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Best practices for developing Universal Windows Platform (UWP) apps

This repository is maintained by Futurice, but contributions from anyone are highly encouraged! If you are interested in iOS or Android development, be sure to check our iOS and Android documents as well.

To keep this document easily approachable, it aims to be concise and practical: Each subtitle is an actual practice and contains short, but very practical description of what to do and what not to do. Some reasoning is included, but more detailed explanations and discussions are only included as external links. The listing tries to start of by taking care of the most common issues and end with the rarest ones.

Everything in the document should apply to either or both of the two latest stable versions of C#, Visual Studio, or the Universal Windows Platform, or to the latest publicly available version of the Windows Store. If there are relevant differences between the two latest versions, the implications should be described in the text. Practices that don't apply to either of the two latest versions should be removed.

Feedback and contributions are wholeheartedly welcomed! Feel free to fork and send a pull request, or just participate in the discussion at Issues.

Table of contents

Tooling and Documentation

Debugging and Exceptions

Architecture

Code Quality

Gotchas

Troubleshooting

Tooling and Documentation

Use Visual Studio Community/Pro or greater

Visual Studio is the de facto IDE for developing Windows apps. The Community edition gives you the same features as the Pro editon for free, but has some restrictions on use in organizations. Premium mainly adds built in testing support beyond simple unit testing, and Ultimate adds enhanced debugging, architecture, and code analysis tools.

The free express versions are good for getting started, but lack some important features, such as support for extensions, the ability to have all the different project types in a single installation and some team collaboration features. Since the Community editon has all the features of the Express editions we recommend not using the Express editions.

Use Productivity Power Tools on VS 2015

A free visual studio productivity extension from Microsoft. It lacks some features of the commercial counterparts like JustCode or ReSharper, but doesn't seem to slow the IDE down at all either.

There's also a version for VS 2017. However, as VS 2017 has some of the features built-in and the renewed Power Tools seem to have some stability and performance issues, the value of installing the addon should be evaluated on case-by-case basis.

Use NuGet

Nuget is Microsoft's take on a package manager. There's a Visual Studio extension called NuGet Package Manager preinstalled in newer Visual Studios. Bottom line: Use it for external references when you don't need to include the source code in your Solution.

Use Package Restore

According to NuGet docs:

Beginning with NuGet 2.7, the NuGet Visual Studio extension integrates into Visual Studio's build events and restores missing packages when a build begins.

This approach to package restore offers several advantages:

You are using the old package restore if you have clicked the "Enable NuGet Package Restore" -button in Visual Studio. If so, you should migrate: NuGet doc or with pictures.

Refer to the right documentation

There are three sets of APIs for UWP apps and the landing base for the documentations can be found at: https://docs.microsoft.com/en-us/uwp/. The three sets of APIs are:

Notice that F# is currently not supported.

Many of the APIs have existed (possible with differences) in incompatible .NET versions. Therefore, if you search e.g. for UIElement, you might end up at http://msdn.microsoft.com/en-us/library/system.windows.uielement%28v=vs.110%29.aspx, while the correct documentation for the UWP version can be found at https://docs.microsoft.com/en-us/uwp/api/Windows.UI.Xaml.UIElement.

To make sure you're in the correct documentation, check the page for Windows Store Apps, Universal Windows Platform, or Windows 10 device family support.

There's also a generic landing page for UWP development, as well as specific landing pages for Design, Develop, and Publish related content. Design/Controls and patterns can be especially helpful for picking the right controls for your UI.

Use the Live Visual Tree and the Live Property Explorer

The VS 2015+ XAML inspection tools are a great asset when tweaking and debugging your UI. Unfortunately the Live Property Explorer doesn't save the changes you make to your XAML. However, after you find a good value for a property, you can click 'go to source' and modify the XAML while debugging. The XAML changes you make won't take effect until you relaunch the app, but at least you have now 'saved' the value you want to use. Notice that you have to be running your app on Windows 10 (device or emulator) to be able to use these tools.

You can find the official documentation on how to use them here.

Use the Windows Community Toolkit

The Windows Community Toolkit is the first place you should look for any UI controls or other functionality missing from the core UWP framework. It is an open source project maintained by both Microsoft and the community. There's also an excellent sample app in the Windows Store that enables you to easily evaluate whether the library offers something valuable for your project.

If you get a certificate error when sideloading an UWP app, install the cert from the package

On Windows 10 Anniversary Update (10.0.14393) or newer, you can enable Sideload Apps or Developer Mode in Settings and simply double click an .appx or .appxbundle to install an UWP app. However, if you haven't installed the certificate that was used to create the app package, you will get an error. You can install the certificate by opening the .appx or .appxbundle Properties -> Digital Signatures -> Details -> View Certificate -> Details -> Install Certificate. Select Local Machine as the Store Location Group and Trusted People as the Store Location.

