c# - 在 .NET 中如何检查事件是否已被订阅？

Question

在我的代码中的某个时刻，我订阅了 the following event :-

UploadFolderMessageQueue.ReceiveCompleted += UploadMSMQReceiveCompleted;

效果很好，当消息队列的 Recieved Completed 事件触发时，我的委托(delegate)会处理它。

现在，我想在订阅之前检查事件是否已被订阅。当我这样做时出现编译时错误:-

// Compile Time Errors...
if (UploadFolderMessageQueue.ReceiveCompleted == null)
    {
        UploadFolderMessageQueue.ReceiveCompleted += UploadMSMQReceiveCompleted;
        UploadFolderMessageQueue.Formatter = 
            new XmlMessageFormatter(new[] {typeof (string)});
    }

The event 'System.Messaging.MessageQueue.ReceiveCompleted' can only appear on the left hand side of += or -=

我知道这非常简单..但我很难过:(有什么建议吗？

Answer 1

如果您需要确保只有一个订阅者可以使用以下代码:

UploadFolderMessageQueue.ReceiveCompleted -= UploadMSMQReceiveCompleted;
UploadFolderMessageQueue.ReceiveCompleted += UploadMSMQReceiveCompleted;

如果 UploadFolderMessageQueue.ReceiveCompleted 是 null 那么第一行什么都不做，否则事件处理程序将被删除。这意味着 UploadFolderMessageQueue.ReceiveCompleted 将始终只有一个订阅者(当然如果 UploadMSMQReceiveCompleted 是该事件的唯一一个事件处理程序)。