https://learn.microsoft.com/ja-jp/dotnet/api/system.diagnostics.contracts.contractreferenceassemblyattribute?view=netframework-4.8.1&viewFallbackFrom=net-10.0
ContractReferenceAssemblyAttribute Class (System.Diagnostics.Contracts) | Microsoft Learn
Specifies that an assembly is a reference assembly that contains contracts.
class systemdiagnosticscontractsmicrosoftlearn