Audit by Deep Research

Impact of Extended Property Patterns on StyleCopAnalyzers

Extended property patterns (introduced in C# 10) allow pattern matching directly on nested properties using dot notation (for example, obj is { Inner.Value: 0 } instead of obj is { Inner: { Value: 0 } }). This new syntax impacts several StyleCopAnalyzers rules, especially those dealing with member access and spacing in patterns.

This checklist covers each impacted rule, how extended property patterns interact with it, and what is required for full support.

SA1101 – Prefix local calls with `this`

Interaction

SA1101 requires instance member accesses to be prefixed with this. for clarity. Extended property patterns caused false positives because the analyzer treated nested property references in the pattern as instance members of the current class instead of members of the pattern input.

Example:

return arg is { Inner.Value: "" };

Earlier, SA1101 incorrectly flagged Inner.Value and suggested this.Inner.Value, which is wrong. In this context, Inner.Value is a property of arg (the pattern input), not of this.

Required changes

Implementation

Update SA1101 to recognize property pattern contexts and exclude them from the “must be prefixed with this” rule.

Specifically:
- When analyzing an extended property pattern, the expression to the left of the : in a subpattern must not be required (or allowed) to use this..
- The analyzer should treat these as pattern members of the input expression, not as members of the containing type.
Tests

Add or extend tests to ensure:
- Patterns like:
```
return obj is { Prop.SubProp: value };
```
  do not produce SA1101 diagnostics.
- Genuine missing this. in non-pattern contexts are still reported.
- Pattern usage in different contexts is covered:
  - is expressions:
```
if (item is { Child.Name: "X" })
{
}
```
    No SA1101 diagnostic on Child.Name.
  - switch statements and expressions:
```
return arg switch
{
    { Inner.Value: > 0 } => 1,
    _ => 0,
};
```
  - Combinations with other patterns (and, or, parenthesized patterns) to ensure SA1101 ignores all pattern contexts.
Documentation

Update the SA1101 documentation to:
- State that property pattern syntax is excluded from the rule.
- Include an example like:
```
if (item is { Outer.InnerProp: value })
{
}
```
  and explicitly note that Outer.InnerProp is valid without this. in this context.

Test scenarios to cover

if (item is { Child.Name: "X" }) – no SA1101 diagnostic.
case MyType { Prop.Value: > 0 }: ... – no SA1101 diagnostic.
Nested property patterns inside:
- switch expressions,
- switch statements,
- patterns combined with and / or,
- parenthesized patterns.

All should be free of SA1101 diagnostics as long as the access occurs in the property pattern portion of the match.

SA1024 – ColonsMustBeSpacedCorrectly

Interaction

SA1024 governs spacing around : tokens. Extended property patterns introduce a new colon usage: separating a nested property path from the subpattern or constant, for example:

obj is { Inner.Value: "" }

The expected style is:

No space before the colon.
Single space after the colon (in most situations), e.g.:

obj is { Inner.Value: "" }

Originally SA1024 did not account for colons appearing in the new pattern grammar and could:

Ignore spacing around these colons, or
Apply spacing rules from unrelated contexts.

Required changes

Implementation

Extend SA1024’s logic to recognize colons inside extended property patterns:
- In Roslyn, the colon in a property subpattern can appear in a SubpatternSyntax (rather than just NameColonSyntax).
- When the colon’s parent is a pattern clause or SubpatternSyntax, SA1024 should:
  - Forbid a space before the colon.
  - Generally require a single space after the colon (consistent with other named patterns and colon usage in the rule).
Tests

Add or extend tests to cover:
- Correct spacing:
```
return x is { Prop.Value: 1 };
```
  No SA1024 diagnostics.
- Space before colon:
```
return x is { Prop.Value : 1 };
```
  Expect SA1024 (“Colon should not be preceded by a space”).
- Missing space after colon:
```
return x is { Prop.Value:1 };
```
  Expect SA1024 (“Colon should be followed by a space”).
- Single-level vs multi-level property names:
```
obj is { Prop: val }
obj is { Prop.SubProp: val }
```
- Multiple subpatterns:
```
obj is { X.Y: 1, Z: 2 }
```
  Each colon must be checked independently.
Documentation

Update SA1024 documentation to:
- Explicitly call out property patterns, for example:
  
  In patterns (e.g. obj is { Property.SubProperty: value }), the colon is treated like a named element separator: no space before, and one space after.
- Include at least one sample showing compliant and non-compliant property pattern spacing.

Test scenarios to cover

Correctly spaced pattern:
```
if (o is { A.B: 5 })
{
}
```
No SA1024 diagnostic.

Mis-spaced patterns:

if (o is { A.B : 5 }) { } // space before colon
if (o is { A.B:5 }) { }   // missing space after colon

Both should produce SA1024 diagnostics.

Mixed pattern with multiple subpatterns:
```
if (o is { X.Y: 1, Z: 2 }) { }
```
All colons should be evaluated according to the rule.

Other rules

Other StyleCopAnalyzers rules were evaluated and found not to be meaningfully affected by extended property patterns. General pattern-matching support (introduced in earlier C# versions) already covered most relevant syntax:

Spacing around is keyword.
Braces in patterns.
Other pattern shapes.

After implementing the changes to SA1101 and SA1024 (with tests and documentation updates), StyleCopAnalyzers should correctly handle extended property pattern syntax without false positives or missed violations.

Sources

SA1101CSharp10UnitTests.cs
Issue “SA1101 in property pattern” (#3472)
StyleCopAnalyzers releases
SA1024ColonsMustBeSpacedCorrectly.cs

Impact of Extended Property Patterns on StyleCopAnalyzers

SA1101 – Prefix local calls with this

Interaction

Required changes

Test scenarios to cover

SA1024 – ColonsMustBeSpacedCorrectly

Interaction

Required changes

Test scenarios to cover

Other rules

Sources

SA1101 – Prefix local calls with `this`

Review impact of extended property patterns on existing rules #3984

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