Polyfills

Modern C# features for netstandard2.0 targets.

ANcpLua.Roslyn.Utilities provides comprehensive polyfills that enable modern C# features when targeting netstandard2.0. All polyfills are #if-gated and compile only on older target frameworks.

Packages

Two packages include polyfills:

Package	Use case
ANcpLua.Roslyn.Utilities.Sources	Analyzers/generators that need both utilities AND polyfills
ANcpLua.Roslyn.Utilities.Polyfills	Projects that need polyfills only (no Roslyn dependency)

# For generators/analyzers
dotnet add package ANcpLua.Roslyn.Utilities.Sources

# For polyfills only
dotnet add package ANcpLua.Roslyn.Utilities.Polyfills

Available Polyfills

Language Features

Polyfill	Enables	Available from
IsExternalInit	record types and init properties	.NET 5+
RequiredMemberAttribute	required modifier	.NET 7+
CompilerFeatureRequiredAttribute	Compiler feature gating	.NET 7+
SetsRequiredMembersAttribute	Constructor sets all required members	.NET 7+
CallerArgumentExpressionAttribute	Argument name capture	.NET 6+
ParamCollectionAttribute	C# 13 params collections	.NET 9+

Nullable Attributes

Polyfill	Enables	Available from
AllowNull, DisallowNull	Nullable input annotations	.NET Core 3.1+
MaybeNull, NotNull	Nullable output annotations	.NET Core 3.1+
MaybeNullWhen, NotNullWhen	Conditional null annotations	.NET Core 3.1+
NotNullIfNotNull	Linked nullability	.NET Core 3.1+
DoesNotReturn, DoesNotReturnIf	Unreachable code flow	.NET Core 3.1+
MemberNotNull, MemberNotNullWhen	Member null state	.NET 5+

Trim/AOT Attributes

Polyfill	Enables	Available from
DynamicallyAccessedMembersAttribute	Trim-safe reflection	.NET 5+
DynamicallyAccessedMemberTypes	Member type flags	.NET 5+
RequiresUnreferencedCodeAttribute	Trim warning	.NET 5+
RequiresDynamicCodeAttribute	AOT warning	.NET 7+
UnconditionalSuppressMessageAttribute	Trim suppression	.NET 5+

Index and Range

Polyfill	Enables	Available from
Index	array[^1] syntax	.NET Core 3.0+
Range	array[1..3] syntax	.NET Core 3.0+

Other

Polyfill	Enables	Available from
StackTraceHiddenAttribute	Hide from stack traces	.NET 6+
UnreachableException	Unreachable code paths	.NET 7+
ExperimentalAttribute	Mark APIs as experimental	.NET 8+
TimeProvider	Testable time abstraction	.NET 8+
Lock	System.Threading.Lock	.NET 9+
String extensions	string.Contains(StringComparison)	.NET Core 2.1+

Opt-Out Control

All polyfills are enabled by default. Use MSBuild properties to opt out:

<PropertyGroup>
  <!-- Disable individual categories -->
  <InjectIsExternalInitOnLegacy>false</InjectIsExternalInitOnLegacy>
  <InjectNullabilityAttributesOnLegacy>false</InjectNullabilityAttributesOnLegacy>
  <InjectTrimAttributesOnLegacy>false</InjectTrimAttributesOnLegacy>
  <InjectIndexRangeOnLegacy>false</InjectIndexRangeOnLegacy>
  <InjectStackTraceHiddenOnLegacy>false</InjectStackTraceHiddenOnLegacy>
  <InjectUnreachableExceptionOnLegacy>false</InjectUnreachableExceptionOnLegacy>
  <InjectExperimentalAttributeOnLegacy>false</InjectExperimentalAttributeOnLegacy>
  <InjectTimeProviderPolyfill>false</InjectTimeProviderPolyfill>
  <InjectLockPolyfill>false</InjectLockPolyfill>
  <InjectStringExtensionsPolyfill>false</InjectStringExtensionsPolyfill>
  <InjectRequiredMemberOnLegacy>false</InjectRequiredMemberOnLegacy>
  <InjectCallerAttributesOnLegacy>false</InjectCallerAttributesOnLegacy>
  <InjectCompilerFeatureRequiredOnLegacy>false</InjectCompilerFeatureRequiredOnLegacy>
  <InjectSetsRequiredMembersOnLegacy>false</InjectSetsRequiredMembersOnLegacy>
  <InjectParamCollectionOnLegacy>false</InjectParamCollectionOnLegacy>

  <!-- Or disable ALL polyfills at once -->
  <InjectAllPolyfillsOnLegacy>false</InjectAllPolyfillsOnLegacy>
</PropertyGroup>

Polyfills are only active for netstandard2.0 and netstandard2.1 targets. .NET 5+ projects don’t need them — the #if guards ensure they compile away.

Conflict Resolution

If another package provides the same polyfills (e.g., PolySharp), disable the overlapping categories to avoid duplicate type definitions:

<PropertyGroup>
  <InjectIsExternalInitOnLegacy>false</InjectIsExternalInitOnLegacy>
  <InjectNullabilityAttributesOnLegacy>false</InjectNullabilityAttributesOnLegacy>
  <InjectIndexRangeOnLegacy>false</InjectIndexRangeOnLegacy>
</PropertyGroup>