csharp-linq

Use when lINQ query and method syntax, deferred execution, and performance optimization. Use when querying collections in C#.

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Repositório: TheBushidoCollective/han
Última atividade na origem: 11 de fevereiro de 2026 às 17:47
Idioma detectado do SKILL.md: inglês
Estrelas: 189
Forks: 20

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// 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);

// 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
              select s)
    .Take(10)
    .ToList(); // Force execution

var numbers = new List<int> { 1, 2, 3, 4, 5 };

// Query is defined but NOT executed
var query = numbers.Where(n => n > 2);

// Add more numbers
numbers.Add(6);
numbers.Add(7);

// Query executes NOW when enumerated
foreach (var num in query) // Gets: 3, 4, 5, 6, 7
{
    Console.WriteLine(num);
}

// Query executes AGAIN (sees current state)
var count = query.Count(); // 5

// Force immediate execution with ToList(), ToArray(), etc.
var snapshot = numbers.Where(n => n > 2).ToList();
numbers.Add(8);
Console.WriteLine(snapshot.Count); // Still 5, not 6

public class DeferredExecutionExample
{
    public void Demonstrate()
    {
        var data = new List<int> { 1, 2, 3, 4, 5 };

        // Deferred - query not executed yet
        var deferred = data.Where(x => x > 2);

        // Immediate - query executed now
        var immediate = data.Where(x => x > 2).ToList();

        // Modify source
        data.Add(6);

        Console.WriteLine(deferred.Count());  // 4 (includes 6)
        Console.WriteLine(immediate.Count()); // 3 (snapshot before 6 was added)
    }

    // Dangerous: query is rebuilt each iteration
    public void DangerousPattern()
    {
        var data = GetData(); // Expensive

        // ❌ BAD - GetData() called multiple times
        foreach (var item in GetData().Where(x => x.IsActive))
        {
            Process(item);
        }

        // ✅ GOOD - GetData() called once
        var items = GetData().Where(x => x.IsActive).ToList();
        foreach (var item in items)
        {
            Process(item);
        }
    }
}

public class InMemoryQueries
{
    public void QueryInMemory()
    {
        var products = new List<Product>
        {
            new Product { Id = 1, Name = "Laptop", Price = 999 },
            new Product { Id = 2, Name = "Mouse", Price = 25 },
            new Product { Id = 3, Name = "Keyboard", Price = 75 }
        };

        // IEnumerable - executes in memory
        IEnumerable<Product> query = products
            .Where(p => p.Price > 50)
            .OrderBy(p => p.Name);

        // All filtering happens in C# code
        foreach (var product in query)
        {
            Console.WriteLine($"{product.Name}: ${product.Price}");
        }
    }
}

public class QueryableExamples
{
    private readonly DbContext _context;

    // IQueryable - translates to SQL
    public async Task<List<Product>> GetExpensiveProductsAsync()
    {
        // Query builds expression tree
        IQueryable<Product> query = _context.Products
            .Where(p => p.Price > 50)
            .OrderBy(p => p.Name);

        // SQL generated and executed here
        return await query.ToListAsync();
        // SQL: SELECT * FROM Products WHERE Price > 50 ORDER BY Name
    }

    // Composable queries
    public IQueryable<Product> GetActiveProducts()
    {
        return _context.Products.Where(p => p.IsActive);
    }

    public async Task<List<Product>> GetExpensiveActiveProductsAsync()
    {
        // Compose queries - still generates single SQL
        var products = await GetActiveProducts()
            .Where(p => p.Price > 100)
            .ToListAsync();

        // SQL: SELECT * FROM Products WHERE IsActive = 1 AND Price > 100
        return products;
    }
}

public class MixingQueries
{
    private readonly DbContext _context;

    public async Task<List<ProductDto>> GetProductsDangerousAsync()
    {
        // ❌ BAD - ToList() brings ALL products to memory first
        var products = await _context.Products.ToListAsync();

