ASP.NET 2.0 – Validating nonvisible Controls on the same page

If you set Visible to false, validation for that control will not fire. From ASP.Net Validation in Depth:

Why not just use Visible=false to have an invisible validator? In ASP.NET the Visible property of a control has a very strong meaning: a control with Visible=false will not be processed at all for pre-rendering or rendering. As a result of this stronger meaning, Visible=false for a validator means that not only does not it not display anything, it is does not function either. It is not evaluated, does not affect page validity, and does not put errors in the summary.

If you want a control to validate but have it hidden on the page, use CSS to set display to none.

The first case is a using directive (using namespace X), and what it means is that the names from namespace X will be available for regular lookup, in the first common namespace of X and the current scope. In this case, the first common namespace ancestor of ::N and ::std is ::, so the using directive will make std::swap available only if lookup hits ::.

The problem here is that when lookup starts it will look inside the function, then inside the class, then inside N and it will find ::N::swap there. Since a potential overload is detected, regular lookup does not continue to the outer namespace ::. Because ::N::swap is a function the compiler will do ADL (Argument dependent lookup), but the set of associated namespaces for fundamental types is empty, so that won't bring any other overload. At this point lookup completes, and overload resolution starts. It will try to match the current (single) overload with the call and it will fail to find a way of converting from int to the argument ::N::C and you get the error.

On the other hand a using declaration (using std::swap) provides the declaration of the entity in the current context (in this case inside the function itself). Lookup will find std::swap immediately and stop regular lookup with ::std::swap and will use it.