Debugging and Exceptions

Regularly debug UWP apps with a .NET Native Debug build

UWP app Debug builds are JIT-compiled, while Release builds are AOT-compiled using the .NET Native tool chain. JIT builds compile much faster and offer better debugging experience, while native builds offer better runtime performance. The catch is though, that in some (special cases) your Release build might behave differently to your Debug build. To find these cases as early as possible, it's a good idea to regularly debug with a Debug build compiled with the .NET Native tool chain. To enable you to easily switch between "native debug" and other build configurations, you need to add a new solution configuration:

  1. Go to Build -> Configuration Manager
  2. Choose "Debug" as the "Active solution configuration"
  3. Open the "Configuration" drop down menu for your app project and choose "<New...>"
  4. Name the new configuration for example "Debug-native", pick Debug from the Copy Settings drop down menu, check "Create new solution configurations", and click "OK".
  5. Make sure you're active Solution configuration is now the new "Debug-native" configuration
  6. Select your app project and go to Project -> Properties
  7. Go to the Build tab and select "All platforms" from the "Platform" drop down menu
  8. Check "Compile with .NET Native tool chain"

If you run into the fore mentioned exceptions look at this MSDN page for help.

For more information on .NET Native compilation go to: MSDN

Use caller information attributes when tracing

When you add CallerMemberName, CallerFilePath, or CallerLineNumber attributes for optional parameters, the parameters get set with the file path, line number, and member name of the caller. The values are set into the method call at compile time, don't have a performance penalty, and are not affected by obfuscation.

Example from msdn:

// using System.Runtime.CompilerServices 
// using System.Diagnostics; 

public void DoProcessing()
{
  TraceMessage("Something happened.");
}

public void TraceMessage(
  string message,
  [CallerMemberName] string memberName = "",
  [CallerFilePath] string sourceFilePath = "",
  [CallerLineNumber] int sourceLineNumber = 0)
{
  Trace.WriteLine("message: " + message);
  Trace.WriteLine("member name: " + memberName);
  Trace.WriteLine("source file path: " + sourceFilePath);
  Trace.WriteLine("source line number: " + sourceLineNumber);
}

// Sample Output: 
//  message: Something happened. 
//  member name: DoProcessing 
//  source file path: c:\Users\username\Documents\Visual Studio 2012\Projects\CallerInfoCS\CallerInfoCS\Form1.cs 
//  source line number: 31

Log Exception.ToString()

Do not just log Exception.Message, it lacks a lot of useful details. Use Exception.ToString() instead, it contains all the necessary information, including exception type, message, stacktrace, and inner exceptions.

To log and handle unhandled exceptions subscribe to App.UnhandledException and TaskScheduler.UnobservedTaskException

Most of the unhandled exceptions end up in the App.UnhandledException handler, however if the "Always use or await the return value of an async method" -pratice is not followed, they end up in the TaskScheduler.UnobservedTaskException instead.

Sources: filipekberg.se, msdn

Only throw exceptions in exceptional cases

Exceptions are slow and are by definition exceptions. Additionally each exception thrown during the normal execution of the codebase makes using the Visual Studio feature to break the debugger when any or some exceptions are thrown a pain. Ideally you could always have the debugger break when a managed exception is thrown. Then, every time your debugger breaks, there's something noteworthy for you to look at. Either there's a programming error, something wrong with your debugging setup, or a device malfunction.

This applies to both designing APIs and using APIs. There's often a way to run a programmatic check to avoid expected exceptions.

Do:

if (!closable.IsAlreadyClosed) {
  closable.Close()
} else {
  DoSomething();
}

Don't:

try {
  closable.Close()
} catch (AlreadyClosedException e) {
  DoSomething();
}

MSDN on throwing exceptions

Catch exactly the exception you expect

If you expect a NetworkException, write "catch (NetworkException e)", not just "catch (Exception e)". By catching a more general exception, you hide programming errors inside the try-block. For example, a NullReferenceException would be swallowed, and make it a lot harder to notice. Additionally, any recovery code you have in the catch-block might not work as intended for other than the specific exception it was written for.

A good article on handling exceptions

Catch OperationCancelledException to detect cancellation of an async function

When you cancel a Task using a CancellationToken a TaskCancelledException is thrown (when the Task is awaited). However, if you pass a CancellationToken into an async function and it uses CancellationToken.ThrowIfCancellationRequested to implement cancellation, an OperationCanceledException is thrown. Without looking inside, it's impossible to know which exception the async function might throw. However, as TaskCanceledException inherits from OperationCanceledException, you can just always catch OperationCanceledException to be safe.