        // Then filters in memory (inefficient)
        return products
            .Where(p => p.Price > 100) // In memory
            .Select(p => new ProductDto { Name = p.Name })
            .ToList();
    }

    public async Task<List<ProductDto>> GetProductsEfficientAsync()
    {
        // ✅ GOOD - everything in SQL
        return await _context.Products
            .Where(p => p.Price > 100) // In SQL
            .Select(p => new ProductDto { Name = p.Name }) // In SQL
            .ToListAsync(); // Execute once
    }

    public async Task<List<Product>> ComplexFilterAsync()
    {
        // ✅ GOOD - SQL where possible, memory when necessary
        return await _context.Products
            .Where(p => p.Price > 50) // SQL
            .ToListAsync() // Execute SQL
            .ContinueWith(t => t.Result
                .Where(p => ComplexInMemoryCheck(p)) // C# predicate
                .ToList()
            );
    }

    private bool ComplexInMemoryCheck(Product product)
    {
        // Logic that can't be translated to SQL
        return product.Name.Split(' ').Length > 2;
    }
}

var numbers = Enumerable.Range(1, 100);

// Simple filter
var evens = numbers.Where(n => n % 2 == 0);

// Multiple conditions
var filtered = numbers.Where(n => n > 10 && n < 50 && n % 3 == 0);

// Filter with index
var everyThird = numbers.Where((n, index) => index % 3 == 0);

// Complex filtering
var products = GetProducts()
    .Where(p => p.IsActive)
    .Where(p => p.Price >= 10 && p.Price <= 100)
    .Where(p => p.Category.StartsWith("Electronics"));

var students = GetStudents();

// Simple projection
var names = students.Select(s => s.Name);

// Anonymous types
var summary = students.Select(s => new
{
    s.Name,
    s.Grade,
    Status = s.Grade >= 80 ? "Pass" : "Fail"
});

// Projection with index
var indexed = students.Select((s, i) => new
{
    Index = i,
    Student = s
});

// DTOs
var dtos = students.Select(s => new StudentDto
{
    FullName = $"{s.FirstName} {s.LastName}",
    GradePoint = s.Grade / 100.0
});

var departments = new List<Department>
{
    new Department
    {
        Name = "IT",
        Employees = new[]
        {
            new Employee { Name = "Alice" },
            new Employee { Name = "Bob" }
        }
    },
    new Department
    {
        Name = "HR",
        Employees = new[]
        {
            new Employee { Name = "Charlie" }
        }
    }
};

// Flatten nested collections
var allEmployees = departments.SelectMany(d => d.Employees);

// With result selector
var employeesWithDept = departments.SelectMany(
    dept => dept.Employees,
    (dept, emp) => new { Department = dept.Name, Employee = emp.Name }
);

// Multiple levels
var orders = GetOrders();
var allItems = orders
    .SelectMany(o => o.OrderLines)
    .SelectMany(ol => ol.Items);

var sales = GetSales();

// Simple grouping
var byCategory = sales.GroupBy(s => s.Category);

foreach (var group in byCategory)
{
    Console.WriteLine($"{group.Key}: {group.Count()} items");
}

// Grouping with projection
var categoryTotals = sales
    .GroupBy(s => s.Category)
    .Select(g => new
    {
        Category = g.Key,
        TotalSales = g.Sum(s => s.Amount),
        AverageSale = g.Average(s => s.Amount),
        Count = g.Count()
    });

// Multiple key grouping
var grouped = sales.GroupBy(s => new { s.Category, s.Region });

// GroupBy with custom comparer
var byNameIgnoreCase = students.GroupBy(
    s => s.Name,
    StringComparer.OrdinalIgnoreCase
);

var customers = GetCustomers();
var orders = GetOrders();

// Inner join
var customerOrders = from c in customers
                     join o in orders on c.Id equals o.CustomerId
                     select new { c.Name, o.OrderDate, o.Total };

