https://learn.microsoft.com/en-us/dotnet/api/system.diagnostics.contracts.pureattribute?view=netframework-4.8.1&redirectedfrom=MSDN
PureAttribute Class (System.Diagnostics.Contracts) | Microsoft Learn
Indicates that a type or method is pure, that is, it does not make any visible state changes.
class systemdiagnosticscontractsmicrosoftlearn