If you need the TaskCanceledException, for example to access its Task property, you can catch that as well (before catching the OperationCanceledException). Or you can try to cast the caught OperationCanceledException into it.

Set Visual Studio to break debugger every time a CLR exception is thrown

If you have followed the practices above, exceptions should only be thrown in/into your code as a result of a programming error or something unrecoverable such as an OutOfMemoryException. Generally, When you make an error, you want to be notified about it as loud and clear as possible. The default behavior for Visual Studio is to only break debugger on uncaught exceptions. Now, if you have some generic catches in place to swallow exceptions, for example from some of your secondary components, such as analytics, you'd miss the unwanted behavior.

On Visual Studio 2013 go to: Debug -> Exceptions... and check the "Thrown" cloumn checkbox on the Common Language Runtime Exceptions. On Visual Studio 2015 and 2017 go to: Debug -> Windows -> Exception Settings.

If you are using any synchronous APIs that throw exceptions even in expected cases, you might have to leave those unchecked. Also, you might want to uncheck TaskCanceledException and OperationCanceledException.

When rethrowing an exception use just "throw" or include the original exception in the new exception

Sometimes you want to catch an exception and rethrow it as is or with additional information. For example:

try {
  ExceptionThrowingOperation();
} catch (Exception e) {
  if (CanRecover) {
    Recover();
  }
  else {
    // Different options for rethrowing
    throw e; // Don't do this. It basically throws a new exception, with a new stacktrace.
    throw; // Do this if you want to rethrow the same exception. It keeps the stacktrace (Except if the original exception occured in the same method. In which case you will loose the line information. There are ways around this however).
    throw new SpecificException("my message", e); // Do this if you want to throw a more specific exception with additional information. It sets the original exception as the inner exception of the new exception. Therefore, the stacktrace will be preserved.
  }
}

Use ContinueWith and Task.Exception to handle exceptions from async methods in expected cases

Some APIs like to throw exceptions even on expected cases. Check, for example, the next practice for explanation on why this is not necessarily the best kind of behavior. One good example of such API is Windows.Web.Http.HttpClient, which throws Exceptions on network errors. A network error is hardly an unexpected event. Especially if your app is supposed to fallback to a cached/alternative value in such cases. Fortunately, there's a way to utilize Task on asynchronous methods to avoid getting the exception thrown into your code while still handling it. For example, here we convert all network errors to HttpStatusCode.RequestTimeout without letting an exception to be thrown:

var client = new HttpClient();
var request = new HttpRequestMessage(HttpMethod.Get, new Uri("http://www.futurice.com"));

var responseTcs = new TaskCompletionSource<HttpResponseMessage>();
await client.SendRequestAsync(request)
  .AsTask()
  .ContinueWith(task => {
    HttpResponseMessage response = null;

    if (task.Status == TaskStatus.Faulted) {
      // You will have to access the task.Exception property to mark the Exception 'observed' otherwise it'll end up in TaskScheduler.UnobservedTaskException
      // Use GetBaseException() to get the original exception from the AggregateException.
      var exception = task.Exception.GetBaseException();
      response = new HttpResponseMessage(HttpStatusCode.RequestTimeout);
      response.ReasonPhrase = exception.ToString();
      response.RequestMessage = request;
    }
    else {
      response = task.Result;
    }

    responseTcs.TrySetResult(response);
  });

HttpResponseMessage responseMessage = await responseTcs.Task;

Utilize the memory dumps when debugger doesn't cut it

On newer WP8 firmwares you can to got Settings->Feedback and turn on storing the dumps on the device. The dumps will be stored in Documents\Debug folder on your phone. On Windows 8 you can merge this registry entry to enable storing dumps into C:\crashdumps. Remember to replace the YOUR_EXE_NAME in the .reg with the name of your app's exe. On Window 8, you can also generate dumps manually by right clicking your app in the Task Manager and choosing "create dump file".

Dumps can be analyzed for example using Visual Studio or WinDbg.

Architecture

Use Windows.Web.Http.HttpClient for your HTTP needs

Out of the different HTTP APIs available, HttpClient(s) are the newest ones that support async/await and automatic request decompression. Notice that there are two of them now: System.Net.Http.HttpClient and Windows.Web.Http.HttpClient. The latter was added to Windows 8.1 and is a WinRT API instead of a .NET API. More importantly System.Net.Http might be made unavailable for Windows Store Apps in the future. Additionally, the Windows.Web.Http classes offer HTTP/2 support, better caching, and better configurability all around.

