LINQ to XML 中的 ISNULL()

Question

SELECT * FROM CUSTOMERS WHERE RTRIM(ISNULL([SHORTNAME],'')) LIKE '%john%'

我想使用 Linq 编写此内容，

var persons = from person in xmlDoc.Descendants("Table")
where 
person.Element("SHORTNAME").Value.Contains("123")
select new
{
  shortName = person.Element("SHORTNAME").Value,
  longName = person.Element("LONGNAME").Value,
  address = person.Element("ADDRESS").Value,
  Phone = person.Element("PHONE") != null ? person.Element("PHONE").Value : "",
  zip = person.Element("ZIPCODE") != null ? person.Element("ZIPCODE").Value : "",
};

当 [SHORTNAME] 不为空且 [SHORTNAME] 为空值时，此方法可以正常工作这会破坏代码并弹出“空引用异常”

请帮助我......

Answer 1

假设您试图避免选取任何没有短名称的内容...

var persons = from person in xmlDoc.Descendants("Table")
    let shortNameElement = person.Element("SHORTNAME") 
    where shortNameElement != null && shortNameElement.Value.Contains("123")
    select new
    {
        shortName = person.Element("SHORTNAME").Value,
        longName = person.Element("LONGNAME").Value,
        address = person.Element("ADDRESS").Value,
        Phone = person.Element("PHONE") != null ? 
            person.Element("PHONE").Value : "",
        zip = person.Element("ZIPCODE") != null ? 
            person.Element("ZIPCODE").Value : "",
    };

或者，您可以使用空合并运算符使所有这些变得更简单：

var emptyElement = new XElement("ignored", "");

var persons = from person in xmlDoc.Descendants("Table")
    where (person.Element("SHORTNAME") ?? emptyElement).Value.Contains("123")
    select new
    {
        shortName = person.Element("SHORTNAME").Value,
        longName = person.Element("LONGNAME").Value,
        address = person.Element("ADDRESS").Value,
        Phone = (person.Element("PHONE") ?? emptyElement).Value
        zip = (person.Element("ZIPCODE") ?? emptyElement).Value
    };

或者，您可以编写一个扩展方法：

public static string ValueOrEmpty(this XElement element)
{
    return element == null ? "" : element.Value;
}

然后像这样使用它：

var persons = from person in xmlDoc.Descendants("Table")
    where person.Element("SHORTNAME").ValueOrEmpty().Contains("123")
    select new
    {
        shortName = person.Element("SHORTNAME").Value,
        longName = person.Element("LONGNAME").Value,
        address = person.Element("ADDRESS").Value,
        Phone = person.Element("PHONE").ValueOrEmpty(),
        zip = person.Element("ZIPCODE").ValueOrEmpty()
    };

Answer 2

使用空合并运算符：

Phone = person.Element("PHONE") ?? String.Empty;