// Method syntax
var customerOrders2 = customers.Join(
    orders,
    c => c.Id,
    o => o.CustomerId,
    (c, o) => new { c.Name, o.OrderDate, o.Total }
);

// Left outer join
var leftJoin = from c in customers
               join o in orders on c.Id equals o.CustomerId into customerOrders
               from co in customerOrders.DefaultIfEmpty()
               select new
               {
                   Customer = c.Name,
                   OrderTotal = co?.Total ?? 0
               };

// Multiple joins
var fullData = from c in customers
               join o in orders on c.Id equals o.CustomerId
               join od in orderDetails on o.Id equals od.OrderId
               select new { c.Name, o.OrderDate, od.Product };

var numbers = new[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

// Count
int count = numbers.Count();
int evenCount = numbers.Count(n => n % 2 == 0);

// Sum
int sum = numbers.Sum();
decimal totalPrice = products.Sum(p => p.Price);

// Average
double avg = numbers.Average();
double avgGrade = students.Average(s => s.Grade);

// Min/Max
int min = numbers.Min();
int max = numbers.Max();
var cheapest = products.MinBy(p => p.Price); // C# 9+
var mostExpensive = products.MaxBy(p => p.Price); // C# 9+

// Any/All
bool hasEvens = numbers.Any(n => n % 2 == 0);
bool allPositive = numbers.All(n => n > 0);

// First/Last/Single
var first = numbers.First();
var firstEven = numbers.First(n => n % 2 == 0);
var firstOrNull = numbers.FirstOrDefault(n => n > 100); // 0
var single = numbers.Single(n => n == 5);
var last = numbers.Last();

public class AggregationExamples
{
    public void AdvancedAggregates()
    {
        var sales = GetSales();

        // Aggregate - custom accumulator
        var total = sales.Aggregate(0m, (acc, sale) => acc + sale.Amount);

        // Complex aggregation
        var stats = sales.Aggregate(
            new { Sum = 0m, Count = 0 },
            (acc, sale) => new
            {
                Sum = acc.Sum + sale.Amount,
                Count = acc.Count + 1
            },
            acc => new
            {
                acc.Sum,
                acc.Count,
                Average = acc.Sum / acc.Count
            }
        );

        // Grouped aggregations
        var categorySummary = sales
            .GroupBy(s => s.Category)
            .Select(g => new
            {
                Category = g.Key,
                Count = g.Count(),
                Total = g.Sum(s => s.Amount),
                Average = g.Average(s => s.Amount),
                Min = g.Min(s => s.Amount),
                Max = g.Max(s => s.Amount)
            });
    }
}

var list1 = new[] { 1, 2, 3, 4, 5 };
var list2 = new[] { 4, 5, 6, 7, 8 };

// Distinct - remove duplicates
var unique = new[] { 1, 2, 2, 3, 3, 3 }.Distinct(); // 1, 2, 3

// DistinctBy - C# 9+
var customers = GetCustomers();
var uniqueByEmail = customers.DistinctBy(c => c.Email);

// Union - combine and remove duplicates
var union = list1.Union(list2); // 1, 2, 3, 4, 5, 6, 7, 8

// Concat - combine without removing duplicates
var concatenated = list1.Concat(list2); // 1, 2, 3, 4, 5, 4, 5, 6, 7, 8

// Intersect - common elements
var intersection = list1.Intersect(list2); // 4, 5

// Except - elements in first but not second
var difference = list1.Except(list2); // 1, 2, 3

// Set operations with custom comparer
var products1 = GetProducts();
var products2 = GetMoreProducts();
var uniqueProducts = products1.Union(products2, new ProductComparer());

public class ProductComparer : IEqualityComparer<Product>
{
    public bool Equals(Product? x, Product? y)
    {
        if (x == null || y == null) return false;
        return x.Name.Equals(y.Name, StringComparison.OrdinalIgnoreCase);
    }

    public int GetHashCode(Product obj)
    {
        return obj.Name.ToUpperInvariant().GetHashCode();
    }
}

