[英]null reference exception with linq .where clause
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 null 的对象数组(数组)中获取一个属性,但我总是得到一个 null 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 null 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.我也试过不取空值。 I don't get the mechanism to tell LINQ not to process if the array is null.如果数组为空,我没有得到告诉 LINQ 不要处理的机制。
phone = c.address.contactItems.Where( ci => ci != null && ci.contactType == ContactType.PHONE).First().contactText
You can check if it's null
with ?:
(conditional) operator :您可以使用?:
(条件)运算符检查它是否为null
:
phone = c.address.contactItems == null ? ""
: c.address.contactItems.Where( ci => ci.contactType == ContactType.PHONE).First().contactText
If First
throws an exception because there's no one with ContactType.PHONE
you can use DefaultIfEmpty
with a custom default value:如果First
抛出异常,因为没有人使用ContactType.PHONE
,您可以将DefaultIfEmpty
与自定义默认值一起使用:
c.address.contactItems.Where( ci => ci.contactType == ContactType.PHONE)
.DefaultIfEmpty(new Contact{contactText = ""})
.First().contactText
Note that First
now 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,因为我提供了默认值。
Try the code below (I've assumed that contactText
is a string
).试试下面的代码(我假设contactText
是一个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
});
}
Try:尝试:
var contact = c.address.contactItems.Where( ci => ci.contactType == ContactType.PHONE).FirstOrDefault();
phone = contact != null ? contact.contactText : "";
the null
value is contactType
so we add (ci.contactType != null)
null
值是contactType
所以我们添加(ci.contactType != null)
var phone = c.address.contactItems.Where( ci => (ci.contactType != null) && ci.contactType == ContactType.PHONE).First().contactText
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
}
); ); } }
avoid the argument null exception in linq like below像下面这样在 linq 中避免参数 null 异常
Summaries = (from r in Summaries
where r.Contains(SearchTerm)
orderby r
select r).ToArray();
In this case, if null passed to searchTerm you can check the null expression like below在这种情况下,如果 null 传递给 searchTerm 您可以检查 null 表达式,如下所示
Summaries = (from r in Summaries
where string.IsNullOrEmpty(SearchTerm) ||r.Contains(SearchTerm)
orderby r
select r).ToArray();
It work's for me!这个对我有用!
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