(sdks) Add new page on .NET/C# version compatibility

.vale.ini

@@ -19,6 +19,7 @@ Microsoft.Accessibility = NO
 Microsoft.Passive = NO
 Microsoft.Suspended = NO
 Microsoft.Vocab = NO
+Microsoft.Spacing = NO
 Microsoft.Semicolon = NO # Mostly just picks up code
 Microsoft.SentenceLength = NO # Mostly just picks up code
 Microsoft.Dashes = NO

.vale/styles/FernStyles/Acronyms.yml

@@ -65,6 +65,10 @@ exceptions:
   - XSS
   - YAML
   - ZIP
+  - SEO
+  - TLS
+  - BCL 
+  - LLM 
   - MDX
   - SEO 
   - AWS 

.vale/styles/FernStyles/Headings.yml

@@ -27,6 +27,10 @@ exceptions:
   - Support
   - Markdown
   - Docs
+  - Mono
+  - Unity
+  - SDK
+  - SDKs
   - API Reference
   - API Explorer
   - GitHub

fern/products/sdks/overview/csharp/version-compatibility.mdx

@@ -0,0 +1,227 @@
+---
+title: .NET compatibility
+description: Learn about supported .NET Framework versions (net462, net472, net48) and Unity compatibility for Fern-generated SDKs.
+---
+
+Fern-generated .NET SDKs support older .NET Framework versions (`net462`, `net472`, `net48`) and Unity. 
+
+Because these SDKs are built with modern C# features, you'll need the following even when targeting older .NET Framework targets:
+- A modern compiler (Visual Studio 2022 or .NET SDK)
+- The appropriate .NET Framework Developer Pack or reference assemblies
+
+## Supported frameworks
+
+Fern .NET SDKs support [all officially supported .NET versions](https://dotnet.microsoft.com/en-us/platform/support/policy/dotnet-framework), except for net35.
+
+| Target | Platform |
+|--------|----------|
+| **net9.0** | Cross-platform |
+| **net8.0** | Cross-platform |
+| **net481** | Windows |
+| **net48** | Windows |
+| **net472** | Windows |
+| **net471** | Windows |
+| **net47** | Windows |
+| **net462** | Windows |
+
+<Note>
+  The C# language version is determined by your compiler, while the target framework determines which runtime and base class library APIs are available. Fern SDKs use modern C# features but compile to older framework versions.
+</Note>
+
+<AccordionGroup>
+<Accordion title="IDE 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](#developer-packs).
+- **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](#using-sdks-with-unity).
+
+<Info>
+You can "bring your own compiler" with the `Microsoft.Net.Compilers.Toolset` package on any environment.
+</Info>
+</Accordion>
+<Accordion title="Compiler compatibility">
+
+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.
+
+<Info title="Building on Mono">
+
+Use the .NET SDK (`dotnet`) with the `Microsoft.NETFramework.ReferenceAssemblies` package to supply `net48` references on non-Windows. Use Roslyn via `dotnet build` or Mono's MSBuild.
+
+```bash title="macOS/Linux"
+dotnet build -c Release
+```
+</Info>
+</Accordion>
+<Accordion title="Developer packs">
+
+- [.NET Framework 4.6.2 Developer Pack](https://dotnet.microsoft.com/en-us/download/dotnet-framework/net462)
+- [.NET Framework 4.7.2 Developer Pack](https://dotnet.microsoft.com/en-us/download/dotnet-framework/net472)
+- [.NET Framework 4.8 Developer Pack](https://dotnet.microsoft.com/en-us/download/dotnet-framework/net48)
+</Accordion>
+</AccordionGroup>
+
+## Using SDKs with Unity
+
+Unity controls its own C# compiler and .NET profile, which affects how you use Fern's .NET SDK.
+
+<AccordionGroup>
+<Accordion title="C# language version">
+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.
+</Accordion>
+<Accordion title="API compatibility level">
+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)
+
+</Accordion>
+<Accordion title="Required assemblies">
+
+Unity doesn't support NuGet packages, so you must manually download and add these assemblies to your Unity project:
+
+| Assembly | NuGet Package |
+|----------|---------------|
+| Microsoft.Bcl.AsyncInterfaces | [10.0.0-preview.6.25358.103](https://www.nuget.org/packages/Microsoft.Bcl.AsyncInterfaces/10.0.0-preview.6.25358.103) |
+| OneOf | [3.0.271](https://www.nuget.org/packages/OneOf/3.0.271) |
+| OneOf.Extended | [3.0.271](https://www.nuget.org/packages/OneOf.Extended/3.0.271) |
+| System.Buffers | [4.6.1](https://www.nuget.org/packages/System.Buffers/4.6.1) |
+| System.IO.Pipelines | [10.0.0-preview.6.25358.103](https://www.nuget.org/packages/System.IO.Pipelines/10.0.0-preview.6.25358.103) |
+| System.Memory | [4.6.3](https://www.nuget.org/packages/System.Memory/4.6.3) |
+| System.Runtime.CompilerServices.Unsafe | [6.1.2](https://www.nuget.org/packages/System.Runtime.CompilerServices.Unsafe/6.1.2) |
+| System.Text.Encodings.Web | [10.0.0-preview.6.25358.103](https://www.nuget.org/packages/System.Text.Encodings.Web/10.0.0-preview.6.25358.103) |
+| System.Text.Json | [10.0.0-preview.6.25358.103](https://www.nuget.org/packages/System.Text.Json/10.0.0-preview.6.25358.103) |
