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Merged
BillWagner merged 6 commits into from
Jan 28, 2026
Merged

Add help for various C# 11 diagnostics #51329

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95de5b6
Add CS9049
BillWagner Jan 27, 2026
1586d83
Add CS9027
BillWagner Jan 27, 2026
c2bd0fb
Add CS9041
BillWagner Jan 27, 2026
5a7ac23
Add source generator 11 errors
BillWagner Jan 27, 2026
9ee8406
Edit to provide recommendations.
BillWagner Jan 27, 2026
7845766
Apply suggestions from code review
BillWagner Jan 28, 2026
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3 changes: 3 additions & 0 deletions docs/csharp/language-reference/compiler-messages/feature-version-errors.md
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Original file line number Diff line number Diff line change
Expand Up @@ -39,6 +39,7 @@ f1_keywords:
- "CS8929"
- "CS8957" # ERR_NoImplicitConvTargetTypedConditional Conditional expression is not valid in language version {0} because a common type was not found between '{1}' and '{2}'
- "CS8912" # ERR_InheritingFromRecordWithSealedToString Inheriting from a record with a sealed 'Object.ToString' is not supported
- "CS9041" # ERR_UnsupportedCompilerFeature '{0}' requires compiler feature '{1}', which is not supported by this version of the C# compiler.
- "CS9014" # ERR_UseDefViolationPropertyUnsupportedVersion Use of possibly unassigned property
- "CS9015" # ERR_UseDefViolationFieldUnsupportedVersion Use of possibly unassigned field
- "CS9016" # WRN_UseDefViolationPropertyUnsupportedVersion Use of possibly unassigned property
Expand Down Expand Up @@ -98,6 +99,7 @@ helpviewer_keywords:
- "CS8912"
- "CS8919"
- "CS8929"
- "CS9041"
- "CS9014"
- "CS9015"
- "CS9016"
Expand Down Expand Up @@ -150,6 +152,7 @@ That's be design. The text closely matches the text of the compiler error / warn
- **CS8929**: *Method cannot implement interface member in type because the target runtime doesn't support static abstract members in interfaces.*
- **CS8957**: *Conditional expression is not valid in language version because a common type was not found between types.*
- **CS8967**: *Newlines inside a non-verbatim interpolated string are not supported in C#*
- **CS9041**: *Requires compiler feature, which is not supported by this version of the C# compiler.*
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@gewarren gewarren Jan 28, 2026

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Suggested change
- **CS9041**: *Requires compiler feature, which is not supported by this version of the C# compiler.*
- **CS9041**: *Requires a compiler feature that is not supported by this version of the C# compiler.*

- **CS9014**: *Error: Use of possibly unassigned property. Upgrade to auto-default the property.*
- **CS9015**: *Error: Use of possibly unassigned field. Upgrade to auto-default the field.*
- **CS9016**: *Warning: Use of possibly unassigned property. Upgrade to auto-default the property.*
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4 changes: 4 additions & 0 deletions docs/csharp/language-reference/compiler-messages/overloaded-operator-errors.md
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Expand Up @@ -71,6 +71,7 @@ helpviewer_keywords:
- "CS9023"
- "CS9024"
- "CS9025"
- "CS9027"
- "CS9308"
- "CS9310"
- "CS9311"
Expand Down Expand Up @@ -119,6 +120,7 @@ That's by design. The text closely matches the text of the compiler error / warn
- [**CS9023**](#checked-operators): *Operator cannot be made checked.*
- [**CS9024**](#checked-operators): *Operator cannot be made unchecked.*
- [**CS9025**](#checked-operators): *Operator requires a matching non-checked version to also be declared.*
- [**CS9027**](#checked-operators): *Unexpected keyword 'unchecked'*
- [**CS9308**](#operator-declaration-requirements): *User-defined operator must be declared public.*
- [**CS9310**](#operator-signature-requirements): *The return type for this operator must be void.*
- [**CS9311**](#interface-and-inheritance-requirements): *Type does not implement interface member. The type cannot implement member because one of them is not an operator.*
Expand Down Expand Up @@ -386,11 +388,13 @@ public class C4
- **CS9023**: *Operator can't be made checked*
- **CS9024**: *Operator can't be made unchecked*
- **CS9025**: *Checked operator requires a matching non-checked version to also be declared*
- **CS9027**: *Unexpected keyword 'unchecked'*

To use checked operators correctly, follow these requirements. For more information, see [Arithmetic operators](../operators/arithmetic-operators.md#user-defined-checked-operators) and [User-defined checked operators](~/_csharplang/proposals/csharp-11.0/checked-user-defined-operators.md).

- Apply `checked` or `unchecked` keywords only to supported arithmetic operators: `+`, `-`, `*`, `/`, `++`, `--`, and explicit conversions (**CS9023**, **CS9024**).
- Provide both checked and unchecked versions when declaring a checked operator (**CS9025**). The compiler needs both to handle different contexts.
- Use the `unchecked` keyword only in valid operator declaration contexts (**CS9027**). Don't place it in locations where it's syntactically unexpected.

## Interface and inheritance requirements

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