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Sean Corfield recently blogged about the choice to use IsDefined vs. structKeyExists when trying to determine if a scoped variable (session, application, request, form url etc), exists or not.

Sean talks about how certain dot noted variables will or won’t be seen with IsDefined, BUT I found the fact that IsDefined looks for ANY possible definition even when the variable you’re searching for has been scoped.

Read his blurb to get the full effect.