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 isnon-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！