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Fix null-string conversions to nullable C# extensible enums - #12576
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[WIP] Fix ArgumentNullException for nullable string extensible enum
Fix null-string conversions to nullable C# extensible enums
Nov 13, 2026
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🟢 Approval recommended
No active review issues remain for the nullable extensible-enum conversion fix.
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What changed in this PR
Fixes #12572 in the C# client generator so converting a null string to a nullable extensible enum returns null instead of throwing.
Changes:
- Explicitly casts the conditional’s null branch to the nullable enum type.
- Adds regression tests using string variables with null, known, unknown, and empty values, while preserving exceptions for non-nullable conversions.
- Updates affected generation baselines and sample output.
| File | Description |
|---|---|
packages/http-client-csharp/generator/TestProjects/Local/Sample-TypeSpec/src/Generated/Models/ThingRequiredNullableLiteralString1.cs |
Applies nullable cast in generated fixture. |
packages/http-client-csharp/generator/TestProjects/Local/Sample-TypeSpec/src/Generated/Models/ThingOptionalLiteralString.cs |
Applies nullable cast in generated fixture. |
packages/http-client-csharp/generator/TestProjects/Local/Sample-TypeSpec/src/Generated/Models/StringExtensibleEnum.cs |
Updates enum exercised by regression tests. |
packages/http-client-csharp/generator/TestProjects/Local/Sample-TypeSpec/src/Generated/Models/PreviewExtensibleChoice.cs |
Applies nullable cast in generated fixture. |
packages/http-client-csharp/generator/TestProjects/Local/Sample-TypeSpec/src/Generated/Models/LegacyExtensibleChoice.cs |
Applies nullable cast in generated fixture. |
packages/http-client-csharp/generator/TestProjects/Local/Sample-TypeSpec/src/Generated/Models/DaysOfWeekExtensibleEnum.cs |
Applies nullable cast in generated fixture. |
packages/http-client-csharp/generator/TestProjects/Local.Tests/ExtensibleEnumTests.cs |
Tests nullable conversions and non-nullable rejection. |
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/EnumProviders/TestData/EnumProviderTests/BackCompat_ExtensibleEnumRemovedValueReadded.cs |
Updates restored-value baseline. |
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/EnumProviders/TestData/EnumProviderTests/BackCompat_ExtensibleEnumRemovedValueNotReaddedWhenProvidedByCustomCode.cs |
Updates custom-provided-value baseline. |
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/EnumProviders/TestData/EnumProviderTests/BackCompat_ExtensibleEnumRemovedValueNotReaddedWhenBaselineAccepts.cs |
Updates accepted-removal baseline. |
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/EnumProviders/TestData/EnumProviderTests/BackCompat_ExtensibleEnumNonPublicValueNotReadded.cs |
Updates non-public-value baseline. |
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/EnumProviders/TestData/EnumProviderTests/BackCompat_ExtensibleEnumCustomCodeTakesPrecedenceOverPreservedUnderscores.cs |
Updates customization-precedence baseline. |
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/ExtensibleEnumProvider.cs |
Generates explicitly typed nullable null branch. |
docs/samples/client/csharp/SampleService/SampleClient/src/Generated/Models/ThingRequiredNullableLiteralString1.cs |
Synchronizes sample with corrected conversion. |
docs/samples/client/csharp/SampleService/SampleClient/src/Generated/Models/ThingOptionalLiteralString.cs |
Synchronizes sample with corrected conversion. |
docs/samples/client/csharp/SampleService/SampleClient/src/Generated/Models/StringExtensibleEnum.cs |
Synchronizes sample with corrected conversion. |
docs/samples/client/csharp/SampleService/SampleClient/src/Generated/Models/DaysOfWeekExtensibleEnum.cs |
Synchronizes sample with corrected conversion. |
Review effort: Balanced
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Converting a null string to a nullable extensible enum throws because the conditional’s untyped
nullresolves through the non-nullable implicit operator. Existing null-literal tests bypassed the nullable operator and missed the failure.