如果响应中存在值，请检查。易趣API [重复]

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• What is a NullReferenceException, and how do I fix it? 33 answers

什么是NullReferenceException,我该如何解决? 33个答案

I have a problem with the development of an application. I am using the eBay API to get information about the items a seller has listed. The Call I use is named "GetSellerList".

我在开发应用程序时遇到问题。我正在使用eBay API获取有关卖家列出的商品的信息。我使用的调用名为“GetSellerList”。

The response might contain a value called "CategoryID" (FreeAddedCategory.CategoryID) if there is a FreeAddedCategory available for the item. You would find a XML like this as response:

如果有可用于项目的FreeAddedCategory,则响应可能包含名为“CategoryID”(FreeAddedCategory.CategoryID)的值。你会发现像这样的XML作为响应:

<ItemArray>
  <Item>
    <FreeAddedCategory>
      <CategoryID>XXXXXXXXX</CategoryID>
    </FreeAddedCategory>
  </Item>
</ItemArray>

Because this value is not always returned I am getting the following error message:

因为并不总是返回此值,我收到以下错误消息:

Object reference not set to an instance of an object

你调用的对象是空的

I now want to check if this object is available before using it to prevent getting the error. This is how I startet to solve the issue (but without success):

我现在想在使用它之前检查这个对象是否可用,以防止出现错误。这就是我解决问题的方法(但没有成功):

    foreach (ItemType item in apiCall.ApiResponse.ItemArray)
       {
          if (String.IsNullOrWhiteSpace(item.FreeAddedCategory.CategoryID))
                {
                    Console.WriteLine("FreeAdded: Nicht vorhanden!");
                }
        }

But also this check brings up the same error message. What can I do to make the code working? The logic should be like that:

但是这个检查也会显示相同的错误消息。我该怎么做才能使代码工作?逻辑应该是这样的:

IF item.FreeAddedCategory.CategoryID exist
  {
      do everything that needs to be done
  }
ELSE skip;

1 个解决方案

#1

Try:

XElement root = XElement.Load(file); // .Parse(string);
if (root.Descendants("FreeAddedCategory").Any())
{
   // do exists
}
else
{
   // doesn't exist
}

Or use CategoryId if that makes more sense for your case.

或者使用CategoryId,如果这对你的情况更有意义。