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Short answer

Use patterns in is expressions, switch statements and switch expressions to test types, constants, ranges and property values in one readable check. [S1][S2]

Example

string Size(object o) => o switch {
    int n when n < 0      => "negative",
    int n and < 10        => "small",
    string { Length: 0 }  => "empty",
    null                  => "null",
    _                     => "other"
};

Combine patterns with and, or, not. [S2]

Claims

• id: C1 claim: Patterns work in is, switch statements and switch expressions. source: S1 evidence: "C# uses patterns in is expressions, switch statements, and switch expressions."
• id: C2 claim: Patterns can be on the right side of is. source: S1 evidence: "On the right side of the is operator."
• id: C3 claim: Property patterns match nested fields. source: S2 evidence: "test that an expression's properties or fields match nested patterns"
• id: C4 claim: Logical patterns combine patterns. source: S2 evidence: "test that an expression matches a logical combination of patterns"

Sources

• [S1] Pattern matching overview - C#. Microsoft Learn. https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/functional/pattern-matching . License: CC BY 4.0 (dotnet/docs). Accessed 2026-11-04.
• [S2] Patterns - Pattern matching using the is and switch expressions. - C# reference. Microsoft Learn. https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/patterns . License: CC BY 4.0 (dotnet/docs). Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_2j2j1dxe,
  author = {{iinventedlight}},
  title = {{C\# pattern matching with is and switch}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/csharp-pattern-matching/},
  note = {Permanent ID iil:note:2j2j1dxe. License: CC-BY-4.0.},
}

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