Xamarin Interview Questions and Answers Mobile App Development

While writing Xamarin Cross-Platform Applications, one might encounter a lot of instances where similar functionality needs to be implemented across various platforms, which is generic. To facilitate this and allow code reuse, it makes utmost sense to share some common piece of code between these platforms as it saves time, reduces error scope and is easy for maintenance.

Xamarin provides three common approaches to share code between Cross-Platform Applications:-

1. .NET Standard Libraries
2. Shared Projects
3. Portable Class Libraries (Deprecated)

1. .NET Standard Libraries

.NET Standard Libraries is a way to share common code over multiple runtimes. .NET Standard is a set of specifications which various .NET runtimes adhere to and hence code written for one version of .NET Standard works on multiple versions of .NET runtimes like .NET Core, Mono, etc.  You can write your code and compile it into .NET Class Library and share it with others.

The primary benefits of using this approach are:

1. It allows you to share code across multiple projects
2. When you refactor, all affected references are updated at once.

Disadvantage

1. You cannot do the conditional compilation for specific platforms using the #if directive It is somewhat similar to PCL but with a simpler model for platform support.

2. Using Shared Projects

Shared Projects are the usual .NET application projects that contain code files and assets. Shared Projects are meant to be included in other projects and help in code reuse. It is a code level sharing technique.

A cross-platform application that supports iOS, Android, and Windows would require an application project for each platform and a separate Shared Project for the code common to all.

So, if you are creating a cross-platform app for Android, iOS, and Windows, you will usually have the following projects

• Shared Project – the Shared Project that contains the code which is common for all platforms viz iOS, UWP, Android
• AppAndroid – the Xamarin.Android project that specifically calls the underlying .NET APIs exposed for Android
• AppiOS – Xamarin.iOS application project that specifically calls the underlying .NET APIs exposed for iOS
• AppWindows – Windows application project that utilizes exposed Windows APIs and specific to UWP (Universal Windows Platform)

A Shared Project is not directly compiled. In other words, no DLL file is produced in the compilation process. Instead, the files are compiled into the same DLL as the project that references it. This way, it is possible to write blocks of platform-specific code in the Shared Project that will only be compiled by the specific platform.

The advantages of using this technique are:-

• Allows you to share common code across multiple projects.
• The shared code can be branched based on the platform using compiler directives (eg. using #if __ANDROID__ or #if __IOS__)
• Application projects can include platform-specific references that the shared code can utilize

Disadvantages:-

They do not produce any output assembly of their own. Hence, not used for sharing and distributing to other developers

3. Portable Class Libraries (Deprecated)

When you create an Application Project or a Library Projection compiles into a DLL, it is restricted to work only on a  specific platform, i.e. the one it is created for. This is attributed to the fact that the various platforms use different .NET runtime ecosystems. This prevents you from writing an assembly for one and runs it on all .NET Runtimes.

Portable Class Library allows you to develop a Class Library that can run for multiple platforms. You can choose a set of platforms for the Portable Class Library while creating it. This choice is represented by the  "Profile" identifier and helps in identifying the platforms the Portable Class Library is meant to work with.

 The benefits of using this approach are:-

• Allows you to share code across multiple projects.
• When you refactor, all affected references are updated at once.

Disadvantages:-

• Deprecated in the latest versions of Visual Studio, .NET Standard libraries are recommended instead.
• You cannot do the conditional compilation for specific platforms using the #if directive
• Only a subset of the .NET framework is available to use, determined by the profile selected

It’s possible to convert a PCL to a .NET Standard project by changing the target of your project to .Net Standard.

This is one of the most frequently asked Xamarin interview questions for freshers in recent times.

