C#中的is关键字原来会做null检查

internal class ShouldLimitNameLengthConverter : IValueConverter
    {
        const int mIdealNameLengthMax = 20;

        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (value is string)
                return ((string)value).Length > mIdealNameLengthMax;
            throw (new NotSupportedException());
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            Debug.Assert(false);
            throw (new NotSupportedException());
        }
    }

在改一个用户发来的crash report，通过windbg调试dump文件，确认了crash发生在Convert函数，异常是NotSupportedException，那么原因很简单，就是value is string返回了true。这个converter是WPF的DataTemplate调用的，绑定是一个对象的Text属性，是String类型。照理说没问题啊！百思不得其解！终于查了MSDN发现，in关键字是会check
null的，只要对象为空就返回false：

An is expression evaluates totrue if the provided expression is
non-null, and the provided object can be cast to the provided type without causing an exception to be thrown.

呵呵，长知识了！