Compiler Directive for detecting C# 8/nullable reference types

Question

Is there a compiler directive for C# 8 or detecting if <nullable>enable</nullable> is set?

I have a source code only Nuget package that could stand to help with the nullable reference types story. I'd like to conditionally set the #nullable enable directive if this is set, such as:

#ifdef ICANHASNULLABLE
#nullable enable
#endif

and

#ifdef ICANHASNULLABLE
string Foo(string? val)
#else
string Foo(string val)
#endif

I have a mix of stuff built with the .NET Core 2.1 SDK and am working to bring things to .NET Core 3.1, albeit slowly and not just by flipping a switch. The directive only points to the framework versions, not language version.

Answer 1

Assume you have a multi targeting app. In your .csproj file:

<PropertyGroup>
    <TargetFrameworks>netcoreapp2.2;netcoreapp3.1</TargetFrameworks>
</PropertyGroup>

You could add a conditional PropertyGroup specific to netcoreapp3.1 and have your own constant defined:

<PropertyGroup Condition="'$(TargetFramework)'=='netcoreapp3.1'">
    <DefineConstants>ICANHASNULLABLE</DefineConstants>
    <Nullable>enable</Nullable>
</PropertyGroup>

You can also define your directive based on <Nullable>:

<PropertyGroup Condition="'$(Nullable)'=='enable'">
    <DefineConstants>ICANHASNULLABLE</DefineConstants>
</PropertyGroup>

In your code, you could use ICANHASNULLABLE:

#if ICANHASNULLABLE
void Foo(string? val)
#else
void Foo(string val)
#endif
{
}

Default language version for netcoreapp 3.x is C# 8.0. So any PropertyGroup specifically defined for netcoreapp3.1 in the above example could be considered as specific to C# 8.0 if LangVersion is not set.