How to solve the "The type or namespace 'Packaging' does not exist in the namespace 'System.IO'" error

If you encounter this error "The type or namespace 'Packaging' does not exist in the namespace 'System.IO', here's an easy way to solve it.

Just locate the

<assemblies>

 tag in your web.config file and add this line.  It should resolve the compilation errors straightaway.

<add assembly="WindowsBase, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>