+| System.Threading.Tasks.Extensions | [4.6.3](https://www.nuget.org/packages/System.Threading.Tasks.Extensions/4.6.3) |
+| portable.system.datetimeonly | [9.0.0](https://www.nuget.org/packages/portable.system.datetimeonly/9.0.0) |
+
+</Accordion>
+</AccordionGroup>
+
+## Project configuration examples
+
+Use SDK-style projects where possible, as they provide modern compilers and simpler multi-targeting. If you have a classic project format, consider converting to SDK-style. The [legacy projects section](#legacy-projects) covers legacy setups that can't be converted. 
+
+<Note title="Building on macOS/Linux">
+  When building for `net48` on macOS/Linux, you won't have the Windows targeting packs. Add the reference assemblies package:
+
+  ```xml title="YourProject.csproj"
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.3" PrivateAssets="all" />
+  </ItemGroup>
+  ```
+  This provides the `net48` reference assemblies so Roslyn can compile for `net48` on any platform. You'll still need Mono or Wine to execute the result.
+</Note>
+
+<AccordionGroup>
+<Accordion title="SDK-style projects (recommended)">
+
+Targeting `net48` with modern C#
+
+<Steps>
+<Step title="Configure your project">
+
+<Info title="Prerequisite (Windows)">
+    Install the [.NET Framework 4.8 Developer Pack](#developer-packs) so the `net48` reference assemblies are available to the compiler.
+</Info>
+
+```xml title="YourProject.csproj"
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <!-- Target .NET Framework 4.8 -->
+    <TargetFramework>net48</TargetFramework>
+
+    <!-- Use a modern C# version -->
+    <LangVersion>latest</LangVersion>
+
+    <!-- Optional, but recommended for better warnings -->
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+</Project>
+```
+</Step>
+
+<Step title="Build your project">
+```bash
+dotnet build -c Release
+```
+</Step>
+
+<Step title="Multi-target (optional)">
+To support multiple frameworks, use `<TargetFrameworks>`:
+{/* <!-- vale off --> */}
+```xml title="YourProject.csproj"
+<TargetFrameworks>net48;net8.0</TargetFrameworks>
+```
+{/* <!-- vale on --> */}
+</Step>
+</Steps>
+</Accordion>
+<Accordion title="Legacy projects">
+
+For older MSBuild/Visual Studio versions, Visual Studio 2019, or legacy `packages.config` setups, add a modern Roslyn toolset and target the old framework.
+
+<Steps>
+<Step title="Install target packs">
+   Install the appropriate [.NET Framework Developer Pack](#developer-packs) so Visual Studio can find the reference assemblies.
+</Step>
+
+<Step title="Set language version">
+   ```xml title="YourProject.csproj"
+   <PropertyGroup>
+  <!-- Unlock modern C# -->
+  <LangVersion>latest</LangVersion>
+</PropertyGroup>
+   ```
+   Or use a specific version: `9`, `10`, `11`, `12`, or `preview`.
+</Step>
+<Step title="Add modern compiler">
+
+   ```xml title="YourProject.csproj"
+    <ItemGroup>
+    <PackageReference Include="Microsoft.Net.Compilers.Toolset" Version="x.y.z">
+        <!-- Keep it private: -->
+        <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+    </ItemGroup>
+   ```
+   This ensures your build uses a modern `csc.exe` even if your IDE/MSBuild is older. 
+
+</Step>
+</Steps>
+</Accordion>
+</AccordionGroup>
+
+## Troubleshooting
+
+<AccordionGroup>
+<Accordion title="Type or namespace IsExternalInit not found">
+  Add a small shim in your project or reference a helper package.
+</Accordion>
+<Accordion title="Reference assemblies for .NETFramework,Version=v4.8 were not found">
+  - Windows: install the [.NET Framework 4.8 Developer Pack](#developer-packs).  
+  - Cross-platform/CI: add `Microsoft.NETFramework.ReferenceAssemblies` to the project (as `PrivateAssets=all`).
+</Accordion>
+<Accordion title="VS 2019 can't parse newer syntax">
+  Add `Microsoft.Net.Compilers.Toolset` to the project, or move to VS 2022+.
+</Accordion>
+<Accordion title="CS0619: Constructors of types with required members are not supported">
+This error occurs when using the `required` keyword with older Visual Studio versions. Follow the instructions at [Legacy projects](#legacy-projects) to add a modern compiler.
+
+<Note>
+  Some versions of Visual Studio may show this error in the IDE but compile the project successfully when using the package-provided compiler.
+</Note>
+</Accordion>
+</AccordionGroup>
+
+
+
+
+
+
+
+
+

fern/products/sdks/sdks.yml

@@ -130,6 +130,8 @@ navigation:
           - page: Publishing to NuGet
             path: ./overview/csharp/publishing-to-nuget.mdx
             slug: publishing
+          - page: Version compatibility
+            path: ./overview/csharp/version-compatibility.mdx
           - page: Adding custom code
             hidden: true
             path: ./overview/csharp/custom-code.mdx