使用insertBefore进行HTML5拖放(HTML5 Drag and Drop with insertBefore)

 如果属性中有空值，那就没那么糟了。 让我们考虑这个例子： 
 
var items = from obj in "Hello".ToCharArray()
            select new
            {
                First = (string)null
            };    
var listTemp = items.ToList();
 
 当你调用ToList方法时，它完全没问题。 
 
 可能，异常来自列表中的null元素，您可以从CommonList.ItemList()方法获得该元素。 我扩展了前面的示例，并将null元素添加到列表中： 
 
var listTemp = items.ToList();
listTemp.Add(null);

var itemsProcessed = from item in listTemp
                     select new
                     {
                         item.First 
                     };    
var list = itemsProcessed.ToList();
 
 在这种情况下，将抛出NullReferenceException ，因为当我们创建新元素并尝试获取First属性时，我们没有真正的元素。 
 
 如果在查询中添加where子句 ，则可以轻松解决此问题： 
 
var itemsProcessed = from item in listTemp
                     where item != null
                     select new
                     {
                         item.First 
                     };

If you have null values in the properties, it's not so bad. Let's consider this example:
 
var items = from obj in "Hello".ToCharArray()
            select new
            {
                First = (string)null
            };    
var listTemp = items.ToList();
 
When you call the ToList method it's totally fine.
 
Probably, the exception comes from the null element in the list, which you get from the CommonList.ItemList() method. I extend the previous example and add the null element into the list:
 
var listTemp = items.ToList();
listTemp.Add(null);

var itemsProcessed = from item in listTemp
                     select new
                     {
                         item.First 
                     };    
var list = itemsProcessed.ToList();
 
In this case the NullReferenceException will be thrown, because when we create the new element and try to get the First property we don't have a real element.
 
This issue can be easily solved if you add the where clause in the query:
 
var itemsProcessed = from item in listTemp
                     where item != null
                     select new
                     {
                         item.First 
                     };