Variants

EmptyParam can be used for Variants that represent optional parameters whose value is unassigned. This is useful when the code that marshals method calls requires a fixed number of parameters, even if some of them are optional.  

When marshaling interface calls that include optional parameters, COM requires a value for those parameters even when they are not used. EmptyParam returns an OleVariant you can pass as that value to indicate that the parameter is not used.  

Marshaling interfaces also exist under Linux, but we can't spare the resources to document them right now. 

VarIsEmptyParam