Entity Framework Core: N+1 query problem with lazy loading disabled

When loading a list of entities and accessing a navigation property in a loop, EF Core issues one query per row by default (if lazy loading proxies are enabled) or throws a NullReferenceException (if disabled). Either way, related data is not loaded efficiently.

Use eager loading with Include() / ThenInclude() to load related data in a single query:

// BAD — N+1 if lazy loading on, null ref if off
var orders = await db.Orders.ToListAsync();
foreach (var o in orders)
    Console.WriteLine(o.Customer.Name); // extra query per row

// GOOD — single JOIN query
var orders = await db.Orders
    .Include(o => o.Customer)
    .ThenInclude(c => c.Address)
    .ToListAsync();

For projections that don't need the full entity, use Select to avoid loading unneeded columns:

var names = await db.Orders
    .Select(o => new { o.Id, CustomerName = o.Customer.Name })
    .ToListAsync();

EF Core translates LINQ to SQL and executes queries only when enumerated. Navigation properties are null until explicitly loaded. Lazy loading proxies intercept property access and issue additional queries per access — one per entity in the collection.