The TrySkipIisCustomErrors property is used only when your application is hosted in IIS 7.0. When running in Classic mode in IIS 7.0 the TrySkipIisCustomErrors property default value is true. When running in Integrated mode, the TrySkipIisCustomErrors property default value is false.
TrySkipIisCustomErrors is introduced in the .NET Framework version 3.5. For more information, see .NET Framework 3.5 Architecture.