While developing apps with Xamarin.Forms, you will find that certain native platform-specific functionalities are not present in the Xamarin.Forms API. This is because of the generic nature of Xamarin.Forms. Xamarin.Forms allow apps to call into platform-specific functionality from shared code. This functionality enables Xamarin.Forms apps to do anything that a native app can do.   You need to necessarily define an interface and write platform-specific implementations of that interface in the platform project. Dependency service will find the correct implementation of that interface in the various platform projects.   Xamarin.Forms apps need four components to use DependencyService:

• Interface – The required functionality is defined by an interface in shared code. This needs to be implemented by each platform
• Implementation Per Platform – The implementation of the above-mentioned interface for each platform.
• Registration – To find the correct implementation of the interface for the suitable platform at runtime, it is required that each platform implementation  class of the interface be registered with the DependencyService at runtime
• Invoking Dependency Service- It is required that there be an explicit invocation of the Dependency service, which will then allow the Dependency service to choose the appropriate implementation based on the platform.

   The structure of the application is explained by the following diagram:

Custom renderers allow the Generic Xamarin.Forms control to be customized in behavior and look as per the platform they are going to be used on. This enables developers to give a native look and behavior to the otherwise Generic Xamarin.Forms Control.  

Xamarin Forms controls are NOT rendered directly on the native platform. Every Xamarin.Forms control has an accompanying renderer for each platform that creates an instance of a native control. The properties from the Xamarin Forms control are translated across to the native control, then the native control is placed in the native layout, which then gets rendered by the platform.  

Custom Renderers allow developers to customize the appearance and/or behavior of the control by writing their custom classes. Custom Renderers can be defined to have a custom behavior or appearance for one platform while allowing the default behavior on other platforms or they can be defined for each platform and provide customization for each different platform like iOS, Android, and the Universal Windows Platform (UWP).  If instead of changing the complete behavior and appearance only some trivial changes are required then 'Effects' can be used as an alternative.

5 Effects, like Custom Renderers, allow a developer to customize controls for a specific platform. Effects are preferred over Custom Renderers when small styling changes are required instead of a complete layout or behavior change. Custom Effects are created for platform-specific projects by extending the base class PlatformEffect. Once created, they can be attached to the control it is meant for.  

Effects don't have any type related information about the control they are attached to and hence if they are specified with a wrong control they should gracefully degrade. Effects are reusable and can be parameterised to extend its reusability.  

Eg The sample application demonstrates a Focus Effect that changes the background color of a control when it gains focus.  

 Xamarin allows two different ways of creating applications, based on the amount of code reusability and customization.  

The first approach is the Traditional Native Approach wherein platform-specific apps using Xamarin.iOSiOS and Xamarin.Android can be made. This way of creating apps is generally used when there is a lot of customization specific to the platform is required as it allows direct access to platform-specific APIs. Xamarin.iOS is used for iOS applications and Xamarin.Android is used to create Android applications.

The second approach is creating apps through Xamarin.Forms approach. Xamarin.Forms are used when there is a possibility of reuse of a lot of platform-independent code and the focus is less on custom UI. The platform-independent code is separated and kept in Shared Project or PCL or .NET Standard Library and Platform Specific projects consume this common code by including it.

Pages are Xamarin Forms generic representation of Cross Mobil Application Screens. A Page occupies most or all of a screen and contains a single child.

On IOS, the Page is mapped to ViewController, on Android, it is mapped to somewhat like Activity and on Universal Windows Platform, it is mapped to  Page. Pages can be of several types, viz. Master /Detail Page, navigational Page, Carousel Page, Tabbed Page, Template Page, etc.

• Content Page

A Content Page is the most basic type of page which contains the content in a  Stak Layout Grid Layout or Scroll View. The content is usually text or images

• Master-Detail Page

The Master-Detail Page is divided into two sections: Master and Detail. Master usually contains a list of items or Menu. Clicking on the item would show details about the item on the Detail Page

• Navigation Page

Navigation Page allows navigating from one page to the next by keeping a stack of all the pages. As the user goes deeper inside the app the pages get stacked one on top of the other and when the user tries to come back to the entry page thee pages are popped out in the reverse order of stacking

• Tabbed Page

Tabbed Image allows viewing multiple child pages across the same page wherein each Tab represents a child page. One child page is active at a time and it is easier to navigate to the other Tab child page by clicking on the Tab

• Carousal Page

Just like Tabbed page, Carousel Page is also Multipage. It is similar to the gallery. Only the difference is Instead of clicking on the tab, the Carousal allows swiping gestures on the pages to navigate between the multiple child pages

• Templated Page

When a user wants to show full mobile screen say for a Game or for showing splash Screen, Template Pages are used. They cover the entire screen area.

