rgdispidNamedArgs

rgdispidNamedArgs is a field used with the COM IDispatch interface, specifically within the DISPPARAMS structure that governs late-bound method invocation. It is a pointer to an array of DISPIDs that identify which of the provided arguments are named arguments. The array length is given by cNamedArgs, and the total number of arguments is given by cArgs. The rgvarg array holds the actual argument values and is organized in reverse order (the last argument is at index 0).

During an invocation, the named arguments are identified by the DISPID values in rgdispidNamedArgs. The i-th element in rgdispidNamedArgs corresponds to the i-th named argument as it appears in the call (left to right). The value for that named argument is found in rgvarg at index cArgs - i, while the remaining unnamed arguments occupy the other positions in rgvarg. This mapping allows a caller to supply some arguments by name while others are supplied by position.

This mechanism is commonly used in languages that host COM objects and support named parameters, enabling more flexible calls to Automation interfaces. It also supports optional parameters by allowing arguments to be specified out of the typical positional order.

In summary, rgdispidNamedArgs identifies which arguments are named and provides their DISPIDs, while rgvarg holds the corresponding values in a reverse-ordered array. The relationship among cArgs, cNamedArgs, rgdispidNamedArgs, and rgvarg is what enables named-parameter invocations in IDispatch::Invoke.