# Referensi Cepat C#

*Tipe, LINQ, async/await, koleksi, esensial OOP*

> Source: Microsoft C# Documentation (learn.microsoft.com) · MIT

## Dasar

### Hello World

```
Console.WriteLine("Hello, World!");  // top-level (C# 10+)
// Classic: class Program { static void Main() { ... } }
```

### Build & Jalankan

```
dotnet new console -n MyApp   # create project
dotnet run                     # compile and run
dotnet build                   # compile only
```

### Variabel & Konstanta

```
int x = 42;
var name = "Alice";           // type inference
const double Pi = 3.14159;
readonly int maxRetries = 3;  // set once, in ctor
```

## Tipe

### Value Type

| Command | Description |
|---------|-------------|
| `int` | Integer 32-bit bertanda |
| `long` | Integer 64-bit bertanda |
| `float` | Floating point 32-bit (suffix `f`) |
| `double` | Floating point 64-bit |
| `decimal` | Presisi tinggi 128-bit (suffix `m`) |
| `bool` | `true` / `false` |
| `char` | Karakter Unicode 16-bit |

### Reference Type

| Command | Description |
|---------|-------------|
| `string` | Teks UTF-16 yang immutable |
| `object` | Tipe dasar semua tipe |
| `dynamic` | Lewati pemeriksaan tipe compile-time |
| `int[]` | Array integer |
| `List<T>` | Daftar generik (System.Collections.Generic) |

### Nullable & Tuple

```
int? age = null;               // nullable value type
string? name = null;           // nullable reference (C# 8+)
var point = (X: 1, Y: 2);     // named tuple
Console.WriteLine(point.X);
```

### Fitur String

```
string name = "World";
string msg = $"Hello, {name}!";       // interpolation
string path = @"C:\Users\file.txt";   // verbatim
string raw = """raw "string" here""";  // raw (C# 11+)
```

## Control Flow

### If / Else

```
if (x > 0) Console.WriteLine("positive");
else if (x == 0) Console.WriteLine("zero");
else Console.WriteLine("negative");
```

### Switch & Pattern Matching

```
string label = x switch {
    > 0 => "positive", 0 => "zero", _ => "negative"
};
if (obj is string s && s.Length > 0) { }  // pattern match
```

### Loop

```
for (int i = 0; i < 10; i++) { }
foreach (var item in collection) { }
while (condition) { }
do { } while (condition);
```

## Class

### Definisi Class

```
public class Person {
    public string Name { get; set; }
    public int Age { get; init; }       // init-only (C# 9+)
    public Person(string name, int age) { Name = name; Age = age; }
}
```

### Record (C# 9+)

```
public record Point(double X, double Y);
var p1 = new Point(1, 2);
var p2 = p1 with { X = 3 };  // non-destructive copy
// auto: Equals, GetHashCode, ToString, deconstruct
```

### Inheritance

```
public abstract class Shape { public abstract double Area(); }
public class Circle(double r) : Shape {
    public override double Area() => Math.PI * r * r;
}
```

### Access Modifier

| Command | Description |
|---------|-------------|
| `public` | Dapat diakses dari mana saja |
| `private` | Hanya class yang sama (default untuk member) |
| `protected` | Class yang sama dan class turunan |
| `internal` | Hanya assembly yang sama (default untuk class) |
| `protected internal` | Assembly yang sama atau class turunan |

## Interface

### Definisi Interface

```
public interface IShape {
    double Area();
    double Perimeter() => 0;  // default impl (C# 8+)
}
public class Rect(double w, double h) : IShape { public double Area() => w * h; }
```

### Interface Umum

| Command | Description |
|---------|-------------|
| `IEnumerable<T>` | Dukungan iterasi (foreach, LINQ) |
| `IDisposable` | Pembersihan deterministik (pernyataan `using`) |
| `IComparable<T>` | Pengurutan alami untuk sorting |
| `IEquatable<T>` | Perbandingan kesetaraan nilai |
| `ICloneable` | Kloning objek |

## LINQ

### Sintaks Method

```
var result = numbers
    .Where(n => n > 3)
    .OrderBy(n => n)
    .Select(n => n * 2)
    .ToList();
```

### Sintaks Query

```
var result = from n in numbers
             where n > 3
             orderby n
             select n * 2;
```

