.NET compatibility

All major IDEs support Fern-generated SDKs when targeting net462, net472, and net48 versions:

• Visual Studio 2022 (recommended) - Works out of the box. Set <LangVersion> in your project and install the appropriate developer pack.
• Visual Studio 2019 - Supports C# 9 natively. For newer C# versions, add the Microsoft.Net.Compilers.Toolset package.
• Rider - Uses Roslyn. Set <LangVersion> and ensure reference assemblies are available.
• VS Code - Works via OmniSharp and .NET SDK builds.
• Unity - Uses Unity’s own C# compiler. See Using SDKs with Unity.

Use the Roslyn compiler (csc) that comes with Visual Studio 2022+ or the .NET SDK. Set your C# version via <LangVersion> to latest, preview, or a specific version (minimum 9).

Fern’s .NET SDK doesn’t support the legacy Mono compiler (mcs, gmcs) or the pre-Roslyn csc.exe compiler. Use Roslyn even when building on Mono.

$
dotnet build -c Release

• .NET Framework 4.6.2 Developer Pack
• .NET Framework 4.7.2 Developer Pack
• .NET Framework 4.8 Developer Pack

Unity compiles projects using Roslyn, typically supporting C# 9 (varies by Unity version). You can’t force Unity to use a newer C# compiler by changing your package because Unity controls the compiler inside the Editor.

Configure this in Unity: Edit → Project Settings → Player → Other Settings → Api Compatibility Level

• .NET Standard 2.1 (recommended for cross-platform plugins)
• .NET Framework 4.x (Unity’s “4.x equivalent” profile)

Unity doesn’t support NuGet packages, so you must manually download and add these assemblies to your Unity project:

Targeting net48 with modern C#

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<Project Sdk="Microsoft.NET.Sdk">
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  <PropertyGroup>
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    <!-- Target .NET Framework 4.8 -->
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    <TargetFramework>net48</TargetFramework>
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    <!-- Use a modern C# version -->
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    <LangVersion>latest</LangVersion>
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    <!-- Optional, but recommended for better warnings -->
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    <Nullable>enable</Nullable>
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  </PropertyGroup>
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</Project>

$
dotnet build -c Release

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<TargetFrameworks>net48;net8.0</TargetFrameworks>

For older MSBuild/Visual Studio versions, Visual Studio 2019, or legacy packages.config setups, add a modern Roslyn toolset and target the old framework.

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<PropertyGroup>
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<!-- Unlock modern C# -->
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<LangVersion>latest</LangVersion>
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</PropertyGroup>

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 <ItemGroup>
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 <PackageReference Include="Microsoft.Net.Compilers.Toolset" Version="x.y.z">
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     <!-- Keep it private: -->
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     <PrivateAssets>all</PrivateAssets>
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 </PackageReference>
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 </ItemGroup>

Add a small shim in your project or reference a helper package.

• Windows: install the .NET Framework 4.8 Developer Pack.
• Cross-platform/CI: add Microsoft.NETFramework.ReferenceAssemblies to the project (as PrivateAssets=all).

Add Microsoft.Net.Compilers.Toolset to the project, or move to VS 2022+.

This error occurs when using the required keyword with older Visual Studio versions. Follow the instructions at Legacy projects to add a modern compiler.