[Mono-list] Error message

[Julien Sobrier]

Hello,
what does this error message means when I compile my code:
syntax error, got token `OP_ADD_ASSIGN', expecting ABSTRACT EXTERN 
INTERNAL NEW OPERATOR OVERRIDE PRIVATE PROTECTED PUBLIC READONLY SEALED 
STATIC THIS UNSAFE VIRTUAL VOLATILE OPEN_BRACKET OPEN_PARENS DOT STAR 
IDENTIFIER

It happens for this line:
PluginListen += new EventListen(EventListenHandler);


The code is :
class Plugin: PluginInterface
{
     [...]
     public event EventListen PluginListen;

     PluginListen += new EventListen(EventListenHandler);

     private void EventListenHandler(string pluginName, string 
eventName, Session userSession, Hashtable parameters)
     {
     	//
     	
     }

}

Where EventListen is defined in another file:
public delegate void EventListen(string pluginName, string eventName, 
Session userSession, Hashtable parameters);

interface PluginInterface
{
	[...]
	event EventListen PluginListen;
}

Thanks
Julien