// Usage
var distinct = products.Distinct(new ProductComparer());

var products = GetProducts();

// OrderBy - ascending
var ascending = products.OrderBy(p => p.Price);

// OrderByDescending
var descending = products.OrderByDescending(p => p.Price);

// ThenBy - secondary sort
var sorted = products
    .OrderBy(p => p.Category)
    .ThenByDescending(p => p.Price)
    .ThenBy(p => p.Name);

// Reverse
var reversed = products.OrderBy(p => p.Price).Reverse();

// Custom comparer
var customSort = products.OrderBy(p => p.Name, StringComparer.OrdinalIgnoreCase);

public class PaginationExamples
{
    public PagedResult<Product> GetPage(int pageNumber, int pageSize)
    {
        var query = _context.Products
            .Where(p => p.IsActive)
            .OrderBy(p => p.Name);

        var total = query.Count();

        var items = query
            .Skip((pageNumber - 1) * pageSize)
            .Take(pageSize)
            .ToList();

        return new PagedResult<Product>
        {
            Items = items,
            PageNumber = pageNumber,
            PageSize = pageSize,
            TotalCount = total,
            TotalPages = (int)Math.Ceiling(total / (double)pageSize)
        };
    }

    // Efficient pagination with keyset
    public List<Product> GetNextPage(int? lastId, int pageSize)
    {
        var query = _context.Products.Where(p => p.IsActive);

        if (lastId.HasValue)
        {
            query = query.Where(p => p.Id > lastId.Value);
        }

        return query
            .OrderBy(p => p.Id)
            .Take(pageSize)
            .ToList();
    }
}

public static class LinqExtensions
{
    // WhereIf - conditional filtering
    public static IEnumerable<T> WhereIf<T>(
        this IEnumerable<T> source,
        bool condition,
        Func<T, bool> predicate)
    {
        return condition ? source.Where(predicate) : source;
    }

    // Batch - split into chunks
    public static IEnumerable<List<T>> Batch<T>(
        this IEnumerable<T> source,
        int batchSize)
    {
        var batch = new List<T>(batchSize);

        foreach (var item in source)
        {
            batch.Add(item);

            if (batch.Count == batchSize)
            {
                yield return batch;
                batch = new List<T>(batchSize);
            }
        }

        if (batch.Count > 0)
        {
            yield return batch;
        }
    }

    // ForEach
    public static void ForEach<T>(this IEnumerable<T> source, Action<T> action)
    {
        foreach (var item in source)
        {
            action(item);
        }
    }

    // DistinctBy (built-in in C# 9+)
    public static IEnumerable<T> DistinctBy<T, TKey>(
        this IEnumerable<T> source,
        Func<T, TKey> keySelector)
    {
        var seenKeys = new HashSet<TKey>();

        foreach (var item in source)
        {
            if (seenKeys.Add(keySelector(item)))
            {
                yield return item;
            }
        }
    }
}

// Usage
var filtered = products
    .WhereIf(filterByPrice, p => p.Price > 100)
    .WhereIf(filterByCategory, p => p.Category == "Electronics");

var batches = items.Batch(100);
foreach (var batch in batches)
{
    ProcessBatch(batch);
}

public class PerformanceExamples
{
    // ❌ BAD - Multiple enumerations
    public void MultipleEnumerations(IEnumerable<Product> products)
    {
        var query = products.Where(p => p.Price > 100);

        Console.WriteLine(query.Count()); // Enumeration 1
        Console.WriteLine(query.Sum(p => p.Price)); // Enumeration 2

        foreach (var p in query) // Enumeration 3
        {
            Console.WriteLine(p.Name);
        }
    }

    // ✅ GOOD - Single enumeration
    public void SingleEnumeration(IEnumerable<Product> products)
    {
        var list = products.Where(p => p.Price > 100).ToList();

