however, only platform code is allowed to be marked as "critical" or "safecritical". the silverlight runtime ensures that platform assemblies are loaded only from the silverlight installation directory, and are digitally signed by microsoft. this effectively means that user-application assemblies can only be "transparent" code (run under partial trust and limited rights). platform code can be marked with either attribute. the bcl methods of the .net framework, which have the codice_19 attribute set, allowing them to be called from untrusted code originating from the internet, are exposed in silverlight bcl as "transparent" methods.