Use of a partial class to implement platform-specific logic in Unity.

Login.cs

namespace Login {

	public static partial class Login {
  
		// Call this from the outside to log in
		public static void LogIn() {
			DoLogIn();
		}
		
		// Implementation delegate
		static partial void DoLogIn();
	}
	
	#if UNITY_IOS
	public static partial class Login {
		static partial void DoLogIn() {
			// iOS login logic goes here...
		}
	}
	#endif
	
	#if UNITY_ANDROID
	public static partial class Login {
		static partial void DoLogIn() {
			// Android login logic goes here...
		}
	}
	#endif
}

The advantage to using partial classes is that it lets us split the platform-specific definitions into different files so you don't wind up with one unreadable monolith.

Note: the fact that this class is static is not necessary, and only done here for simplicity of demonstration.