        Console.WriteLine(list.Count);
        Console.WriteLine(list.Sum(p => p.Price));

        foreach (var p in list)
        {
            Console.WriteLine(p.Name);
        }
    }
}

public class QueryOptimization
{
    private readonly DbContext _context;

    // ❌ BAD - N+1 query problem
    public async Task<List<OrderDto>> GetOrdersBadAsync()
    {
        var orders = await _context.Orders.ToListAsync();

        return orders.Select(o => new OrderDto
        {
            Id = o.Id,
            // Triggers separate query for each order!
            CustomerName = o.Customer.Name,
            ItemCount = o.OrderLines.Count
        }).ToList();
    }

    // ✅ GOOD - Eager loading
    public async Task<List<OrderDto>> GetOrdersGoodAsync()
    {
        return await _context.Orders
            .Include(o => o.Customer)
            .Include(o => o.OrderLines)
            .Select(o => new OrderDto
            {
                Id = o.Id,
                CustomerName = o.Customer.Name,
                ItemCount = o.OrderLines.Count
            })
            .ToListAsync();
    }

    // ✅ BETTER - Project in SQL
    public async Task<List<OrderDto>> GetOrdersBestAsync()
    {
        return await _context.Orders
            .Select(o => new OrderDto
            {
                Id = o.Id,
                CustomerName = o.Customer.Name,
                ItemCount = o.OrderLines.Count
            })
            .ToListAsync();
    }
}

public class ExpressionTreeExamples
{
    public void ExpressionTreeDemo()
    {
        // Lambda as delegate
        Func<int, bool> isEvenDelegate = n => n % 2 == 0;

        // Lambda as expression tree
        Expression<Func<int, bool>> isEvenExpression = n => n % 2 == 0;

        // Examine expression structure
        var binary = (BinaryExpression)isEvenExpression.Body;
        Console.WriteLine(binary.NodeType); // Modulo

        // Compile and execute
        var compiled = isEvenExpression.Compile();
        bool result = compiled(4); // true
    }

    // Building expressions dynamically
    public IQueryable<T> ApplyFilter<T>(
        IQueryable<T> query,
        string propertyName,
        object value)
    {
        var parameter = Expression.Parameter(typeof(T), "x");
        var property = Expression.Property(parameter, propertyName);
        var constant = Expression.Constant(value);
        var equality = Expression.Equal(property, constant);
        var lambda = Expression.Lambda<Func<T, bool>>(equality, parameter);

        return query.Where(lambda);
    }
}

name	csharp-linq
user-invocable	false
description	Use when lINQ query and method syntax, deferred execution, and performance optimization. Use when querying collections in C#.
allowed-tools	["Bash","Read","Write","Edit"]

csharp-linq

Informações da origem

Opções de instalação

Revise os arquivos de origem

Mais deste repositório

C# LINQ

LINQ Query Syntax vs Method Syntax

Query Syntax

Mais deste repositório

C# LINQ

LINQ Query Syntax vs Method Syntax

Query Syntax

Method Syntax

When to Use Each Syntax

Deferred Execution

Understanding Deferred Execution

Deferred vs Immediate Execution

IEnumerable vs IQueryable

IEnumerable<T> - In-Memory

IQueryable<T> - Expression Trees

Mixing IEnumerable and IQueryable

Common LINQ Operators

Where - Filtering

Select - Projection

SelectMany - Flattening

GroupBy - Grouping

Join - Combining Collections

Aggregation Operations

Basic Aggregations

Advanced Aggregations

Set Operations

Distinct, Union, Intersect, Except

Custom Equality Comparers

Ordering and Pagination

Ordering

Pagination

Custom LINQ Operators

Extension Methods

Performance Considerations

Materialization

Database Query Optimization

Expression Trees

Best Practices

Common Pitfalls

When to Use

Resources