// DANGEROUS: Materializes entire table into memory, then filters in C#
IEnumerable<Order> orders = dbContext.Orders;
var recent = orders.Where(o => o.CreatedAt > cutoff).ToList();
// SQL: SELECT * FROM Orders  (no WHERE clause!)

// CORRECT: Filter executes on the database server
IQueryable<Order> orders = dbContext.Orders;
var recent = orders.Where(o => o.CreatedAt > cutoff).ToList();
// SQL: SELECT ... FROM Orders WHERE CreatedAt > @cutoff

// MISTAKE 1: AsEnumerable() before filtering
var results = dbContext.Orders
    .AsEnumerable()           // <-- switches to client evaluation
    .Where(o => o.Total > 100)  // runs in memory, not SQL
    .ToList();

// MISTAKE 2: Calling a C# method in IQueryable predicate
var results = dbContext.Orders
    .Where(o => IsHighValue(o))  // Cannot translate to SQL; throws or falls back
    .ToList();

// FIX: Use expression-compatible predicates or call after materialization
var results = dbContext.Orders
    .Where(o => o.Total > 100)   // SQL-translatable
    .AsEnumerable()
    .Where(o => IsHighValue(o))  // C# logic after materialization
    .ToList();

// MISTAKE 3: Projecting too many columns
var names = dbContext.Orders.ToList().Select(o => o.CustomerName);
// Loads ALL columns, then picks one in memory

// FIX: Project before materializing
var names = dbContext.Orders.Select(o => o.CustomerName).ToList();
// SQL: SELECT CustomerName FROM Orders

public sealed class OrderRepository(AppDbContext db)
{
    // Compiled once, reused across all calls
    private static readonly Func<AppDbContext, Guid, Task<Order?>>
        s_findById = EF.CompileAsyncQuery(
            (AppDbContext ctx, Guid id) =>
                ctx.Orders.FirstOrDefault(o => o.Id == id));

    private static readonly Func<AppDbContext, DateTime, IAsyncEnumerable<Order>>
        s_findRecent = EF.CompileAsyncQuery(
            (AppDbContext ctx, DateTime cutoff) =>
                ctx.Orders
                    .Where(o => o.CreatedAt > cutoff)
                    .OrderByDescending(o => o.CreatedAt));

    public Task<Order?> FindByIdAsync(Guid id) =>
        s_findById(db, id);

    public IAsyncEnumerable<Order> FindRecentAsync(DateTime cutoff) =>
        s_findRecent(db, cutoff);
}

// BUG: Enumerates the database query twice
IQueryable<Order> query = dbContext.Orders.Where(o => o.Status == Status.Active);

var count = query.Count();         // Executes SQL (1st query)
var items = query.ToList();        // Executes SQL again (2nd query)

// FIX: Materialize once
var items = dbContext.Orders
    .Where(o => o.Status == Status.Active)
    .ToList();

var count = items.Count;           // In-memory, no SQL

// BUG: All queries capture the same loop variable 'i' by reference
var queries = new List<IQueryable<Order>>();
for (int i = 0; i < statuses.Length; i++)
{
    queries.Add(dbContext.Orders.Where(o => o.Status == statuses[i]));
    // 'i' is captured by reference -- all queries use final value of i
}

// FIX: Copy to a local variable inside the loop body
for (int i = 0; i < statuses.Length; i++)
{
    var localStatus = statuses[i];
    queries.Add(dbContext.Orders.Where(o => o.Status == localStatus));
}

// DANGEROUS: Returns an unevaluated query -- caller may not realize
// the DbContext could be disposed before enumeration
public IEnumerable<Order> GetActiveOrders()
{
    return dbContext.Orders.Where(o => o.Status == Status.Active);
    // Not evaluated yet -- DbContext may be disposed when caller iterates
}

// SAFE: Materialize before returning
public async Task<List<Order>> GetActiveOrdersAsync(CancellationToken ct)
{
    return await dbContext.Orders
        .Where(o => o.Status == Status.Active)
        .ToListAsync(ct);
}

// LINQ: Allocates iterator + delegate + List<T>
var result = items
    .Where(x => x.IsActive)
    .Select(x => x.Name)
    .ToList();

// Manual loop: Single List<T> allocation, no iterator/delegate overhead
var result = new List<string>(items.Count);
foreach (var item in items)
{
    if (item.IsActive)
    {
        result.Add(item.Name);
    }
}

// LINQ: Allocates iterator + delegate + bool boxing (Any)
var hasActive = items.Any(x => x.IsActive);

// Manual loop: Zero allocations beyond the enumerator
var hasActive = false;
foreach (var item in items)
{
    if (item.IsActive)
    {
        hasActive = true;
        break;
    }
}

// 1. Use Array.Find / Array.Exists for arrays (no iterator allocation)
var first = Array.Find(items, x => x.IsActive);
var exists = Array.Exists(items, x => x.IsActive);

// 2. Pre-size collections when count is known
var result = new List<string>(items.Length);
result.AddRange(items.Where(x => x.IsActive).Select(x => x.Name));

// 3. Use static lambdas to avoid delegate allocation (C# 9+)
var result = items.Where(static x => x.IsActive).ToList();
// Note: static lambdas prevent accidental closure capture
// but the delegate is already cached by the compiler for
// non-capturing lambdas; the main benefit is enforcement

// Zero-allocation contains check on an array
ReadOnlySpan<int> values = stackalloc int[] { 1, 2, 3, 4, 5 };
bool found = values.Contains(3);

// Zero-allocation index search
int index = values.IndexOf(4);

// Zero-allocation split and iterate
ReadOnlySpan<char> csv = "alice,bob,charlie";
foreach (var segment in csv.Split(','))
{
    ReadOnlySpan<char> value = csv[segment];
    // Process each value without allocating strings
}

// Zero-allocation trim and compare
ReadOnlySpan<char> input = "  hello  ";
bool match = input.Trim().SequenceEqual("hello");

// BAD: Loads entire entity graph
var orders = await dbContext.Orders
    .Include(o => o.Lines)
    .Include(o => o.Customer)
    .ToListAsync(ct);

var summaries = orders.Select(o => new
{
    o.Id,
    o.Customer.Name,
    Total = o.Lines.Sum(l => l.Price * l.Quantity)
});

// GOOD: Project in the query -- single SQL with computed columns
var summaries = await dbContext.Orders
    .Select(o => new
    {
        o.Id,
        CustomerName = o.Customer.Name,
        Total = o.Lines.Sum(l => l.Price * l.Quantity)
    })
    .ToListAsync(ct);

// Offset pagination: O(N) -- server must skip rows
var page = await dbContext.Orders
    .OrderBy(o => o.Id)
    .Skip(pageSize * pageNumber)
    .Take(pageSize)
    .ToListAsync(ct);

// Keyset pagination: O(1) -- index seek
var page = await dbContext.Orders
    .Where(o => o.Id > lastSeenId)
    .OrderBy(o => o.Id)
    .Take(pageSize)
    .ToListAsync(ct);

// BAD: N UPDATE statements (one per tracked entity change)
foreach (var order in orders)
{
    order.Status = OrderStatus.Archived;
}
await dbContext.SaveChangesAsync(ct);
// Generates N individual UPDATE statements in a single round-trip

// GOOD: EF Core 7+ ExecuteUpdateAsync (single SQL statement)
await dbContext.Orders
    .Where(o => o.CreatedAt < cutoff)
    .ExecuteUpdateAsync(
        s => s.SetProperty(o => o.Status, OrderStatus.Archived),
        ct);