C# LangVersion (dotnet build -p:LangVersion): fix "Feature is not available in C# N"

<LangVersion> in a .csproj, or dotnet build -p:LangVersion=11 for one build, selects which C# language version the compiler accepts. Code that uses a newer feature fails with an error that names the feature and the version it needs, for example error CS9058: Feature 'collection expressions' is not available in C# 11.0. Please use language version 12.0 or greater. Without the property, the compiler uses the default version of the project's target framework.

A modern file compiled with older versions

dotnet new console -n App -o . >/dev/null 2>&1
cat > Program.cs <<'EOF'
namespace Demo;

record Point(int X, int Y);

static class Program
{
    static void Main()
    {
        int[] nums = [1, 2, 3];
        Console.WriteLine($"{nums.Length} {new Point(1, 2)}");
    }
}
EOF
build() { dotnet build --no-incremental "$@" 2>&1 | grep -oE "(warning|error) CS[0-9]+[^[]*" | sort -u; echo "exit: ${PIPESTATUS[0]}"; }
for v in default 11 9 7.3 99; do
  echo "== LangVersion=$v"
  if [ "$v" = default ]; then build; else build -p:LangVersion=$v; fi
done

== LangVersion=default
exit: 0
== LangVersion=11
error CS9058: Feature 'collection expressions' is not available in C# 11.0. Please use language version 12.0 or greater.
exit: 1
== LangVersion=9
error CS8773: Feature 'file-scoped namespace' is not available in C# 9.0. Please use language version 10.0 or greater.
error CS8773: Feature 'global using directive' is not available in C# 9.0. Please use language version 10.0 or greater.
exit: 1
== LangVersion=7.3
error CS8630: Invalid 'nullable' value: 'Enable' for C# 7.3. Please use language version '8.0' or greater.
exit: 1
== LangVersion=99
error CS1617: Invalid option '99' for /langversion. Use '/langversion:?' to list supported values.
exit: 1

The default (C# 14 on net10.0) compiles everything. C# 11 rejects only the collection expression [1, 2, 3]. C# 9 already fails on the file-scoped namespace and on the global using directives that ImplicitUsings generates. C# 7.3 does not even know nullable reference types, which the template enables, so the error is about the nullable option. An unknown value fails with CS1617.

Values

Errors that point to LangVersion

The error number depends on the feature (older features use other numbers, such as CS8370 for features of C# 8.0); the message text always names the feature and the minimum version.

Where to set it

<PropertyGroup>
  <TargetFramework>net10.0</TargetFramework>
  <!-- Usually leave LangVersion unset; pin it only when you must -->
  <LangVersion>12</LangVersion>
</PropertyGroup>

Put it in Directory.Build.props to give a whole repository the same version. Avoid latest in shared builds: a newer SDK on a developer machine then accepts code that the CI SDK rejects.

Pitfalls

• Pinning an old LangVersion does not make the code run on an old runtime; that is the job of TargetFramework.
• Changing only the property on the command line may not recompile in an incremental build; use --no-incremental or dotnet build -t:Rebuild when experimenting, as the example does.
• A missing namespace or type is not a language-version problem: CS0246 means a reference or using is missing.

FAQ