   So start VS 2008 express and from the templates select the WPF application as show in the below figure below.   Once we have created the WPF application project you will see two file types one the XAML file and the other is the behind code i.e. XAML.cs. XAML files are nothing but XML files which has all the elements needed to display the windows UI. Every of the XAML elements maps to come class. For instance the ‘Window’ element maps to ‘WpfApplication1.Window1’ class , ‘Button’ elements in XAML file maps to ‘System.Windows.Control.Button’ class and ‘Grid’ XAML element is mapped to ‘System.Windows.Control.Grid’. The ‘App.XAML’ and ‘App.XAML.CS’ are the entry point files. If we see the code for ‘App.XAML.CS’ you will see the reference to XAML file which needs to be loaded. So the first code which runs in the application is ‘void main()’ method from ‘App.XAML.CS’ which in turn loads the ‘Window1.XAML’ file for rendering. We can now connect the behind code method and function to events in XAML file elements. You can see from the above code snippet how the button element has the click event linked to the ‘MyButton_Click’ function. ‘MyButton_Click’ is the method which is in the XAML.CS behind code. So now if you run the code you can see the button and if you click on it you can see the message box for the same.  

Question 9 : Which are the different namespaces and classes in WPF?

Answer 9 : There are ten important namespaces / classes in WPF. System.Threading.DispatcherObject All WPF objects derive from the DispatcherObject. WPF works on STA model i.e. Single Threading Apartment Model. The main duty of this object is to handle concurrency and threading. When any message like mouse clicks, button clicks etc are initiated they are sent to the DispatcherObject who verifies whether code is running on the correct thread. In the coming section we will look in to detail how WPF threading works.   System.Windows.DependencyObject When WPF was designed property based architecture was considered. In other words rather than using methods, functions and events object behavior will interact using properties. For now we will only restrict ourselves to this definition. In the coming section we have dedicated question for the same.   System.Windows.Media.Visual Visual class is a drawing object which abstracts drawing instructions, how drawing should be drawn like clipping, opacity and other functionalities. Visual class also acts like a bridge between unmanaged MilCore.dll and WPF managed classes. When any class derived from visual it can be displayed on windows. If you want to create your own customized user interface then you can program using visual objects.   System.Windows.UIElement UIElement handled three important aspects layout, input and events. System.Windows.FrameworkElement FrameWorkElement uses the foundation set by UIElement. It adds key properties like HorizontalAlignment , VerticalAlignment , margins etc. System.Windows.Shapes.Shape This class helps us to create basic shapes such as Rectangle, Polygon, Ellipse, Line, and Path. System.Windows.Controls.Control This class has controls like TextBox,Button,ListBox etc. It adds some extra properties like font,foreground and background colors. System.Windows.Controls.ContentControl It holds a single piece of content. This can start from a simple label and go down to a unit level of string in a layout panel using shapes. System.Windows.Controls.ItemsControl This is the base class for all controls that show a collection of items, such as the ListBox and TreeView.  System.Windows.Controls.Panel This class is used for all layout containers—elements that can contain one or more children and arrange them as per specific layout rules. These containers are the foundation of the WPF layout system, and using them is the key to arranging your content in the most attractive, flexible way possible.

Question 10 : How does hardware acceleration work with WPF?

Answer 10 : Hardware acceleration is a process in which we use hardware to perform some functions rather than performing those functions using the software which is running in the CPU. WPF exploits hardware acceleration in a two tier manner. WPF API first detects the level of hardware acceleration using parameters like RAM of video card , per pixel value etc. Depending on that it either uses Tier 0, Tier 1 or Tier 2 rendering mode. Tier 0:- If the video card does not support hardware acceleration then WPF uses Tier 0 rendering mode. In other words it uses software acceleration. This corresponds to working of DirectX version less than 7.0. Tier 1:- If the video card supports partial hardware acceleration then WPF uses Tier 1 rendering mode. This corresponds to working of DirectX version between 7.0 and 9.0. Tier 2:- If the video card supports hardware acceleration then WPF uses Tier 2 rendering mode. This corresponds to working of DirectX version equal or greater than 9.0.

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