[英]how to check List<string> is null
I wanna to check the values (its type is list) and if it is null, it starts again from at the beginning of the loop and if it is not, counter++. 我想检查值(它的类型是list),如果它为null,它会从循环的开头重新开始,如果不是,则是counter ++。 I mean if values is like: "values": []
the compile back at the beginning of loop. 我的意思是,如果值就像: "values": []
在循环开始时重新编译。
I tried with if (id==null) and also I used this one (id.Contains(""))
but the error happens. 我尝试了if (id==null) and also I used this one (id.Contains(""))
但是发生了错误。 Error:Index was out of range. 错误:索引超出范围。 Must be non-negative and less than the size of the collection.Parameter name: index (Argument out of range exception was unhandled). 必须为非负数且小于集合的大小。参数名称:index(未处理参数超出范围的异常)。
Data that I want to check (when the values is null) 我要检查的数据(当值为空时)
{
"cid": "241",
"fnames": [
"id",
"name",
"hash",
"score",
"rank"
],
"values": [],
"tal": 0,
"sion": "v3"
}
Full data (when the values is not null) 完整数据(值不为null时)
{
"cid": "64",
"fnames": [
"id",
"name",
"hash",
"score",
"rank"
],
"values": [
[
"126",
"126",
"126",
"1",
"2.77"
],
"tal": 0,
"sion": "v3"
}
Code 码
foreach (DataRow row in dt.Rows)
{
string url = "http://hgsfe/<uid>?groups=<uid>
var test = url.Replace("<uid>", Convert.ToString(row[uid]));
System.Diagnostics.Process.Start(test);
string client = (new WebClient()).DownloadString(test);
var pi = JsonConvert.DeserializeObject<read_json>(client);
List<string> id = pi.values[0];
if (id==null) //also I used this one (id.Contains(""))
continue;
if (id.Contains(Convert.ToString(row[tid])))
{
counter++;
}
尝试检查pi.Count
或pi.Length
以获取列表中的项目数。
You have to check both: 您必须同时检查:
pi == null
pi.values[0] == null
because you try the assignment, but pi doesn't event exist (probably) 因为您尝试分配,但是pi不存在事件(可能)
When you debug your code, you can clearly see, where the application crashes. 在调试代码时,您可以清楚地看到应用程序崩溃的位置。
Hope i could help you with that 希望我能帮你
Try the following 尝试以下
List<string> id = pi.values.ToList();
And after this just check the Count
property of the list 然后,只需检查列表的Count
属性
if(id.Count<1)
continue;
...
...
you did provide code with a lot of 'unknowns', but here is what could be wrong and you should check it to make your code safer: 您确实为代码提供了很多“未知数”,但是这可能是错误的,应该检查一下以使代码更安全:
uid
? 什么是uid
? You are accessing row[uid]
. 您正在访问row[uid]
。 row
is of type DataRow
where accessing item which does not exist (specified by uid
) throws exception row
类型为DataRow
,其中不存在的访问项(由uid
指定)将引发异常 pi
? pi
是什么? what is pi.values
? pi.values
是pi.values
? Maybe you should check if pi
is null, pi.values
is null and you should definately check if pi.values
contains any items (so if item values[0]
is present). 也许您应该检查pi
是否为null, pi.values
是否为null,并且应该确定检查pi.values
包含任何项目(因此,如果item values[0]
存在)。 If its empty then values[0]
will throw exception you mentioned. 如果其为空,则values[0]
将引发您提到的异常。 pi.values[0]
may be present, but it again may be assigned null. 可能存在pi.values[0]
,但可以再次将其分配为null。 So your line if (id.Contains...
may fail on that you are trying to call id.Contains
on null item id
. Check if its not null before call. 所以您的行if (id.Contains...
可能会因为您尝试调用id
。而id.Contains
null项目id
而失败。请在调用之前检查其是否不为null。 tid
in row[tid]
? 什么是tid
在row[tid]
Again, check if such item is present in the row so you won't get DataRow exception. 再次,检查该项目是否存在于行中,这样就不会出现DataRow异常。
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