dotnet-multi-targeting

Comprehensive guide for .NET multi-targeting strategies with a polyfill-first approach. This skill consumes the structured output from [skill:dotnet-version-detection] (TFM, C# version, preview flags) and provides actionable guidance on backporting language features, handling runtime gaps, and validating API compatibility across target frameworks.

Scope

• Decision matrix: polyfill vs conditional compilation
• PolySharp for compiler-synthesized polyfills
• SimonCropp/Polyfill for BCL API backporting
• Conditional compilation with TFM-based preprocessor symbols
• Multi-targeting .csproj patterns and TFM-specific source files
• API compatibility validation (EnablePackageValidation, ApiCompat tool)

Out of scope

• TFM detection logic -- see [skill:dotnet-version-detection]
• Version upgrade lane selection -- see [skill:dotnet-version-upgrade]
• Platform-specific UI frameworks (MAUI, Blazor) -- see respective framework skills
• Cloud deployment configuration