Lesson 3: Filtering & Projection with LINQ

Filtering and projection are two of the most important LINQ patterns. Filtering selects the subset of data you need, and projection shapes each item into the form you want.

Filter with Where

Where filters a sequence by a condition. Each item is passed to a lambda function that returns true or false.

var users = new[]
{
    new { Name = "Ava", Age = 23 },
    new { Name = "Leo", Age = 31 },
    new { Name = "Mia", Age = 19 }
};

var adults = users.Where(user => user.Age >= 21);

Shape Data with Select

Select projects each item into a new form, such as an anonymous type or a primitive value.

var names = users.Select(user => user.Name);
var summaries = users.Select(user => new { user.Name, user.Age });

Use SelectMany for Nested Collections

SelectMany flattens nested collections into a single sequence.

var orders = new[]
{
    new { OrderId = 101, Items = new[] { "Pen", "Notebook" } },
    new { OrderId = 102, Items = new[] { "Mouse", "Keyboard" } }
};

var allItems = orders.SelectMany(order => order.Items);

Distinct, Take, and Skip

LINQ also includes operators for deduplication and paging:

• Distinct() removes duplicates.
• Take(n) returns the first n items.
• Skip(n) skips the first n items.

Projection Example

Projection is useful when you only need specific fields or when you want to transform data for UI or reporting.

var emailList = users
    .Where(user => user.Age >= 21)
    .Select(user => $"{user.Name.ToLower()}@example.com");

Lesson Summary