One useful thing to know about the Windows.Web.Http.HttpClient is that it throws an Exception for network errors that prevent a succesful HTTP-connection. You can get the actual reason with Windows.Web.WebError.GetStatus(HRESULT). For example:

var client = new HttpClient();
var request = new HttpRequestMessage(HttpMethod.Get, new Uri("http://www.futurice.com"));
try {
  // Http-errors are returned in the response, and no exception is thrown
  HttpResponseMessage response = await client.SendRequestAsync(request);
}
// It really is just an exception, can't catch a more specific type
catch (Exception ex) {
  WebErrorStatus error = WebError.GetStatus(ex.HResult);
  // For example, if your device could not connect to the internet at all, the error would be WebErrorStatus.HostNameNotResolved
}

Use CultureInfo.InvariantCulture for serializations

When ever you are serializing values that can be represented differently in different cultures, make sure that you serialize and deserialize them with the same culture. If you don't define the culture explicitly, the APIs normally use the culture of the current thread, which is often set by a setting in the operating system. CultureInfo.InvariantCulture is an IFormatProvider that exists exactly for the purpose of hardocoding the culture for serializations.

For example, serialization of DateTime, double, float, and decimal depend on culture. If you serialize these values using the OS provided culture, and the culture has changed when you deserialize, you are likely to run into runtime exceptions. as the expected date format, or the decimal separator might've changed.

So, don't do this:

string serializedDateTime = dateTime.ToString();
// Write serializedDateTime to a presisten storage.
...
// Load serializedDateTime from presistent storage.
// If the thread culture was, let's say, en-us when you serialized the datetime, and now it has been changed for example to fi-fi, you would get an FormatException: "String was not recognized as a valid DateTime".
DateTime dateTime = DateTime.Parse(serializedDateTime);

Do this:

string serializedDateTime = dateTime.ToString(System.Globalization.CultureInfo.InvariantCulture);
// Write serializedDateTime to a presisten storage.
...
// Load serializedDateTime from presistent storage.
// No matter what the current thread culture during the serialization and deserialization, it will use the special InvariantCulture date formatting settings, and just work.
DateTime dateTime = DateTime.Parse(serializedDateTime, System.Globalization.CultureInfo.InvariantCulture);

Sources: MSDN, Channel9

Always add en-US store description for your app if your users know English

Windows 10 store limits the visibility of apps based on the store description languages and the selected language of the device. For example, if your app only has a store description in Finnish, it won't show up on devices that have their primary language set to English or Swedish - A very common case in Finland. Exempt to this rule seems to be en-US, which seems to act as a 'locale for everybody'. If your app (also) has an en-US store description, it is (apparently) visible for all users.

Know the timers

There are different timers for different purposes. For example DispatcherTimer for WinRT, DispatcherTimer for WP Silverlight and ThreadPoolTimer. Additionally there are Observable.Timer, Task.Delay, and last (and least) Thread.Sleep.

Use lazy loading

Lazy loading is a concept of deferring loading of an object until the object is actually needed. It doesn't necessary mean creation of a new instance of an object, but often involves a heavy operation that returns an object. Lazy can save memory and unnecessary processing in cases where running a heavy operation or creation of a heavy object is only needed in optional or late execution paths.

C# 6 allows for a very concise and low overhead lazy properties for the simplest cases. Note that this approach isn't thread safe and doesn't consider null to be a legitimate 'loaded' value, but should work just fine for constrained cases.

private MyHeavyClass _heavy;
public MyHeavyClass Heavy => _heavy ?? (_heavy = HeavyOperation());

Now, accessing Heavy would either return the instance from _heavy, or if it's null, call HeavyOperation, set the return value to _heavy and return it to the caller of Heavy. Notice that if HeavyOperation returns null, it will be called again on the next time Heavy is accessed.

There are at least three other ways to implement lazy loading in .NET: Lazy<T>, LazyInitializer, and ThreadLocal<T>. All of these are explained at MSDN, but to summarize:

Use yield when returning an IEnumerable

Rather than writing something like:

public System.Collections.Generic.IEnumerable<Galaxy> Galaxies {
  get {
    return new List<Galaxy>() {
      new Galaxy { Name = "Tadpole", MegaLightYears = 400 },
      new Galaxy { Name = "Pinwheel", MegaLightYears = 25 },
      new Galaxy { Name = "Milky Way", MegaLightYears = 0 },
      new Galaxy { Name = "Andromeda", MegaLightYears = 3 },
    };
  }
}

Write this instead:

public System.Collections.Generic.IEnumerable<Galaxy> Galaxies {
  get {
    yield return new Galaxy { Name = "Tadpole", MegaLightYears = 400 };
    yield return new Galaxy { Name = "Pinwheel", MegaLightYears = 25 };
    yield return new Galaxy { Name = "Milky Way", MegaLightYears = 0 };
    yield return new Galaxy { Name = "Andromeda", MegaLightYears = 3 };
  }
}

