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Creates an event source.

[ event_source(
   optimize=[speed | size],
   decorate=[true | false]
) ]


An enumeration of one of the following values:

  • native for unmanaged C/C++ code (default for unmanaged classes).

  • com for COM code. You must use coclass when type=com. This value requires that you include the following header files:

       #define _ATL_ATTRIBUTES
       #include <atlbase.h>
       #include <atlcom.h>

When type is native, you can specify optimize=size, to indicate that there is 4 bytes of storage (minimum) for all events in a class or optimize=speed (the default) to indicate that there is 4 * (# of events) bytes of storage.


When type is native, you can specify decorate=false, to indicate that the expanded name in the merged (.mrg) file should not include the enclosing class name. /Fx lets you generate .mrg files. decorate=false, which is the default, results in fully-qualified type names in the merged file.

The event_source C++ attribute specifies that the class or structure to which it is applied will be an event source.

event_source is used in conjunction with the event_receiver attribute and the __event keyword. Use event_receiver to create event receivers. Use __event on methods within the event source to specify those methods as events.


A templated class or struct cannot contain events.

Attribute Context

Applies to

class, struct



Required attributes

coclass when type=com

Invalid attributes


For more information, see Attribute Contexts.