IDispatch is the
interfaceInterface generally refers to an abstraction that an entity provides of itself to the outside. This separates the methods of external communication from internal operation Interface generally refers to an abstraction that an entity provides of itself to the outside. This separates the methods of...
that exposes the
OLE AutomationIn Microsoft Windows applications programming, OLE Automation , is an inter-process communication mechanism based on Component Object Model that was intended for use by scripting languages – originally Visual Basic – but now are used by languages run on Windows...
protocol. It is one of the standard interfaces that can be exposed by
COMComponent Object Model is a binary-interface standard for software componentry introduced by Microsoft in 1993. It is used to enable interprocess communication and dynamic object creation in a large range of programming languages...
objects. The
I in
IDispatch refers to interface. COM distinguishes between three interface types:
custom,
dispatch and
dual interfaces.
IDispatch derives from
IUnknownIn programming, the IUnknown interface is the fundamental interface in the Component Object Model . The published mandates that COM objects must minimally implement this interface...
and extends its set of three
methodsIn object-oriented programming, a method is a subroutine that is exclusively associated either with a class or with an object...
(
AddRef,
Release and
QueryInterface) with an additional four methods –
GetTypeInfoCount,
GetTypeInfo,
GetIDsOfNames and
Invoke.
The Automation (IDispatch) interface allows a client application to find out what properties and methods are supported by an object at run-time.
IDispatch is the
interfaceInterface generally refers to an abstraction that an entity provides of itself to the outside. This separates the methods of external communication from internal operation Interface generally refers to an abstraction that an entity provides of itself to the outside. This separates the methods of...
that exposes the
OLE AutomationIn Microsoft Windows applications programming, OLE Automation , is an inter-process communication mechanism based on Component Object Model that was intended for use by scripting languages – originally Visual Basic – but now are used by languages run on Windows...
protocol. It is one of the standard interfaces that can be exposed by
COMComponent Object Model is a binary-interface standard for software componentry introduced by Microsoft in 1993. It is used to enable interprocess communication and dynamic object creation in a large range of programming languages...
objects. The
I in
IDispatch refers to interface. COM distinguishes between three interface types:
custom,
dispatch and
dual interfaces.
IDispatch derives from
IUnknownIn programming, the IUnknown interface is the fundamental interface in the Component Object Model . The published mandates that COM objects must minimally implement this interface...
and extends its set of three
methodsIn object-oriented programming, a method is a subroutine that is exclusively associated either with a class or with an object...
(
AddRef,
Release and
QueryInterface) with an additional four methods –
GetTypeInfoCount,
GetTypeInfo,
GetIDsOfNames and
Invoke.
The Automation (IDispatch) interface allows a client application to find out what properties and methods are supported by an object at run-time. It also provides the information necessary to invoke these properties and methods. Client applications do not need to be aware of the object members when they are compiled. This allows COM and ActiveX objects to be called by scripting programs platforms such as the
ASPActive Server Pages , also known as Classic ASP or ASP Classic, was Microsoft's first server-side script engine for dynamically-generated web pages...
server and
JavaScriptJavaScript is an object-oriented scripting language used to enable programmatic access to objects within both the client application and other applications. It is primarily used in the form of client-side JavaScript, implemented as an integrated component of the web browser, allowing the...
on
Internet ExplorerWindows Internet Explorer , is a series of graphical web browsers developed by Microsoft and included as part of the Microsoft Windows line of operating systems starting in 1995...
, where calling conventions are not known at the time
IISInternet Information Services - formerly called Internet Information Server - is a set of Internet-based services for servers created by Microsoft for use with Microsoft Windows. It is the world's second most popular web server in terms of overall websites behind the industry leader Apache HTTP...
or IE were built. By contrast, a simple object library is compiled and linked into a program, while a DLL call also needs to know a function name and parameters at compile time.
Each property and method implemented by an object that supports the IDispatch interface has what is called a Dispatch ID, which is often abbreviated DISPID. The DISPID is the primary means of identifying a property or method and must be supplied to the
Invoke function for a property or method to be invoked, along with an array of
VariantVariant is a data type in certain programming languages, particularly Visual Basic and C++ when using the Component Object Model.In Visual Basic the Variant data type is a tagged union that can be used to represent any other data type except fixed-length string type...
s containing the parameters. The
GetIDsOfNames function can be used to get the appropriate DISPID from a property or method name that is in
stringIn mathematics, a string is an sequence of symbols that are chosen from a set or alphabet.In computer programming, a string is, essentially, a sequence of characters...
format.
A script writer can ask the COM object for a method or property it already knows about from documentation. Then, the client can execute the function with Invoke provided by the IDispatch interface, a form of late-binding. This sort of capability was also supported by
Dynamic Data ExchangeDynamic Data Exchange is a technology for communication between multiple applications under Microsoft Windows or OS/2.- Overview :...
(DDE), which never became popular due to being too low-level.
It is flexible, but suffers from the overhead of checking validity of method and parameters at run time.
interface IDispatch : public IUnknown
{
virtual ULONG GetTypeInfoCount(unsigned int FAR* pctinfo) = 0;
virtual HRESULT GetTypeInfo(unsigned int iTInfo,
LCID lcid,
ITypeInfo FAR* FAR* ppTInfo
) = 0;
virtual ULONG GetIDsOfNames(REFIID riid,
OLECHAR FAR* FAR* rgszNames,
unsigned int cNames,
LCID lcid,
DISPID FAR* rgDispId
) = 0;
virtual ULONG Invoke(DISPID dispIdMember,
REFIID riid,
LCID lcid,
WORD wFlags,
DISPPARAMS FAR* pDispParams,
VARIANT FAR* pVarResult,
EXCEPINFO FAR* pExcepInfo,
unsigned int FAR* puArgErr
) = 0;
};
The
IDispatch interface ID is {00020400-0000-0000-C000-000000000046}.
External links