It will automatically return empty IEnumerable if no "yield return" is called. This will avoid null reference exceptions when you're expecting to get an IEnumerable. The yield approach also only runs as far as is required by the possible iterator. For example, the following only creates one Galaxy instance, avoiding unnecessary processing.

var firstGalaxy = Galaxies.First();

Explicitly convert LINQ queries into collections to avoid unnecessary re-evaluation

Probably the #1 thing to know about LINQ is that it creates a query that is executed whenever its items are accessed. Sometimes this is what you actually want. However, often you just want to run the query once but access the resulting items multiple times. To avoid unnecessary re-evaluation and bugs resulting from the query result changing, use ToArray, ToList, etc. extension methods to run the query and store the results into a collection.

So rather than:

var sItems = MyItems.Where(i => i.Name.Contains("s"));
var firstSItem = sItems.First(); // Query executed
var lastSItem = sItems.Last(); // Query executed
MyItems.Add(new MyItem("spoon"));
Handle(sItems.Last()); // Query executed, returns the spoon item

Do this instead:

var sItems = MyItems.Where(i => i.Name.Contains("s")).ToList(); // Query executed
var firstSItem = sItems.First(); 
var lastSItem = sItems.Last();
MyItems.Add(new MyItem("spoon"));
Handle(sItems.Last()); // returns the lastSItem

Use ItemsStackPanel over VirtualizingStackPanel

ItemsStackPanel was added into Windows 8.1 and should be used over VirtualizingStackPanel. If item grouping is used, VirtualizingStackPanel realizes the whole group of items even if only the first one was required. ItemsStackPanel handles items virtualization correctly also when groups are used and will therefore offer better performance.

Source: MSDN Blog

Use templated controls over user controls

There are two main ways to 'package' a piece of UI into a reusable component. User controls inherit from UserControl and are composed of a C#/VB file and an attached XAML file. Essentially both of them are partial definitions of the same class. Templated controls are classes inherited from Control and have their Template property set to a ControlTemplate defined in XAML.

The issue with UserControls is that their XAML is parsed every time an instance of the control is created. Additionally, UserControls are loaded even if they are created with Collapsed Visibility, while Templated Controls are only be loaded when their Visibility is set to Visible. These details can have significant effect on performance, especially if the controls are used in list items. Templated controls also offer better reusability with re-templating and styling.

To create a new templated control select Project -> Add -> Add New Item -> Templated Control. This will add a new class that inherits from Control and sets the DefaultStyleKey in it's constructor, as well as adds an empty default style for the control into Generic.xaml file in your project.

To avoid having to create a new control every time you just want to make a piece of XAML reusable, you can create a templated control that doesn't set the DefaultStyleKey and then just set the control's Style/Template to your reusable Style/Template when you instantiate the control.

For example:

...
<ResourceDictionary>
  <ControlTemplate x:Key="LogoWithBordersTemplate" TargetType="local:TemplateControl">
    <Border Padding="15" BorderThickness="2" BorderBrush="Black"
      <Image Source="Logo.png" />
    </Border>
  </ControlTemplate>
</ResourceDictionary>
...

<local:TemplateControl Template={StaticResource LogoWithBordersTemplate} />

Enable more reusability with data bindings, template bindings and styling:

...
<ResourceDictionary>
<Style x:Key="LogoWithBordersStyle" TargetType="local:TemplateControl">
  <Setter Property="Foreground" Value="Red" />
  <Setter Property="Template>
    <Setter.Value>
      <ControlTemplate x:Key="LogoWithBordersTemplate" TargetType="local:TemplateControl">
        <Border Padding="15" BorderThickness="2" BorderBrush="{TemplateBinding Foreground}"
          <Image Source="{Binding Logo}" />
        </Border>
      </ControlTemplate>
    </Setter.Value>
  </Setter>
</ResourceDictionary>
...

<!-- Borders are Red -->
<local:TemplateControl
  Style={StaticResource LogoWithBordersStyle} 
  DataContext={Binding SelectedCompany}/>

<!-- Borders are Yellow -->
<local:TemplateControl
  Style={StaticResource LogoWithBordersStyle} 
  DataContext={Binding SelectedCompany}
  Foreground="Yellow"/>

Templated controls and user controls have also been discussed in this issue.

Use independent animations over dependent ones

Dependent animations are animations that depend on the UI thread, the major drawback is performance. Independent animations are animations that can be run independent of the UI thread and therefore don't burden it and remain smooth even if the UI thread is blocked.

For implementing animations you should use Storyboards.

According to MSDN, all of the following types of animations are guaranteed to be independent:

Additionally you should use animations from the Windows.UI.Xaml.Media.Animation namespace when possible. The animations have "Theme" in their class name, and are described in Quickstart: Animating your UI using library animations.

