The purpose and use of DispInterface #217

Some COM objects (not the Delphi ones) don't support VTable binding or in other words a normal interface. They only allow access via a Dispatch interface (as defined in IDispatch). This requires the code to do a name lookup to find the index (DispID) for a function and then call it passing the parameters as an array. Some languages like VB Script require this interface. If you assign this type of interface to a variant you can call the methods on the object. For example:

var
  v:variant;
begin
  v := CreateOleObject('Word.Application');
  v.Open('mydoc.doc');
end;

Internally Delphi looks up the index of the Open command and sends all the parameters. This is done at run time so v.MakeMoney will compile but will give an error at run time (as will any incorrect parameters).

The type library importer also creates a dispinterface for this type of object. This allows Delphi to check names and parameter types at compile time (and also gives you tool tips). It also looks up the DispID at compile time so it is a bit faster.

A normal interface (derived from IUnknown) gives direct access (no lookups, no parameter parsing) so it is generally the best option if it is available, which is pretty much every where these days.

An object which supports both IUnknown derived interfaces and the IDispatch mechanism is called a Dual interface.