What's the Difference Between LINQ's Select and SelectMany Operators?

LINQ to SQL: Understanding Select and SelectMany

Mastering LINQ to SQL requires understanding the key differences between Select and SelectMany. Both project data, but their approaches differ significantly.

Select performs a one-to-one transformation. It takes each element from an input sequence and applies a function to produce a new element in the output sequence. This is ideal for extracting specific properties or performing calculations on individual items.

SelectMany, on the other hand, flattens a sequence of sequences into a single sequence. This is invaluable when working with hierarchical or nested data structures. It allows you to traverse multiple levels and access the underlying elements.

Let's illustrate with a Person class containing a collection of PhoneNumber objects:

<code class="language-csharp">public class PhoneNumber
{
    public string Number { get; set; }
}

public class Person
{
    public IEnumerable<PhoneNumber> PhoneNumbers { get; set; }
    public string Name { get; set; }
}</code>

Select Example:

<code class="language-csharp">IEnumerable<IEnumerable<PhoneNumber>> phoneLists = people.Select(p => p.PhoneNumbers);</code>

This uses Select to get a sequence of phone number sequences—one for each person. The result remains nested.

SelectMany Example:

<code class="language-csharp">IEnumerable<PhoneNumber> phoneNumbers = people.SelectMany(p => p.PhoneNumbers);</code>

SelectMany flattens the nested structure, producing a single sequence containing all phone numbers from all people.

Combining Parent and Child Data with SelectMany:

A powerful feature of SelectMany is its ability to incorporate parent data into the projection. Using the overload with a second parameter, you can create a new object combining parent and child properties:

<code class="language-csharp">var directory = people
    .SelectMany(p => p.PhoneNumbers, (parent, child) => new { parent.Name, child.Number });</code>

This generates a directory listing each person's name and their associated phone numbers. The output combines data from both the Person and PhoneNumber classes.

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