我在这里用List做错了什么

问题描述：

List<int> processedList = new List<int>();
            processedList = (List<int>) Session["processedUsers"];

我在这里获取参数null引用异常吗???

I am getting argument null reference exception here????

if(processedList.Count != 0)
{
  Boolean exists =  processedList.Contains(i);
    if (!exists)
    {
    }

我甚至收到Null参考异常

布尔值存在= processingList.Exists(element => element == i)

会话状态列表可以为空.我仍然进行了此测试.
请告知.

I am getting Null reference exception even

Boolean exists = processedList.Exists(element => element ==i)

The session state list can be empty. Still I have run this test.
Please advise.

答

您用Session中的值覆盖了processedList实例.因此，如果会话中没有列表实例，则会出现此异常.

因此，您可能没有正确初始化Session["processedUsers"]，因此会遇到异常.

You overwrite your processedList instance with a value from Session. So if there is no list instance in the session, you will get this exception.

So you probably don''t have correctly initialized Session["processedUsers"] and thus you have exception.

简单地放置

Put simply,

List<int> processedList = new List<int>();

创建处理列表并为其分配一个新的空列表.

Creates processedList and assignes a new, empty list to it.

processedList = (List<int>) Session["processedUsers"];

用强制转换返回的内容替换空列表.

Replaces the empty list with whatever the cast returns.

if(processedList.Count != 0)

使用处理列表中包含的列表来检查条目数.

因此，如果Session["processedUSers"]不返回列表，则:
如果返回非列表项，则将引发无法转换"异常.
如果返回null，则processedUsers 将变为null，并且分配给您的空列表将被丢弃.

如果processedUsers 为null，则尝试访问任何元素，即使项目计数也将给出null引用异常.

因此:在尝试使用Session["processedUSers"]之前，先检查Session["processedUSers"]返回的内容，并确保processedItems 是有效列表.

Uses the list contained in processedList to check the number of entries.

So if Session["processedUSers"] does not return a list then:
if it returns a non-list item it will throw a "cannot convert" exception.
if it returns a null, processedUsers will become null, and the empty list you assigned will be discarded.

If processedUsers is null, then attempting to access any element, even the items count, will give a null reference exception.

So: Check what Session["processedUSers"] is returning, and make sure that processedItems is a valid list before you try and use3 it.

我在这里用List做错了什么

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