https://learn.microsoft.com/en-us/dotnet/api/system.diagnostics.codeanalysis.notnullattribute?view=netstandard-2.1&viewFallbackFrom=net-9.0
NotNullAttribute Class (System.Diagnostics.CodeAnalysis) | Microsoft Learn
Specifies that an output is not null even if the corresponding type allows it. Specifies that an input argument was not null when the call returns.
class systemdiagnosticsmicrosoftlearn