FreshMvvm is a super light Mvvm Framework designed specifically for Xamarin.Forms only. It's designed to be Easy, Simple and Flexible. It is easy to learn and uses convention over configuration.

Fresh MVVM deviates a little from MVVM as it uses the concept of Page and PageModel instead of View and ViewModel.

The various features of Fresh MVVM are:

• Every Page must be associated with a Page Model
• The PageModel can have an Init method to initialize Model variables and a ReversInit() method (a destructor) that is called when a model is popped with object
• Page Model allows dependencies to be automatically injected into the Constructor
• A Page must have a corresponding PageModel, with naming important so an ImagePageModel must have an ImagePage
• PageModel can have dependencies automatically injected into the Constructor
• The navigation in Fresh MVVM takes place between Page Models.
• Supports automatic wiring of BindingContext
• Supports automatic wiring of Page events (eg. appearing)
• FreshMvvm comes with a built-in Inversion of Control Container
• Page Model Important Methods:
  • Init(): The Init() method is called when the page is about to be loaded. User can pass an "object" to this method to allow setting up the Page with external data before it is being loaded
  • ReversInit(): The ReverseInit()  method is called when the page I about to be popped from the stack. It allows returning an "object" which can contain the data returned by the Page
  • ViewIsDisappearing(): ViewisDisappearing() is called when the view is about to disappear from the screen
  • ViewIsAppearing(): ViewisIsAppearing() is called when the view is about to appear on the screen.

MVVMCross

MVVMCross is a .NET cross-platform MVVM framework. It allows making cross-platform solutions, for platforms such as Xamarin.Forms, Xamarin.Android, Xamarin.iOS, Xamarin.Mac, Xamarin.tvOS, UWP, and WPF. It is currently inactive state of development.

MVVM Cross requires the application to be divided into two parts: the Core and the UI. The Core part contains the View Models, Service, Models, and the Business logic whereas the UI parts consist of the different Views and platform-specific code that interacts with the Core. The views are View Screens that contain the graphical content. In addition to Core and UI, the application may contain additional libraries for various functionalities.

The various high-level features that MvvmCross provides are:

• An MVVM architecture pattern
• A flexible Navigation system
• Data Binding to allow source-target automatic binding
• Platform specifics support
• Inversion of Control container
• Dependency Injection engine
• Plugins for common functionalities. A lot of customizable plugins are available for common tasks.

The cons of using MVVM Cross are:

• It has a steep learning curve and lacks a Wiki for how to get started developing apps.

MVVM Light

MVVM Light is another Framework that allows the creation of Enterprise-Grade apps using the MVVM Architectural pattern on Xamarin. MVVM Light enables developers to create and develop  MVVM applications on a variety of platforms like Xamarin.Forms, Xamarin.Android, Xamarin.iOS, UWP, and WPF. It allows you to separate the View from Model and hence right more testable and extensible applications.  It is not in active development now. It does not support async commands.

A typical MVVM Light application is divided into the following parts:

• View: It is a platform-specific user interface that is associated with the screen
• View Model: IT exposes the data and the properties related to data
• ViewModel Locator: It registers custom ViewModels.

MVVM Light advocates the use of a Messenger for loosely-coupled communication everywhere in the app, including data binding, dialogs, and navigation:

• Supports two-way data binding
• Supplies a DelegateCommand class for wiring up ViewModel commands. 

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XAML(Extensible Application Markup Language) allows developers to define the UI in Xamarin.Forms application using markup instead of code. The Designer allows previewing this UI thereby simplifying the design process.

