IDispatch is the interface that exposes the OLE Automation protocol. Interface generally refers to an abstraction that an entity provides of itself to the outside In Microsoft Windows applications programming OLE Automation (later renamed by Microsoft to just Automation, although the old term remained in widespread It is one of the standard interfaces that can be exposed by COM objects. Component Object Model ( COM) is an interface standard for Software componentry introduced by Microsoft in 1993 The I in IDispatch refers to interface. COM distinguishes between three interface types: custom, dispatch and dual interfaces.
IDispatch derives from IUnknown and extends its set of three methods (AddRef, Release and QueryInterface) with an additional four methods – GetTypeInfoCount, GetTypeInfo, GetIDsOfNames and Invoke. In Programming, the IUnknown interface is the fundamental interface in the Component Object Model (COM In Object-oriented programming, the term method refers to a Subroutine that is exclusively associated either with a class (called class methods
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 ASP server and JavaScript on Internet Explorer, where calling conventions are not known at the time IIS or IE were built. Active Server Pages ( ASP) is Microsoft 's first server-side script engine for dynamically-generated web pages JavaScript is a Scripting language most often used for Client-side web development Windows Internet Explorer (formerly Microsoft Internet Explorer abbreviated MSIE) commonly abbreviated to IE, is a series of graphical 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 Variants containing the parameters. Variant is a Data type in certain programming languages particularly Visual Basic and C++ when using the Component Object Model. The GetIDsOfNames function can be used to get the appropriate DISPID from a property or method name that is in string format. In Computer programming and some branches of Mathematics, a string is an ordered Sequence of Symbols.
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. In Programming languages name binding is the association of values with Identifiers An identifier bound to a value is said to reference This sort of capability was also supported by Dynamic Data Exchange (DDE), which never became popular due to being too low-level. Dynamic Data Exchange ( DDE) is a technology for communication between multiple applications under Microsoft Windows or OS/2.
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}. A Globally Unique Identifier or GUID (ˈguːɪd or /ˈgwɪd/ is a special type of identifier used in Software applications in order to provide a reference number
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