这个语句如何理解?+=

这个语句怎么理解?+=
workflowRuntime.WorkflowCompleted += delegate(object sender, WorkflowCompletedEventArgs e) {waitHandle.Set();}; 
这个语句怎么理解?+=和 {}谁先呀?

[解决办法]
delegate 建立委托阿
WorkflowCompleted 先
[解决办法]
delegate 建立委托阿 
WorkflowCompleted 就执行waitHandle.Set();

+= 是建立委托阿