Select vs. SelectMany in LINQ to SQL: When Should I Use Which?

The differences and application scenarios of SelectMany in Linq to SQL

When using Linq to SQL, it is important to understand the difference between

and

. Select It is used to retrieve a single element from the data set, and SelectMany The query results used to flatten the nested set. Select SelectMany The following examples will explain the differences between the two more clearly:

Assuming we have a

class that contains a

attribute, which returns a set of a Person instance. In order to retrieve all the phone numbers of all personnel, we can use PhoneNumbers: PhoneNumber Select

However, contains the collection of each phone number list, which may not be the result we want. In order to flatten this nested data and obtain a complete phone number list, we use

<code class="language-csharp">IEnumerable<Person> people = new List<Person>();

// Select: 返回一个电话号码列表的列表
IEnumerable<IEnumerable<PhoneNumber>> phoneLists = people.Select(p => p.PhoneNumbers);</code>

:

phoneLists SelectMany In addition, if we want to include the data of the father

to the result, we can use the

<code class="language-csharp">// SelectMany: 将集合扁平化为一个电话号码列表
IEnumerable<PhoneNumber> phoneNumbers = people.SelectMany(p => p.PhoneNumbers);</code>

heavy load:

Person SelectMany By understanding the difference between and

, you can effectively operate the data in the Linq to SQL query to accurately retrieve and aggregate the required information.

<code class="language-csharp">// 带结果选择器的 SelectMany: 在结果中包含父数据
var directory = people
    .SelectMany(p => p.PhoneNumbers,
                (parent, child) => new { parent.Name, child.Number });</code>

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