Can I define constants based on the runtime identifier in .NET Core?

Question

I have a .NET Core Console application. My goal here is to be able to conditionally DLLImport a function and call it, but only on Windows runtimes.

I thought maybe if I could access the runtime identifier in the csproj file, I could conditionally define a constant for that runtime, then in my c# I could surround the DLLImport and calls in #if/#endif blocks.

Is it possible to set compilation constants within a csproj based on the runtime the project is being built for? This is specifically for an SDK-style Project format (that starts with <Project Sdk="Microsoft.NET.Sdk">) that is targeting .NET Core.

Note: this question gets close, but is for project.json style projects.

Alternately, is there a better approach to accomplish my goal?

Answer 1

If you are building and publishing for different runtimes by passing different --runtime options (MSBuild property RuntimeIdentifier), you can condition on that property in the csproj file (allowing you to use #if BUILT_FOR_WINDOWS in your C# code):

<PropertyGroup>
  <DefineConstants Condition="'$(RuntimeIdentifier)' == 'win-x64'">$(DefineConstants);BUILT_FOR_WINDOWS</DefineConstants>
</PropertyGroup>

However you can also test the current OS at run time using:

using System.Runtime.InteropServices;
…

if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
{
    // call windows function here
}
else
{
    // do something else here
}

As long as a function marked with [DllImport(…)] is not called on an OS where the library / method cannot be found, there shouldn't be any problems. Do note that DllImport() can also probe for different libraries depending on the os - so DllImport("foo") would check for foo.dll, foo.dylib, libfoo.so etc.

Answer 2

Adding to a Martin Ullrich's answer: if you want to define constants based on RuntimeIdentifier in a referenced library project as opposed to a project with application entry point make sure that you include the list of identifiers which you use in a RuntimeIdentifiers property in the project's .csproj file, for example:

<PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
    <RuntimeIdentifiers>linux-x64;linux-arm</RuntimeIdentifiers>
</PropertyGroup>

If you don't do it then the constants will not be defined as RuntimeIdentifier property will not be passed to the csproj, as was in my case.

Source: https://github.com/dotnet/core/issues/2678#issuecomment-498967871