LINQ FirstOrDefault检查默认值 [英] LINQ FirstOrDefault check for default value

问题描述

如何检查FirstOrDefault LINQ函数返回的对象是否实际上是默认对象?

How can you check to see whether the object returned by the FirstOrDefault LINQ function is in fact the default?

例如:

Contact contact = dbo.contact
                     .Where(m => m.contactName == "Stackoverflow")
                     .FirstOrDefault();

是否有其他方法可以检查上面的联系人是否为默认值，而不是使用以下内容?

Is there an alternative way to check whether the contact above is default value instead of using the following?

if (!contact.contactName.Equals("Stackoverflow"))
    // do something

推荐答案

您不需要执行相等检查，因为您的查询仅返回contantName为Stackoverflow的对象.当您使用FirstOrDefault时，如果未找到任何对象，它将返回null，您可以这样做

You wouldn't need to perform that equals check because your query only returns objects where the contantName is Stackoverflow. When you use FirstOrDefault it returns null if no objects were found so you can do

if(contact == null)
    do something

如果Contact是一个类，您知道它是引用类型，因此其默认值将为null.但是，您可以使用default检查它是任何对象(引用或值)的默认类型.

You know it's a reference type if Contact is a class so it's default value would be null. You can, however, check it's the default type of any object (reference or value) by using default.

if(contact == default(Contact))
    do something

如注释中所述，您可以通过使用带谓词的FirstOrDefault重载来提高代码的效率.

As mentioned in the comments, you can possibly make your code more efficient by using the overload of FirstOrDefault that takes a predicate.

FirstOrDefault(m => m.contactName == "Stackoverflow") 

如果程序需要与null或0以外的其他程序一起使用，则还可以更改返回的默认值.例如

You can also change the default value returned if your program needs to work with something other than a null or 0. For example

Contact defaultContact = new Contact();
defaultContact.ContactName = "StackExchange";

Contact contact = dbo.contact.Where(m=>m.contactName == "Stackoverflow")
                             .DefaultIfEmpty(defaultContact).First();

如果未找到其他对象，则以上内容将返回defaultContact对象(而不是返回null).如果执行此操作，则无需检查null或default(T)，因为您知道自己有一个Contact对象.

The above will return the defaultContact object if no other object was found (instead of returning null). If you do this then you don't need to check for null or default(T) because you know you have a Contact object.

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