### Method LINQ Umum

| Command | Description |
|---------|-------------|
| `.Where(pred)` | Filter elemen |
| `.Select(func)` | Proyeksi / transformasi elemen |
| `.OrderBy(key)` | Urutkan ascending |
| `.GroupBy(key)` | Kelompokkan elemen berdasarkan key |
| `.First() / .FirstOrDefault()` | Elemen pertama (atau default) |
| `.Any(pred)` | `true` jika ada elemen yang cocok |
| `.Count()` | Jumlah elemen |
| `.Sum() / .Average()` | Agregasi nilai numerik |
| `.Distinct()` | Hapus duplikat |
| `.SelectMany(func)` | Ratakan koleksi bersarang |

## Async/Await

### Method Async

```
public async Task<string> FetchAsync(string url) {
    using var client = new HttpClient();
    return await client.GetStringAsync(url);
}
```

### Kombinator Task

```
var results = await Task.WhenAll(task1, task2, task3);
var first = await Task.WhenAny(task1, task2);
```

### Pola Async

| Command | Description |
|---------|-------------|
| `Task` | Return async void (tanpa hasil) |
| `Task<T>` | Return async dengan hasil bertipe T |
| `ValueTask<T>` | Task ringan untuk jalur sync-fast |
| `await foreach` | Iterasi async atas `IAsyncEnumerable<T>` |
| `CancellationToken` | Pembatalan kooperatif untuk operasi async |

## Koleksi

### Koleksi Umum

| Command | Description |
|---------|-------------|
| `List<T>` | Array dinamis, akses indeks cepat |
| `Dictionary<K,V>` | Hash map, lookup O(1) berdasarkan key |
| `HashSet<T>` | Elemen unik, lookup O(1) |
| `Queue<T>` | Koleksi FIFO |
| `Stack<T>` | Koleksi LIFO |
| `LinkedList<T>` | Linked list dua arah |
| `SortedDictionary<K,V>` | Diurutkan berdasarkan key (berbasis tree) |

### Penggunaan Dictionary

```
var dict = new Dictionary<string, int> {
    ["Alice"] = 90, ["Bob"] = 85
};
dict.TryGetValue("Alice", out int score);
foreach (var (key, val) in dict) { }
```

### Koleksi Immutable

```
using System.Collections.Immutable;
var list = ImmutableList.Create(1, 2, 3);
var newList = list.Add(4);  // returns new list
```

## Property

### Sintaks Property

```
public string Name { get; set; }
public int Age { get; private set; }
public string Email { get; init; }        // init-only
public string Display => $"{Name} ({Age})"; // computed
```

### Indexer

```
public double this[int row, int col] {
    get => data[row, col];
    set => data[row, col] = value;
}
```

### Pola Property

| Command | Description |
|---------|-------------|
| `{ get; set; }` | Auto-property read-write |
| `{ get; }` | Read-only (hanya bisa di-set dalam constructor) |
| `{ get; init; }` | Read-only setelah inisialisasi (C# 9+) |
| `{ get; private set; }` | Bisa dibaca publik, ditulis privat |
| `=> expression` | Property expression-bodied (computed) |

## Exception

### Try / Catch / Finally

```
try { int result = int.Parse(input); }
catch (FormatException ex) { Console.Error.WriteLine(ex.Message); }
catch (Exception ex) when (ex is not OutOfMemoryException) { }
finally { /* always executes */ }
```

### Pernyataan Using

```
using var file = File.OpenRead("data.txt");
// file.Dispose() called automatically at scope end
// equivalent to try/finally with Dispose()
```

### Exception Umum

| Command | Description |
|---------|-------------|
| `ArgumentNullException` | Argumen null dikirim ke method |
| `ArgumentOutOfRangeException` | Argumen di luar rentang valid |
| `InvalidOperationException` | Operasi tidak valid untuk kondisi saat ini |
| `NullReferenceException` | Dereference objek null |
| `KeyNotFoundException` | Key tidak ditemukan dalam dictionary |
| `NotImplementedException` | Method belum diimplementasikan |

### Exception Kustom

```
public class AppException : Exception {
    public int Code { get; }
    public AppException(string msg, int code)
        : base(msg) { Code = code; }
}
```