VB Shell Programming

The PIDL

Explorer needs a way to uniquely identify each item in the namespace unambiguously in relation to other items in the namespace. It must be able to enumerate these items in a consistent, generic manner, even though these items represent a wide variety of data. It does this with a PIDL.

A PIDL is a pointer to an item identifier list, or ITEMIDLIST. An ITEMIDLIST is an array of shell item IDs. Each one of these identifiers is an array of bytes that contains information that is specific to the namespace extension using it.

How can Explorer use PIDLs if they are different in respect to every extension? Well, as it turns out PIDLs are pretty simple creatures. Let’s look at how a PIDL is defined, and you’ll be able to see this for yourself. Here is what an ITEMIDLIST (just remember a PIDL is a pointer to one of these) looks like, as defined by the Platform SDK:

typedef struct _ITEMIDLIST {
    SHITEMID mkid;
} ITEMIDLIST, * LPITEMIDLIST;

As you can see, an ITEMIDLIST is nothing more than a structure that contains one member of type SHITEMID. This structure looks like so:

typedef struct _SHITEMID { 
    USHORT cb; 
    BYTE abID[1]; 
} SHITEMID, * LPSHITEMID;

The first member of SHITEMID, cb, contains the number of bytes of the SHITEMID structure. SHITEMID is a variable-length structure, and cb contains two bytes specifying its size. For those of you who have never done any C programming, you probably have never seen this technique before: the first member of a structure is used to define the ...