The Xamarin.Forms pages have markup which has the responsibility of visual design for the UI and associated code to specify some action or logic based on interaction with the UI.

Xamarin.Android additionally supports AXML which is also a markup language specifically made for Xamarin.Android.

The main advantage of using XAML is it provides a clean separation of markup and code, so while the developer can concentrate on the behavior of the application (i.e. the code), the graphic designers can focus on the look and feel of the application. XAML can be compiled, and it takes full advantage of the Longhorn graphics subsystem, helping developers to produce great visual effects. In a XAML file, there is a clear hierarchy of graphical elements and it is easy to understand the GUI design of the app i.e. which element encloses which other elements. The separation of concerns ensures that any changes to the visual aspects of the application may not affect the business logic.

The drawbacks of using XAML are, XAML cannot contain any logic, all the logic has to go into separate code files. It does not directly support conditional processing or loops for repetitive processing of tasks

The lifecycle of an app contains the sequence of methods that are called since the time the app is launched until the time it is in the memory. The Lifecycle methods allow developers to write code for initialization or logic or transition so that the app components are initialized or show a specific behavior as and when they transit from one state to another. The developer may want to save state, initialize graphical components with data or reset apps state or free memory during these App methods.

The various life cycle methods of XamarinForms Apps are:

• OnStart: when the application initializes onStart is called. It is the best place to initialize objects
• OnSleep: Once the application goes to the background, the state is called ‘resume’. The app does not die here rather is kept in memory but with reduced priority. OnSleep() is similar to OnPause() in Android
• OnResume: OnResume is called when the application is resumed, i.e after being sent to the background. Similar to OnResume on Android.

While writing an application with MVVM, developers specify data bindings in the View Model to bind the UI and the underlying code. Sometimes it may be required that instead of simple properties, the app may have to react to commands initiated by a user that affect something in the View Model. Such commands are generally associated with button clicks of a button or Tap event of a gesture and are handled by the appropriate background handler. These Commands, that implement the ICommand interface, define the operation to be performed when the appropriate Click or Tap is performed. To use the command interface, one has to define a data binding that targets the Command Property of the Button whereas the source is a property in the ViewModel of type ICommand.

The Xamarin.Forms Entry is used to specify single-line text input. It is like a TextBox where the user can enter text characters. It, just like the Editor view, supports multiple keyboard types. Entry can also be used as a password field where each character entered is replaced by a * on the Entry thereby hiding the actual characters.

Editor, just like entry is used to enter the text input, the difference being the Editor allows multiple- line input. Editor also has a Tet property that represents the text that is associated with the Editor control. Editor and Entry can be both used to specify placeholder text thereby allowing the developer to give hints with sample text to make it easy for the developer to enter text in a correct format.

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Triggers allow the developer to specify actions declaratively in XAML to change the appearance of controls based on events or property changes. For eg, you may want to change the appearance of a button when it is in pressed state vs when it is not pressed. A trigger can be directly assigned to control.

There are namely four types of triggers:

• Property Trigger: Property Trigger comes into action when the value of the property of a control is assigned a value
• Data Trigger: It uses data binding to associate changes for some other control. When there are changes to another control, Data Trigger triggers changes in the control
• Event Trigger: An Event trigger causes an action to happen when an event takes place
• Multi-Trigger: This is a special type of trigger which gets triggered when multiple trigger conditions get satisfied resulting in the raising of this trigger.

To provide a seamless experience to users iOS provides metadata to the system in the form of info.plist (information property list file). This metadata is used by the system to identify the app, document types it supports, and to facilitate the launch of apps.

