What is a DispInterface? #216

Short answer

It is is a specification for an IDispatch interface.

Long answer

The IDispatch interface is the basis of all automation. It has two methods that allow pointerless scripting languages to call methods by name, instead of using method pointers. These methods are GetIDsOfNames, and Invoke.

GetIdsOfNames retrieves the numerical ID of a method with a given name (provided that the object implements another interface that has a method with that name).

Invoke uses the numerical ID of a method to call the method. This numerical ID is known as the DispID.

For example, suppose you create an interface that looks like this:

type
  IMyIntf = Interface(IDispatch)
    ['{4D733280-C514-11D4-8481-A68F52CBDB56}']
    procedure DoThis;
  end;

and an object that implements the interface that looks like this:

type
  TMyDispatchObj = class(TAutoObject, IMyIntf)
  public
    procedure DoThis;
  end;

Delphi will call both GetIDSOfNames and Invoke for you – all you need to do is use a variant. Like this:

{ ... }
var
  AVar: OleVariant;
  { ... }
 
{ ... }
  AVar := CreateComObject(CLASS_TMyAutoObj) as IDispatch;
  AVar.DoThis;
  { ... }

Every time you call a method of a variant referencing an IDispatch interface, GetIDsOfNames and Invoke are called for you behind the scenes. However, calling GetIdsOfNames for every method is quite slow. And since all it does is find a numerical ID for a given method name, it might be nice if you could look up that ID in advance and pass it to Invoke directly, rather than go through GetIdsOfNames every time. Enter the DispInterface:

type
  IMyDispInterface = dispinterface
    ['{4D733284-C514-11D4-8481-A68F52CBDB56}']
    procedure DoThis; DispID 1;
  end;

This declaration tells Delphi the DispID of each method in an interface. So if you use a DispInterface variable, rather than a variant, Delphi can call Invoke directly using the Dispinterface, rather than go through GetIdsOfNames:

{ ... }
var
  Disp: IDispatch;
  Dispint: IMyDispInterface;
{ ... }
  Disp := CreateComObject(CLASS_TMyAutoObj) as IDispatch;
  Dispint := IMyDispInterface(Disp);
  Dispint.DoThis;
  { ... }

A DispInterface is not a true interface. When you use a DispInterface, you are actually using the IDispatch interface of an object, just as you are when you use a variant. That's why you can cast an IDispatch interface directly to a Dispinterface, as in the second line above. All you're doing here is telling the compiler that you already know what other methods a particular object will implement, and what DispIDs can be used to invoke them.