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title: "event_source | Microsoft Docs"
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---
# event_source
Creates an event source.  
  
## Syntax  
  
```  
  
      [ event_source(  
   type,  
   optimize=[speed | size],  
   decorate=[true | false]  
) ]  
```  
  
#### Parameters  
 `type`  
 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>  
    ```  
  
 **optimize**  
 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.  
  
 **decorate**  
 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](../build/reference/fx-merge-injected-code.md) lets you generate .mrg files. **decorate=false**, which is the default, results in fully-qualified type names in the merged file.  
  
## Remarks  
 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](../windows/event-receiver.md) attribute and the [__event](../cpp/event.md) keyword. Use **event_receiver** to create event receivers. Use `__event` on methods within the event source to specify those methods as events.  
  
> [!NOTE]
>  A templated class or struct cannot contain events.  
  
## Requirements  
  
### Attribute Context  
  
|||  
|-|-|  
|**Applies to**|**class**, `struct`|  
|**Repeatable**|No|  
|**Required attributes**|**coclass** when `type`=**com**|  
|**Invalid attributes**|None|  
  
 For more information, see [Attribute Contexts](../windows/attribute-contexts.md).  
  
## See Also  
 [Compiler Attributes](../windows/compiler-attributes.md)   
 [event_receiver](../windows/event-receiver.md)   
 [__event](../cpp/event.md)   
 [__hook](../cpp/hook.md)   
 [__unhook](../cpp/unhook.md)   
 [Class Attributes](../windows/class-attributes.md)