Quick Answer

Process collections with Language Integrated Query.

Understanding the Issue

LINQ provides SQL-like query capabilities for in-memory and external data with deferred execution.

The Problem

This code demonstrates the issue:

Csharp Error 
List<Person> people = GetPeople();
// Need to query data

The Solution

Here's the corrected code:

Csharp Fixed 
// Filtering
var adults = people.Where(p => p.Age >= 18);

// Projection
var names = people.Select(p => p.Name);

// Sorting
var ordered = people.OrderBy(p => p.Name)
                   .ThenByDescending(p => p.Age);

// Aggregation
double avgAge = people.Average(p => p.Age);
int oldest = people.Max(p => p.Age);

// Grouping
var byAge = people.GroupBy(p => p.Age / 10 * 10);

// Join
var joined = from p in people
             join a in addresses on p.Id equals a.PersonId
             select new { p.Name, a.City };

// Query syntax
var query = from p in people
            where p.Age > 21
            orderby p.Name
            select p.Name;

Key Takeaways

Use LINQ for declarative data processing with deferred execution.