当.count()大于0时，从.select()中获取NullReferenceException

本文关键字：select 获取 NullReferenceException 0时 count 大于 | 更新日期: 2023-09-27 18:18:35

我有一个XML文件:

<Org href="https://vcloudserver/api/v1.0/org/272521719" type="application/vnd.vmware.vcloud.org+xml" name="blah-blah" xmlns="http://www.vmware.com/vcloud/v1">
     <Link href="https://vcloudserver/api/v1.0/vdc/1093121285" type="application/vnd.vmware.vcloud.vdc+xml" name="blah-haha" rel="down"/>
     <Link href="https://vcloudserver/api/v1.0/vdc/1213262741" type="application/vnd.vmware.vcloud.vdc+xml" name="blah-hoho" rel="down"/>
     <Link href="https://vcloudserver/api/v1.0/tasksList/272521719" type="application/vnd.vmware.vcloud.tasksList+xml" rel="down"/>
     <Link href="https://vcloudserver/api/v1.0/catalog/1309520800" type="application/vnd.vmware.vcloud.catalog+xml" name="blah-hehe" rel="down"/>
     <Link href="https://vcloudserver/api/v1.0/org/272521719/catalog/1309520800/controlAccess/" type="application/vnd.vmware.vcloud.controlAccess+xml" rel="down"/>
     <Link href="https://vcloudserver/api/v1.0/org/272521719/catalog/1309520800/action/controlAccess" type="application/vnd.vmware.vcloud.controlAccess+xml" rel="controlAccess"/>
     <Link href="https://vcloudserver/api/v1.0/network/1435818199" type="application/vnd.vmware.vcloud.network+xml" name="blah-whodat" rel="down"/>
     <Link href="https://vcloudserver/api/v1.0/network/2048048931" type="application/vnd.vmware.vcloud.network+xml" name="blah-disis" rel="down"/>
     <Description/>
     <FullName>Blah diddy Blah-Blah</FullName>
</Org>

那么，给定以下内容:

XNamespace nameSpace = "http://www.vmware.com/vcloud/v1";
var doc = XDocument.Parse(xml);

当我在VS即时窗口中调试以下代码时:

doc.Root.Elements(nameSpace + "Link").Count()

得到一个期望值，8。但是当我进一步使用:

var vdcs = doc.Root.Elements(nameSpace + "Link")
  .Select(x => new vDC()
  {
    Name = x.Attribute("name").Value,
    Type = x.Attribute("type").Value,
    Href = x.Attribute("href").Value
  }).Where(x=>x.Type.Contains("vdc"));

我得到一个NullReferenceException试图访问vdcs.Count()。我已经在这上面折腾太久了……我在其他地方也用得很好，所以这没什么用。:(尝试在Where()之前填充一个ToList()，并且只是将NullReferenceException移动到该调用。

如果有帮助，vDC当前定义为:

public class vDC
{
  public string Name { get; set; }
  public string Type { get; set; }
  public string Href { get; set; }
}

当.count()大于0时，从.select()中获取NullReferenceException

部分Link元素没有name属性，所以:

var vdcs = doc.Root.Elements(nameSpace + "Link")
  .Select(x => new vDC()
  {
    Name = x.Attribute("name").Value,
    Type = x.Attribute("type").Value,
    Href = x.Attribute("href").Value
  }).Where(x=>x.Type.Contains("vdc"));

窒息

Name = x.Attribute("name").Value

因为在某些情况下x.Attribute("name")是null。

var vdcs = doc.Root.Elements(nameSpace + "Link")
  .Select(x => new 
  {
    NameAttr = x.Attribute("name"),
    TypeAttr = x.Attribute("type"),
    HrefAttr = x.Attribute("href")
  }).Select(x => new vDC()
  {
    Name = x.NameAttr == null ? null : x.NameAttr.Value,
    Type = x.TypeAttr == null ? null : x.TypeAttr.Value,
    Href = x.HrefAttr == null ? null : x.HrefAttr.Value, 
  }).Where(x=>x.Type.Contains("vdc"));

你的一些链接没有name -他们有rel代替。NPE就是这样来的。

只有在调用Count时才会看到它，因为实际的求值在此之前不会发生。

要更改代码以防止NPE，请添加Where条件:

var vdcs = doc.Root.Elements(nameSpace + "Link")
.Where(x => x.Attribute("name") != null && x.Attribute("type") != null && x.Attribute("value") != null)
.Select(x => new vDC {
    Name = x.Attribute("name").Value,
    Type = x.Attribute("type").Value,
    Href = x.Attribute("href").Value
}).Where(x=>x.Type.Contains("vdc"));
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