筆記 LINQ 的使用方式。
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說明
為什麼我們不需要使用 system.Linq 來使用 Linq?因為 .NET 專案預設透過 ImplicitUsing,已經將 System.Linq 命名空間隱含的加入專案中。
在 Linq (Language Integrated Query) 中,有兩種主要的語法 (syntaxes) 來撰寫查詢,Query Syntax 和 Method Syntax。
Query Syntax
Query Syntax 使用類似 SQL 的語法來撰寫查詢,但有些操作無法使用 Query Syntax 來達成。
var query =
from student in students
where student.Age > 20
orderby student.LastName ascending
select student.FirstName;
Method Syntax:
Method Syntax 使用 fluent API 的方式來撰寫查詢,能夠支援所有的查詢操作。
var query = students
.Where(student => student.Age > 20)
.OrderBy(student => student.LastName)
.Select(student => student.FirstName);
Groupby in Linq
// Linq how to group by example
var names = new string[] { "Alice", "Allen", "Bob", "Cindy", "Cathy" };
// Method Syntax
var query = names.GroupBy(name => name[0])
.Select(g => new { FirstLetter = g.Key, Names = g });
// Query Syntax
query = from name in names
group name by name[0] into g
select new { FirstLetter = g.Key, Names = g };
foreach (var group in query)
{
Console.WriteLine(group.FirstLetter);
foreach (var name in group.Names)
{
Console.WriteLine(" " + name);
}
}
Advanced Groupby in Linq Example
// Category for game products
List<Category> categories = [
new Category(1, "Action"),
new Category(2, "Adventure"),
new Category(3, "RPG"),
new Category(4, "Simulation"),
new Category(5, "Strategy")
];
// Game Products
List<Product> products = [
new(1, "GTA V", 29.99m, categories[0]),
new(2, "The Witcher 3", 19.99m, categories[2]),
new(3, "Cyberpunk 2077", 49.99m, categories[2]),
new(4, "FIFA 22", 59.99m, categories[0]),
new(5, "Assassin's Creed Valhalla", 39.99m, categories[1]),
new(6, "Civilization VI", 29.99m, categories[4])
];
var query = from p in products
group p by p.Category.Name;
foreach (var g in query)
{
Console.WriteLine(g.Key);
foreach (var p in g)
{
Console.WriteLine($" {p.Name} - {p.Price}");
}
Console.WriteLine();
}
record Category(int CategoryId, string Name);
record Product(int ProductId, string Name, decimal Price, Category Category);
Group Values by Key in Linq
var query = from p in products
group p by p.Category.Name into g
where g.Count() >= 2
select g;
What difference between groupby and tolookup in linq
Key Differences Summary:
- Return Type: GroupBy returns
IEnumerable<IGrouping<TKey, TElement>>
, while ToLookup returnsILookup<TKey, TElement>
. - Execution: GroupBy uses deferred execution, ToLookup executes immediately.
- Indexing: ToLookup allows indexed access to groups by key efficiently.
Choosing Between GroupBy and ToLookup:
- Use GroupBy when you need deferred execution or plan to perform further aggregation or operations on each group.
- Use ToLookup when you need immediate execution and efficient keyed access to groups.
OfType in Linq
OfType is able to filter the collection by a specific type.
Account[] accounts = [
new SavingsAccount(1, "Savings Account 1"),
new SavingsAccount(2, "Savings Account 2"),
new CreditCardAccount(3, "Credit Card Account 1"),
new CreditCardAccount(4, "Credit Card Account 2")
];
var result = accounts.OfType<SavingsAccount>();
foreach (var item in result)
{
Console.WriteLine((item.AccountId, item.Name));
}
record Account(int AccountId, string Name);
record SavingsAccount(int AccountId, string Name): Account(AccountId, Name);
record CreditCardAccount(int AccountId, string Name): Account(AccountId, Name);