csharp-linq

# C# LINQ

Master Language Integrated Query (LINQ) for querying and transforming data in C#.
This skill covers query syntax, method syntax, deferred execution, performance
optimization, and advanced LINQ patterns from C# 8-12.

## LINQ Query Syntax vs Method Syntax

LINQ supports two syntaxes: query syntax (SQL-like) and method syntax (fluent).
Both compile to the same code.

### Query Syntax

```csharp
var students = new List<Student>
{
    new Student { Name = "Alice", Grade = 85, Age = 20 },
    new Student { Name = "Bob", Grade = 92, Age = 21 },
    new Student { Name = "Charlie", Grade = 78, Age = 20 }
};

// Query syntax - SQL-like
var topStudents = from student in students
                  where student.Grade >= 80
                  orderby student.Grade descending
                  select new { student.Name, student.Grade };

foreach (var student in topStudents)
{
    Console.WriteLine($"{student.Name}: {student.Grade}");
}
```

### Method Syntax

```csharp
// Method syntax - fluent API
var topStudents = students
    .Where(s => s.Grade >= 80)
    .OrderByDescending(s => s.Grade)
    .Select(s => new { s.Name, s.Grade });

// Method syntax is more flexible for complex queries
var result = students
    .Where(s => s.Age >= 20)
    .GroupBy(s => s.Age)
    .Select(g => new
    {
        Age = g.Key,
        AverageGrade = g.Average(s => s.Grade),
        Count = g.Count()
    })
    .OrderBy(x => x.Age);
```

### When to Use Each Syntax

```csharp
// Query syntax better for joins
var query1 = from student in students
             join course in courses on student.Id equals course.StudentId
             where course.Grade > 80
             select new { student.Name, course.Title };

// Method syntax better for chaining and complex logic
var query2 = students
    .Where(s => s.Age >= 20)
    .SelectMany(s => s.Courses)
    .Where(c => c.Grade > 80)
    .Distinct()
    .Take(10);

// Mixed approach
var query3 = (from s in students
              where s.Age >= 20