事件处理程序和空条件运算符

Question

For example, implement INotifyPropertyChanged interface:

public event PropertyChangedEventHandler PropertyChanged;

private void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
    var handler = PropertyChanged;
    if (handler != null)
        handler.Invoke(this, new PropertyChangedEventArgs(propertyName));
}

Two things:

1. Copy event to local variable to prevent errors with multithreading ( here are some examples). Resharper gives notification, if you don't copy to local variable:

2. Check it for null, to prevent NullReferenceException

But now, we can use ?. operator for null-checking. And if I use it, Resharper is idle:

So, question is: should I copy event ProperyChanged to local variable, if I use null-conditional operator?

Answer 1

should I copy event ProperyChanged to local variable, if I use null-conditional operator?

No, there's no need. In fact, one of the main reasons the null-conditional operator was introduced was to simplify code using this pattern. It has the same effect as copying the source value to a local variable and inherently avoids the "check and use" concurrency trap that the "copy to local variable" technique is intended to address.

See related posts:
Invoking Events, h(args) vs EventName?.Invoke() (almost an exact duplicate…it does approach the question from a slightly different angle though)
Why should I check for null before I invoke the custom event?
Raising C# events with an extension method - is it bad?
Is there any reason to assign an event to a local variable before raising it?

Answer 2

还有其他方法可以进行空检查 - 简单地将委托{}分配给您的事件，因此它永远不会为空

public event PropertyChangedEventHandler PropertyChanged = delegate{};