Directory.build.targets

<Project>
  <PropertyGroup Condition="$(TargetFramework.StartsWith('netstandard'))">
    <DefineConstants>$(DefineConstants);NETSTANDARD;PORTABLE</DefineConstants>
  </PropertyGroup>
  <PropertyGroup Condition="$(TargetFramework.StartsWith('Xamarin.iOS'))">
    <DefineConstants>$(DefineConstants);MONO;UIKIT;COCOA;IOS</DefineConstants>
  </PropertyGroup>
   <PropertyGroup Condition="$(TargetFramework.StartsWith('Xamarin.Mac'))">
    <DefineConstants>$(DefineConstants);MONO;COCOA;MAC</DefineConstants>
  </PropertyGroup>
  <PropertyGroup Condition="$(TargetFramework.StartsWith('Xamarin.TVOS'))">
    <DefineConstants>$(DefineConstants);MONO;COCOA;TVOS</DefineConstants>
  </PropertyGroup>
  <PropertyGroup Condition="$(TargetFramework.StartsWith('Xamarin.WatchOS'))">
    <DefineConstants>$(DefineConstants);MONO;COCOA;WATCHOS</DefineConstants>
  </PropertyGroup>
  <PropertyGroup Condition="$(TargetFramework.StartsWith('MonoAndroid'))">
    <DefineConstants>$(DefineConstants);MONO;ANDROID</DefineConstants>
  </PropertyGroup>
</Project>