Platform-specific C# code

Uno Platform allows you to reuse views and business logic across platforms. Sometimes though, you may want to write different code per platform. You may need to access platform-specific native APIs and 3rd-party libraries, or want your app to look and behave differently depending on the platform.

This guide covers multiple approaches to managing per-platform code in C#. See this guide for managing per-platform XAML.

Project structure

There are two ways to restrict code or XAML markup to be used only on a specific platform:

Use conditionals in a source file
Place the code in a separate file which is only included in the desired platform.

The structure of an Uno app created with the default Visual Studio template is explained in more detail here.

`#if` conditionals

The most basic means of authoring platform-specific code is to use #if conditionals:

#if HAS_UNO
Console.WriteLine("Uno Platform - Pixel-perfect WinUI apps that run everywhere");
#else
Console.WriteLine("Windows - Built with Microsoft's own tooling");
#endif

If the supplied condition is not met, e.g. if HAS_UNO is not defined, then the enclosed code will be ignored by the compiler.

The following conditional symbols are predefined for each Uno platform:

Platform	Symbol	Remarks
Android	`__ANDROID__`
iOS	`__IOS__`
Catalyst	`__MACCATALYST__`
macOS	`__MACOS__`
WebAssembly	`__WASM__`	Only available in the `net8.0-browserwasm` target framework, see below
Skia	`HAS_UNO_SKIA`	Only available in the `net8.0-desktop` target framework, see below
Non-Windows	`HAS_UNO`	To learn about symbols available when `HAS_UNO` is not present, see below

Tip

Conditionals can be combined with boolean operators, e.g. #if __ANDROID__ || __IOS__. It is also possible to define custom conditional compilation symbols per project in the 'Build' tab in the project's properties.

Windows-specific code

On net8.0-windows10 target framework, an Uno Platform application isn't using Uno.UI at all. It's compiled using Microsoft's own tooling. For that reason, the HAS_UNO symbol is not defined on Windows. This aspect can optionally be leveraged to write code specifically intended for Uno.

Apps generated with the default unoapp solution template use Windows App SDK when targeting Windows. While this is the recommended path for new Windows apps, some solutions instead use UWP to target Windows. Both app models define a different conditional symbol:

App model	Symbol	Remarks
Windows App SDK	`WINDOWS10_0_18362_0_OR_GREATER`	Depending on the `TargetFramework` value, the 18362 part may need adjustment
Universal Windows Platform	`NETFX_CORE`	No longer defined in new apps by default

WebAssembly considerations

The Uno Platform templates differentiate platforms using the target framework, yet it's possible to determine the WebAssembly platform at runtime using:

if (OperatingSystem.IsBrowser())
{
   // Do something WebAssembly specific
}

Note

JSImport/JSExport are available on all platforms targeting .NET 7 and later, and this code does not need to be conditionally excluded.

Type aliases

Defining a type alias with the using directive, in combination with #if conditionals, can make for cleaner code. For example:

#if __ANDROID__
using _View = Android.Views.View;
#elif __IOS__
using _View = UIKit.UIView;
#else
using _View = Windows.UI.Xaml.UIElement;
#endif

...

public IEnumerable<_View> FindDescendants(FrameworkElement parent) => ...

Partial class definitions

Heavy usage of #if conditionals in shared code makes it hard to read and comprehend. A better approach is to use partial class definitions to split shared and platform-specific code.

Starting from Uno Platform 5.2, in project or class libraries using the Uno.Sdk, a set of implicit file name conventions can be used to target specific platforms:

*.wasm.cs is built only for net8.0-browserwasm
*.skia.cs is built only for net8.0-desktop
*.reference.cs is built only for net8.0-desktop
*.iOS.cs is built only for net8.0-ios and net8.0-maccatalyst
*.macOS.cs is built only for net8.0-macos
*.iOSmacOS.cs is built only for net8.0-ios and net8.0-macos
*.Android.cs is built only for net8.0-android

Using file name conventions allows for reducing the use of #if compiler directives.

A simple example

Shared code in PROJECTNAME/NativeWrapperControl.cs:

public partial class NativeWrapperControl : Control {

...

  protected override void OnApplyTemplate()
  {
    base.OnApplyTemplate();
   
      _nativeView = CreateNativeView();
  }

Platform-specific code in PROJECTNAME/NativeWrapperControl.Android.cs:

#if __ANDROID__
public partial class NativeWrapperControl : Control {

...

  private View CreateNativeView() {
   ... //Android-specific code
  }

Platform-specific code in PROJECTNAME/NativeWrapperControl.iOS.cs:

#if __IOS__
public partial class NativeWrapperControl : Control {

...

  private UIView CreateNativeView() {
   ... //iOS-specific code
  }

You can use partial methods when only one platform needs specialized logic.