The property list can be accessed by the system at runtime. It containskey-value pairs. These key-value pairs describe the various behaviors and configuration options for the app. For every app, the Xcode provides info.pList files with default key values which can be later edited to suit the needs of the application

The various types of keys defined in info.plist files are:

1. Core Foundation Keys: The keys specified in use the prefix 'CF' to distinguish them with other keys. They provide information related to bundles which help in loading them and knowing their content
2. Launch Services Keys: They use the prefix LS to distinguish them from other keys. They provide metadata related to the launch time behavior of the app
3. Cocoa Keys: Cocoa keys use the prefix NS to distinguish them from other keys. Cocoa keys define metadata related to Cocoa Touch Apps
4. iOS Keys: UIKit keys use the prefix UI to distinguish them from other keys
5. App Extension Keys: App extensions enable you to provide features to other apps in iOS and macOS. App Extension Keys provide system information about an app extension's capabilities and intents.

Xamarin.iOS allows you to use the existing .NET System.IO class to carry out file-related operations on iOS.

The File class provided by .NET lets you create, delete or read files, and the Directory class allows you to create, delete or enumerate the contents of directories. You can additionally also use the Stream classes for finer access to files.

But, iOS imposes certain restrictions regarding what an application can do with the file system to maintain the security and integrity of the file system and prevent malignant apps from creating havoc. It is restricted to mostly access its home directory to read and write files and cannot interfere or read other applications directories. A set of rules apply on iOS to limit an app's access to files, preferences, network resources, hardware, etc.

It is important to understand that the iOS file system is case-sensitive so 'file' and 'File' are interpreted differently on iOS. iOS uses the forward-slash ‘/'as the path separator, so to maintain a common flow, System.IO.Path.Combine is preferred instead of explicitly specifying the path separator.

Sometimes a developer has a requirement to store some basic user data dealing with user settings and app configuration. There are plenty of ways that Xamarin.iOS provides to save this data in your app. One such way is through NSUserDefaults.

The NSUserDefaults class is very much like plist in the sense it stores key-value pairs.

It is not recommended storing anything with sensitive user information (username, password, etc) as these values are saved in a plain text .plist file in the documents directory.

To read from or write to NSUserDefaults, one has to get a reference to it.  In the simplest use of NSUserDefaults, this is done via a method that returns a reference to an object capable of interacting with NSUserDefaults's data store.

Xamarin.iOS supports linking with both native C libraries and Objective-C libraries.

For linking with third-party C libraries, it is required that all the libraries be linked statically.

Following are the steps one needs to follow to consume third party C Libraries:

• Bring the Library into your project: Select your project from the Solution Explorer and locate the file on your disk and add it to the project. When prompted, tell Visual Studio for Mac or Visual Studio to copy it into the project. After adding it, find the library in the project, right-click on it, and set the Build Action to none.
• Configure Xamarin.iOS to link the library: Now on the project options of your project,  add in iOS Build's extra argument, the "-gcc_flags" option followed by a quoted string that contains all the third-party libraries that are required for your project
• Access the methods from the library: For this,  use Mono's Platform Invoke (P/Invoke) to access these methods from C#.

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A Service is an application component that allows a user to run long operations that do not require user interaction. Services are broadly classified into Started and Bound Service.

A Started service is one which is invoked by call StartService(). Once started the service can run indefinitely in the background, even after the component that started it is destroyed. Usually, a started service performs a specific operation (like downloading a file) and stops itself. A  User-defined Started Service can be created by either extending the Service class or by extending the Intent Service. An Intent Service is a special type of Service which stops itself automatically after completing the task assigned.

A Bound Service is created when the application component calls BindService() to bind to a service. Bound Service is like a Client-server interface that allows components to interact with the service, send requests, get results, etc. A Bound service can be accessed by the components within an application and also by components residing in other applications (if it is exported/exposed). Bound Services only keep running till they have a client associated with it. When all clients Unbind the service the service stops

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AsyncTask is a facility given by Android for executing operations in a background thread without having to manually handle thread creation or execution

Aynctask has following important callback methods:

• OnPreExecute:  OnPreExecute is invoked before the actual Background Task is executed. It is used to initialize the Asynctask or setup the task  For example setting up the progress bar to show while the task is being executed
• DoInBackground(Params … params): This is the main call back function which contains the code to do the background operation which may take a long time. The parameters of Asynctask are passed to this step and doInBackground can utilize them
• OnProgressUpdate: On Progress Update is a method which is called repeatedly and can be used to publish the progress that has been made by the background tasks so far
• OnPostExecute (Result result). : This is invoked on the UI thread. When the task is completed in DoInBackground the result is returned to this call-back. The user can display the result or store for an app to process.