Put XAML in Class Libraries into their own ResourceDictionary

In some cases it might be tempting to load a XAML snippet with System.Windows.Markup.XamlReader.Parse(yourXamlString) directly in C#. However, if your XAML references for example a control in another class library, the user of your library has to add a reference to that library as well. Otherwise an exception will be thrown when the XamlReader tries to parse the XAML.

First add for example a Resources.xaml into your project, and add the ResourceDictionary with it's contents into the file. You can then load the ResourceDictionary by creating a new ResourceDictionary instance and setting it's source as follows:

var libraryResources = new ResourceDictionary {
  Source = new Uri($"ms-appx:///NameOfMyClassLibrary/Resources.xaml", UriKind.Absolute)
};

Now you can also merge the ResourceDictionary to the app's App.xaml dictionary. Then the named resources can be accessed via Application.Current.Resources and be overridden in the App.xaml.

// Merging the library resources into the app's main ResourceDictionary
Application.Current.Resources.MergedDictionaries.Add(libraryResources);

// Accessing the resource (or it's overridden counterpart)
Application.Current.Resources["libraryResourceKey"]

If you're using Rx in your ViewModels, use ReactiveProperties and ReactiveCommands as well

If you aren't already using a library that offers you an easy way to bind into your reactive code from XAML, here's one that offers ReactiveProperty, ReactiveCommand and some other useful classes.

Code Quality

Split code into small methods to improve stacktraces and the callstack view

Code is often split into small methods for reusability. However, there are reasons to split your methods even if you don't plan to reuse them. Method name documents the intent of the code it encloses. This gives you more informative callstack view while debugging and better stacktraces from your exceptions. The stacktraces part applies especially to release builds, where source code line information is lost.

Always use or await the return value of an awaitable method

Exceptions from synchronous methods propagate up the call stack regardless if you use the possible return value or not. Awaitable methods work a bit differently. When an unhandled exception is thrown within an awaitable method, the exception is wrapped into the task object returned by the method. The exception is only propagated when you either await the task/method, or try to access a completed task's Result. When you access the Result or await the method within a try block, you can catch the unhandled exceptions from the awaitable method normally. Additionally, you can observe the exception by accessing the task's Exception property. However, be aware that reading the Exception property effectively 'catches' the exception. Be careful to not unintentionally swallow the exception this way. If you let the exception go through unobserved, the TaskScheduler.UnobservedTaskException will be fired when the task is garbage collected.

Use ConfigureAwait to avoid deadlocks when making a blocking call for an awaitable method

It's possible to cause a deadlock by making a blocking call (eg. .Result or .Wait()) on an incompleted Task that has been returned by an awaitable method. You should always avoid blocking on async code, but sometimes it's necessary as an intermediate solution when refactoring synchronous code to be asynchronous.

The following is an example of such deadlock:

var task = FetchData();
// task.Result will block the context until the Task returned by FetchData completes
var result = task.Result;
...
async Task<String> FetchData() {
  // With await, execution tries to continue on the captured context by default. However, as the context is blocked by task.Result, the execution can't continue and a deadlock occurs.
  return await _networkManager.RequestData();
}

In order to prevent the deadlock use Task.ConfigureAwait(false); to change await's continuation behavior to not try to marshal back to the captured context.

The following is an example on how to use ConfigureAwait to avoid the deadlock:

var task = FetchData();
// task.Result will block the context until the Task returned by FetchData completes
var result = task.Result;
...
async Task<String> FetchData() {
  // After RequestData completes, execution won't try to marshal back to the captured context (that is blocked), the Task returned by FetchData can complete, and the execution can continue as expected.
  return await _networkManager.RequestData().ConfigureAwait(false);
}

There is also another way to utilize ConfigureAwait to avoid the deadlock, as demonstrated by a utility method in Azure ActiveDirectory library for DotNet:

private static T RunAsyncTaskAndWait<T>(Task<T> task) {
  try {
    Task.Run(async () => await task.ConfigureAwait(false)).Wait();
    return task.Result;
  }
  catch (AggregateException ae) {
    // Any exception thrown as a result of running task will cause AggregateException to be thrown with actual exception as inner.
    throw ae.InnerExceptions[0];
  }
}

More info on the topic can be found in Stephen Cleary's blog post.

Use CallerMemberName when implementing INotifyPropertyChanged

Many MVVM frameworks already help you with notifying property changes from your viewmodels. However, if you don't use any of those, create a helper with CallerMemberName for yourself.

