CODEWITHSUNDEEP
c#uwp
Oleksa
Oleksa

Reputation: 675

How is UWP main App singleton implemented?

Answers to other questions about singleton implementation state that a constructor should be private for a singleton class. But UWP main App singleton class derived from Windows.UI.Xaml.Application has a public constructor. XML documentation generated by VS for it says:

/// <summary>
/// Initializes the singleton application object.  This is the first line of authored code
/// executed, and as such is the logical equivalent of main() or WinMain().
/// </summary>

When I try to create a second instance on mouse event I get

Catastrophic failure (Exception from HRESULT: 0x8000FFFF (E_UNEXPECTED))

So why is constructor public and how is singleton implemented to force only one instance of App to exist?

Upvotes: 0

Views: 373

Answers (1)

Thomas Weller
Thomas Weller

Reputation: 59410

Singleton pattern

The singleton design pattern restricts the instantiation of a class to one single instance. How that is achieved is not defined by the pattern itself, because the pattern is independent of programming languages. You can e.g. use a .Instance property or a .GetInstance() method - or even name it whatever you like, which may confuse people, but it will still be a singleton.

Using a private constructor seems to be good practice, because it is helpful as it will generate a compiler error when trying to use the private constructor. But: some programming languages don't even have a private keyword.

Obviously, UWP chose to restrict the number of instances in a way that causes an exception at runtime.

Implementation

Looking at the call stack, the catastrophic failure happens in

at Windows.UI.Xaml.Application..ctor()
at UWP.App..ctor() in ...\App.xaml.cs:line 29
at UWP.App.OnLaunched(LaunchActivatedEventArgs e) in ...\App.xaml.cs:line 43

So the App class constructor calls the base class constructor which throws an exception. Since Windows.UI.Xaml.dll is not a .NET DLL, I can't look into it with dotPeek or dnSpy.

Upvotes: 1

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