Resharper声明if / else条件永远不会被击中，为什么？

Question

我从PushSharp示例中获取此代码，该示例详细说明了如何处理异常。出于某种原因，Resharper将所有else条件都显示为灰色，并声明The expression is always false。我不知道这是怎么可能的。

// ex is an Exception passed in to the method
if (ex is NotificationException)
{
    // Deal with the failed notification
    var notification = ((NotificationException)ex).Notification;
    var logItem = new PushLog($"{typePrefix} Notification failed", $"Notification Failed: {notification}");
    _pushLogRepo.Insert(logItem);
}
else if (ex is DeviceSubscriptionExpiredException) // Resharper says this is always false
{
   // exception handling code...
}
else if (ex is RetryAfterException) // Resharper says this is always false
{
   // exception handling code...
}
else
{
    Console.WriteLine("Notification Failed for some (Unknown Reason)");
}

有人可以解释一下这是怎么回事吗？我不明白它是怎么回事。这是VS2015的屏幕截图，语法高亮显示更清晰 - 忽略错误，我正在重构。

Answer 1

如果这些类继承NotificationException，就会发生这种情况，因为那时第一个分支总会命中。