For example:

public class ViewModelBase : INotifyPropertyChanged
{
  public event PropertyChangedEventHandler PropertyChanged;

  /// <summary>
  /// Use this from within a property's setter
  /// For example: if (SetProperty(ref _propertysBackingField, value)){ OptionallyDoThisIfValueWasNotEqual(); }
  /// </summary>
  protected bool SetProperty<T>(ref T storage, T value, [CallerMemberName]String propertyName = "")
  {
    if (object.Equals(storage, value)) {
      return false;
    }

    storage = value;
    NotifyPropertyChanged(propertyName);
    return true;
  }
  
  
  /// <summary>
  /// You can use this from within a property's setter when you don't want to set a backing field. 
  /// For example: NotifyPropertyChanged();
  /// </summary>
  protected void NotifyPropertyChanged([CallerMemberName]string propertyName = "")
  {
    PropertyChangedEventHandler handler = PropertyChanged;
    if (handler != null) {
      handler(this, new PropertyChangedEventArgs(propertyName));
    }
  }  
}

Use the nameof operator when notifying changes to other properties

When notifying a change to a property outside of the property setter, you can't use CallerMemberName. Just hardcoding the property name into a string is dangerous as you might misspell it, or the property name might get changed later on. Use the C# 6 nameof operator instead to get the name of the property as a string.

For example:

NotifyPropertyChanged(nameof(MyPropertyWhoseGetterShouldNowReturnNewValue));

Don't make forward references with StaticResource or ThemeResource keywords

Altough writting something like the following might not fail, it carries a performance penalty compared to defining the MyColor resource before the MyBrush resource.


<ResourceDictionary>
  <SolidColorBrush x:Key="MyBrush" Color="{StaticResource MyColor}" />
  <Color x:Key="MyColor" Value="Black" />

Additionally, in some cases a forward reference will throw a runtime exception.

Source

Use x:Bind instead of Binding when possible

x:Bind relies on code generation instead of reflection and therefore has some advantages over the Binding-keyword. Most notably x:Bind can bind anything that is accessible from its scope (including methods and public fields), is type safe, and has better runtime performance. On the other hand, it can not be used everywhere the Binding can, for example in ControlTemplates.

Gotchas

UI thread is the thread with the smallest Managed ID greater than 0

When looking at the Threads window in Visual Studio, it's not exactly obvious which of the threads is the UI thread in your UWP app. Well, it's the one with the smallest Managed ID greater than 0.

Edit and Continue doesn't modify the deployed build

If you have the debugger attached to your running app and you use the Edit and Continue feature to modify the code, the changes won't be maintained in the deployed app after it is closed (and restarted without rebuilding and redeploying).

So if you:

  1. Start debugging an app
  2. Hit a breakpoint and modify the C#
  3. Continue running the app and see your changes take effect
  4. Close the app and stop the debugger
  5. Restart the app without rebuilding (For example from the all apps list)
  6. The changes you made in Edit and Continue won't be in effect

Visual states have to be defined in the root element of a ControlTemplate, DataTemplate, Page, or UserControl

<ControlTemplate, DataTemplate, Page, or UserControl>
  <Any Control>
    <VisualStateManager.VisualStateGroups>
      <!-- Visual states defined here will work as expected -->
...

<ControlTemplate, DataTemplate, Page, or UserControl>
  <Any Control>
    <Any Control>
      <VisualStateManager.VisualStateGroups>
        <!-- You will not get any kind of an error or exception, but visual states defined here will not work as expected -->
...

VisualStates with AdaptiveTriggers in a DataTemplate have to be wrapped into a UserControl

The AdaptiveTrigger in the following example will not work, and will therefore never activate the Narrow-state.

<DataTemplate>
  <Not a UserControl>
    <VisualStateManager.VisualStateGroups>
      <VisualStateGroup>
        <VisualState x:Name="Narrow">
          <VisualState.StateTriggers>
            <AdaptiveTrigger MinWindowWidth="0" />
...

For the AdaptiveTrigger to work within a DataTemplate, the VisualStates have to be wrapped into a user control:

<DataTemplate>
  <UserControl>
    <Any Control>
      <VisualStateManager.VisualStateGroups>
...

Key times have to be set for key frames in a key framed animation

When using a key framed animation (such as ObjectAnimationUsingKeyFrames), the KeyTime property of the key frames has to be set (even if you want it to be zero), or the key frame will never be applied.

Don't be fooled by the IObservable duration parameters in IObservable extension methods

In System.Reactive.Linq, at least Delay, Timeout, Throttle, and GroupByUntil extension methods for IObservable have overloads that take an IObservable parameter of type TDelay, TTimeout, TThrottle, or TDuration. These parameters are always used to indicate a duration and the duration is considered to be passed when the given IObservable completes. The actual type of the IObservable doesn't matter, but most of the time you'll want to use Observable.Timer to create the parameter.

