C# linq .where 子句的空引用异常
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null reference exception with linq .where clause
提问by Arno 2501
I'm trying to get a property from an array of object that can be null (the array) and I'm always getting a null reference exception.
我试图从一个可以为空的对象数组（数组）中获取一个属性，但我总是收到一个空引用异常。
How can I tell LINQ to not process it in case it's null or to return an empty string?
如果它为空或返回空字符串，我如何告诉 LINQ 不处理它？
foreach (Candidate c in candidates) {
results.Add(new Person
{
firstName = c.firstname, //ok
lastName = c.Name, //ok
// contactItems is an array of ContactItem
// so it can be null that's why I get null exception
// when it's actually null
phone = c.address.contactItems.Where( ci => ci.contactType == ContactType.PHONE).First().contactText
}
);
}
I've also tried that to not take null. I don't get the mechanism to tell LINQ not to process if the array is null.
我也试过不取空。如果数组为空，我没有告诉 LINQ 不处理的机制。
phone = c.address.contactItems.Where( ci => ci != null && ci.contactType == ContactType.PHONE).First().contactText
采纳答案by Tim Schmelter
You can check if it's nullwith ?:(conditional) operator:
您可以检查它是否null与?:（条件）运算符：
phone = c.address.contactItems == null ? ""
: c.address.contactItems.Where( ci => ci.contactType == ContactType.PHONE).First().contactText
If Firstthrows an exception because there's no one with ContactType.PHONEyou can use DefaultIfEmptywith a custom default value:
如果First因为没有人ContactType.PHONE可以使用DefaultIfEmpty自定义默认值而引发异常：
c.address.contactItems.Where( ci => ci.contactType == ContactType.PHONE)
.DefaultIfEmpty(new Contact{contactText = ""})
.First().contactText
Note that Firstnow cannot throw an exception anymore since i've provided a default value.
请注意，First现在不能再抛出异常，因为我已经提供了一个默认值。
回答by Hamlet Hakobyan
foreach (Candidate c in candidates) {
results.Add(new Person
{
firstName = c.firstname, //ok
lastName = c.Name, //ok
// contactItems is an array of ContactItem
// so it can be null that's why I get null exception
// when it's actually null
phone = c.address.contactItems == null
? string.Empty
:c.address.contactItems.Where( ci => ci.contactType == ContactType.PHONE).First().contactText
}
); }
); }
回答by Chris Ayers
Try:
尝试：
var contact = c.address.contactItems.Where( ci => ci.contactType == ContactType.PHONE).FirstOrDefault();
phone = contact != null ? contact.contactText : "";
回答by spajce
the nullvalue is contactTypeso we add (ci.contactType != null)
该null值contactType，所以我们增加(ci.contactType != null)
var phone = c.address.contactItems.Where( ci => (ci.contactType != null) && ci.contactType == ContactType.PHONE).First().contactText
回答by Adrian Thompson Phillips
Try the code below (I've assumed that contactTextis a string).
试试下面的代码（我假设它contactText是 a string）。
You may want to look at standardising the capitalisation of your public property names to all start with an upper-case letter.
您可能希望将公共属性名称的大小写标准化为全部以大写字母开头。
foreach (Candidate c in candidates) {
string contactText =
c.address.contactItems
.Where(ci => ci.contactType == ContactType.PHONE)
.Select(ci => ci.contactText)
.FirstOrDefault()
results.Add(
new Person
{
firstName = c.firstname,
lastName = c.Name,
phone = contactText ?? string.Empty
});
}
