DISPIDVALUE

DISPIDVALUE, more commonly written as DISPID_VALUE, is a predefined dispatch identifier used in Component Object Model (COM) automation to refer to an object's default member. It is defined in the OLE Automation headers with a value of 0 and is part of the IDispatch interface mechanism that enables late binding and scripting languages to interact with COM objects.

In practice, DISPID_VALUE identifies the default property or method of an automation-compatible object. When calling IDispatch::Invoke, a caller can pass DISPID_VALUE as the dispIdMember parameter to invoke the object's default member rather than a specifically named one. This is particularly relevant in environments that rely on late binding or default properties, such as certain scripting languages.

Not all COM objects expose a default member. If an object does not implement a default property or method, attempting to invoke DISPID_VALUE may result in an error such as DISP_E_MEMBERNOTFOUND. Conversely, objects that do expose a default member will route the invocation to that member.

Implementation and usage notes: DISPID_VALUE is defined in standard headers (for example, OleAuto.h) and can be used from languages that interoperate with COM, including C/C++. It remains distinct from other standard DISPIDs, such as DISPID_PROPERTYPUT (used for setting properties) and various library-specific DISPIDs that map to named members.

In summary, DISPID_VALUE serves as the canonical identifier for a COM object's default member, enabling a uniform way to access that member through IDispatch::Invoke in environments that use late binding.