UsingSharedProjects

Using Shared Projects

Some libraries have mostly shared code, but do have a few members that are platform-specific. This keeps all the code together, but still allows for multiple platform libraries.

These libraries are easy to develop using the infamous #if preprocessor directive. This is just like a normal if statement, but runs during compilation to conditionally include/exclude code. Although it is often best to avoid using too many preprocessor directives as they do tend towards chaos, and spaghetti code, they are a very powerful and useful tool.

Although custom definitions can be created, there are a few default preprocessor definitions that are available:

Platform	Definitions	Special Definitions
		DEBUG (only for the Debug configuration)
Android	__ANDROID__, __MOBILE__	__ANDROID_<api>__ (eg: __ANDROID_14__)
iOS	__IOS__, __MOBILE__	__UNIFIED__
macOS	__MACOS__	__UNIFIED__
tvOS	__TVOS__, __MOBILE__	__UNIFIED__
UWP	__WINDOWS_UWP__, NETFX_CORE
watchOS	__WATCHOS__, __MOBILE__	__UNIFIED__

More information on #if can be found in the C# documentation:
https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/preprocessor-directives/preprocessor-if

More information on the platforms and .NET Standard versions can be found in the .NET documentation:
https://docs.microsoft.com/en-us/dotnet/standard/net-standard#net-implementation-support

The Solution

Using an entirely shared approach, there will be 3 types of projects:

• one shared project (for the code)
• one .NET Standard project (for the bait)
• several platform projects - one per desired platform (for the switch)

Since all the code is in the shared project, the .NET Standard project and each of the platform projects just need to reference the shared project. Any other code that is not platform-specific can also exist in the shared project instead of being duplicated for each platform project.

Example Solution

An example scenario would be 5 projects:

• one shared project
• one .NET Standard project targeting netstandard1.0
• an Android project
• an iOS project
• a Windows 10 UWP project

Example Code

An example of what shared code may look like with #if, #elif and #endif preprocessor directive:

// a public class that is visible to any platform
public static class Screen
{
    // a public method that is visible to any platform
    public static double GetDensity()
    {
#if WINDOWS_UWP
        // code that is specific to UWP
        var di = DisplayInformation.GetForCurrentView();
        return di.RawPixelsPerViewPixel;
#elif __ANDROID__
        // code that is specific to Android
        var svc = Application.Context.GetSystemService(Context.WindowService);
        var wm = svc.JavaCast<IWindowManager>();
        var dm = new DisplayMetrics();
        wm.DefaultDisplay.GetMetrics(dm);
        return dm.Density;
#elif __IOS__
        // code that is specific to iOS
        return UIScreen.MainScreen.Scale;
#else
        // code that will run on any other platform
        throw new PlatformNotSupportedException();
#endif
    }
}

Consumer Project

The consumer app or library can then easily make use of the APIs:

void DoScreenThings()
{
    double density = Screen.GetDensity();
}

Example NuGet Package

There are many elements that can be used to customize the package, but only a few are required to create a bait-and-switch package:

<?xml version="1.0"?>
<package>
  <metadata>
    <!-- the unique package ID -->
    <id>PackageId</id>
    <!-- the current version of the package -->
    <version>1.0.0</version>
    <!-- the authors text for the gallery -->
    <authors>FirstName LastName</authors>
    <!-- the description for the package manager UI -->
    <description>Package description here.</description>
  </metadata>
  <files>
    <!-- the platform-specific assembly for Android -->
    <file src="output/android/Library.dll" target="lib/MonoAndroid" />
    <!-- the platform-specific assembly for iOS -->
    <file src="output/ios/Library.dll" target="lib/Xamarin.iOS" />
    <!-- the platform-specific assembly for UWP -->
    <file src="output/uwp/Library.dll" target="lib/uap10.0" />

    <!-- the bait assembly -->
    <file src="output/netstandard/Library.dll" target="lib/netstandard" />
  </files>
</package>