For example, you could think that the following code makes all items of the observable timeout after one second:

.Timeout(vm => Observable.Return(TimeSpan.FromSeconds(1)))

However, that will simply timeout immediately. A correct way to use these parameters is for example:

// Notice the .Timer
.Timeout(i => Observable.Timer(TimeSpan.FromSeconds(1)))

XAML attribute order can matter when using databindings

In most cases the order of element attributes in XAML doesn't matter. However, as the properties are being set (and Bindings evaluated) one by one in the order the attributes appear, it's possible to run into a case where the attribute order does matter.

One such case is when binding to ItemsSource and SelectedItem of a Selector -based control. The following example throws an exception because when the SelectedItem is set the ComboBox doesn't have items (ItemsSource) at all. Switching the attributes around fixes the issue.

<ComboBox
  SelectedItem="{Binding SelectedCar, Mode=TwoWay}"
  ItemsSource="{Binding Cars}"/>

Binding only works for the children of FrameworkElements

While it is possible to create an instance of any class with a parameterless constructor in XAML, data bindings created using the Binding-keyword only work for DependencyProperties of instances that are children of a FrameworkElement (or it's derivate).

For example:

<Page>
  <!-- Class does not inherit from FrameworkElement -->
  <MyCustomClass>
    <MyCustomClass.Content>
      <!-- Binding will not evaluate -->
      <TextBlock Text="{Binding Title}" />

The x:Bind doesn't have this limitation.

ResourceDictionary code-behind has to be modified for x:Bind to work

For x:Bind to work in XAML placed into a ResourceDictionary, InitializeComponent has to be placed into the code-behind constructor of the ResourceDictionary.

More info can be found here.

Additionally, the x:Bindings in a ResourceDictionary will only work while the ResourceDictionary instance is alive. Therefore, if you instantiate the ResourceDictionary in code (new TemplatesResourceDictionary()), make sure that the instance stays alive as long as the realized DataTemplate instances are in use.

Troubleshooting

Uninstall the app installed from the store before trying to sideload the same app

You don't actually always need to uninstall it first, but sometimes you can get an unrelated error when trying to sideload app that is already installed to the same device from store. So, if you get an error that doesn't seem to make sense when sideloading, make sure the app isn't already installed to the device.

Unblock downloaded DLLs before referencing them in your projects

For security reasons, Windows usually 'blocks' files downloaded from the internet. If you try to add a reference to such a DLL in Visual Studio, you get an incorrect error message: "A reference to a higher version or incompatible assembly cannot be added to the project." Whenever you get the error, go to Properties of the DLL file and click Unblock. You should now be able to add the reference.

If your app crashes only when NOT debugging, check your App.OnSuspending/OnResuming

When your app is attached to the debugger, it doesn't get suspended as it normally does. This means that App.OnSuspending and App.OnResuming don't get called when for example using any of the APIs that open a system UI and push your app to background. Now, if you have a bug that causes a crash in either of these methods, you might not get the behavior you expect when NOT debugging.

Easy way to check is to place a breakpoint into the methods and use the Lifecycle Events tool in Visual Studio to get your app suspended while debugging.

If starting up an emulator or deploying to it never completes, try disabling your antivirus

At least Symantec Endpoint Protection is known to cause starting up an (WP8.1) emulator or deploying to it never completing. Just disabling the antivirus for a few seconds when the process seems stuck should enable it to continue. In some cases the problem only occurs when starting the emulator and you can keep the antivirus on while deploying.

If starting up an emulator never completes or gives an error, try deleting it in Hyper-V Manager

If your emulator gets stuck or shows an error when starting up, reinstalling the virtual machine might help. Kill the stuck or faulted emulator. Open Hyper-V Manager, right click the virtual machine for the problematic emulator, and choose Delete. Redeploying to the emulator from Visual Studio should reinstall the virtual machine, and hopefully get rid of the issue.

If you have problems with Visual Studio stability, try disabling the XAML Designer

In some cases XAML designer seems to cause Visual Studio to crash and freeze quite a bit. It might be helpful to disable the designer completely and just use Blend when you need to.

On Visual Studio 2015, go to Tools -> Options -> XAML Designer -> Enable XAML Designer.

On Visual Studio 2013, follow these instructions.

Make sure your packaged builds have code optimization and .Net native enabled

If you try to create an app package with builds for multiple architectures in it, you might get the following unhelpful error message:

Error 80080204: All app package manifests in a bundle must declare the same values under the XPath [local-name()='Package']/[local-name()='Dependencies']

To fix the issue, make sure that you have Optimize Code and Compile with .NET Native tool chain checked in the UWP project properties for all of the architectures.

License

Futurice Oy Creative Commons Attribution 4.0 International (CC BY 4.0)