Asynctasks should always be created and loaded on the GUI thread. They cannot be started more than once.

A Pending Intent wraps another intent object. A Pending Intent means the intent that is being enclosed may not be executed now but some time in the future. Pending Intent can be passed on to a foreign application thereby granting that app the right to perform the operation represented by the intent

A PendingIntent itself is a token referencing the original content of the intent viz the action, data, categories, etc. Even if the owning app is killed by the system, PendingIntent can still be executed and it will again bring back the app back in memory. An app after providing the PendingIntent to a foreign app can cancel the need be. PendingIntent can be created for an Activity(through GetActivity() or Broadcast ( via GetBroadcast Method or service (via GetService() method).

Unlike Android, iOS does not allow users to directly install third-party apps. Only those apps that are signed by Apple can only be installed by the iOS users. The provisioning profile is a link between the Developer of the app (his Developer Account) and the Device on which it is being used. The provisioning profile is downloaded from the developer account and the entire bundle is code-signed by the iOS. A Development Provisioning Profile must be installed on each device on which the developer wishes to run his application. If the information in the provisioning profile doesn't match certain criteria, the app won't launch.

A provisioning profile necessarily consists of:

1. Development Certificates : These are for developers who want to test the app on a physical device while writing code
2. Unique Device Identifiers:  the List of devices that the app can run on
3. An App ID: It is a two-part string used to uniquely identify different apps from a single development team.

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The Navigation Page in Xamarin.Forms provide a Hierarchical Navigation to the user. The Navigation is implemented via a stack on the principle of Last In First Out or LIFO. When a user opens the app a default page sits at the top of the stack of the Navigation system. As the user navigates to a new page, the new page gets pushed onto the Navigation stack and becomes the active page.

In Hierarchical Navigation, NavigationPage Class is used to Navigate through a stack of ContentPage Objects. The Navigation Page is the root Page and content Pages are the child Pages on top of it.

The layout of the NavigationPage is platform-dependent i.e. depending on the platform on which it is displayed it has a different appearance:

• On iOS, it displays a title and has a back button to enable navigation to the previous page
• On Android, A Navigation Br is present at the top. It has a title, an icon, and an Up button that allows the previous page to be accessed
• On Universal Windows Platform, a navigation bar is displayed at the top that displays a title. 

Mobile gestures are the movements made by a user to perform a specific action on a control within a mobile interface majorly through fingers(tap, swipe, drag, slide, etc.). The various Gestures supported by Xamarin.Forms are:

1. Tap Gesture: Xamarin supports the tap gesture. A tap gesture is recognised when the user touches the mobile screen with single or multiple fingers once or twice in quick succession. The Tap Gesture is recognized with TapGestureRecognizer Class class.
2. Pinch  Gesture: Pinch Gesture occurs when the user places two fingers on the screen at a distance and brings them towards each other or keeps them together and moves them away from each other. This gesture is common to zoom in or out in a map mode. The Pinch Gesture is recognized with PinchGestureRecognizer Class.
3. Pan Gesture: A pan gesture is used for detecting the movement of fingers around the screen. These movements are then applied to the content currently at the display. It is recognized with  the PanGestureRecognizer class
4. Swipe Gesture: A swipe gesture occurs when a finger is moved across the mobile screen in a horizontal or vertical direction and is often used to initiate navigation through content. Swipe gestures are recognized with SwipeGestureRecognizer class.

This, along with other Xamarin questions and answers, is a regular feature in Xamarin interviews, be ready to tackle it with the approach mentioned.