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I’ve been wanting to know this for years, and finally discovered it the other day when looking inside the Microsoft Composite Application Blocks class in Microsoft.Practices.CompositeUI.BuilderStrategies.CommandStrategy.

There’s some code in this class to discover if an event handler has already been attached to the event, and it goes a little something like this…

private void RegisterCommandHandlers(IBuilderContext context, WorkItem workItem, object target,
            string targetID, MethodInfo methodInfo)
        {
            foreach (CommandHandlerAttribute attr in methodInfo.GetCustomAttributes(typeof(CommandHandlerAttribute), true))
            {
                Command cmd = workItem.Commands[attr.CommandName];

                if (!cmd.IsHandlerRegistered(target, methodInfo))
                {
                    cmd.ExecuteAction += CreateCommandHandler(target, methodInfo);
                    TraceBuildUp(context, target.GetType(), targetID, Properties.Resources.CommandInjectionBuildUp,
                        methodInfo.Name, attr.CommandName);
                }
            }
        }

The clever part is the IsHandlerRegistered method above, which uses the System.MulticastDelegate.GetInvocationList() method to discover all the targets of the event…

internal bool IsHandlerRegistered(object target, MethodInfo methodInfo)
        {
            if (ExecuteAction != null)
            {
                foreach (Delegate dlg in ExecuteAction.GetInvocationList())
                {
                    if (dlg.Target == target && dlg.Method == methodInfo)
                    {
                        return true;
                    }
                }
            }
            return false;
        }