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Why is .NET Framework considered to be in 'maintenance mode', and what does that mean practically for teams still using it? How does cross-platform support differ between .NET Framework and modern .NET? How does packages.config differ from PackageReference, and which is used in Framework versus modern .NET projects? Can .NET Framework and modern .NET applications be run side by side on the same machine, and how does that work? What major APIs or features exist in .NET Framework but are unavailable or different in modern .NET? What tools does Microsoft provide to help analyze and migrate a .NET Framework project to modern .NET? What is the release cadence and Long-Term Support (LTS) model for modern .NET, and how does it differ from Framework's release history? How does the CLR implementation differ between .NET Framework and modern .NET (CoreCLR)? What migration strategies exist for gradually moving a large .NET Framework application to modern .NET rather than a big-bang rewrite?
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How does language version support differ between .NET Framework and modern .NET, specifically regarding C# language features?

Intermediate

Answer

The C# language version is technically decoupled from the runtime, but practically, .NET Framework projects are typically capped at older C# language versions (commonly C# 7.3 without extra configuration) due to compiler and runtime feature dependencies, while modern .NET projects can freely use the latest C# version (e.g., C# 12/13) with all its newer syntax (records, pattern matching, required members, primary constructors) since Microsoft continuously updates the Roslyn compiler independent of runtime versions -- but some newer syntax also depends on runtime library support only available in modern .NET.
<!-- Framework project: often capped without explicit override -->
<LangVersion>7.3</LangVersion>

<!-- Modern .NET project: latest language features available -->
<LangVersion>latest</LangVersion>
<!-- Enables records, required members, primary constructors, etc. -->
Real-world example A team stuck on .NET Framework 4.8 can't use C# 9's record types because the required runtime support isn't present, becoming a concrete motivating example for migration beyond just performance concerns.

Common follow-ups: Which specific C# features require runtime support versus being purely compiler-level?;How would you force a newer LangVersion on a Framework project?

.NET vs .NET Framework;.NET CLI SDK & Project Structure (csproj)
All 16 .NET vs .NET Framework questions