
## Table of Contents

1. [Pattern Matching (C# 7–13)](#pattern-matching-c-7--13)
2. [Records (C# 9.0 / 10.0)](#records-c-90--100)
3. [`required` Members (C# 11)](#required-members-c-11)
4. [Collection Expressions (C# 12)](#collection-expressions-c-12)
5. [Primary Constructors for Classes (C# 12)](#primary-constructors-for-classes-c-12)
6. [`file`-Scoped Types (C# 11)](#file-scoped-types-c-11)
7. [`global using` & Implicit Usings (C# 10)](#global-using--implicit-usings-c-10)
8. [Raw String Literals (C# 11)](#raw-string-literals-c-11)
9. [`params` Collections (C# 13)](#params-collections-c-13)
10. [Other Notable Features](#other-notable-features)

---

## Pattern Matching (C# 7–13)

**Pattern matching** lets you test a value against a shape, type, or condition — and extract data in the process. It has evolved significantly across C# versions, from simple type checks to complex deconstruction.

### Type Patterns (C# 7.0)

```csharp
// Test and cast in one step
object obj = GetValue();

if (obj is string text)
    Console.WriteLine(text.Length); // text is already a string

if (obj is int number and number > 0)
    Console.WriteLine($"Positive: {number}");

// Switch with type patterns
string Describe(object value) => value switch
{
    int i => $"Integer: {i}",
    string s => $"String: {s}",
    null => "null",
    _ => $"Unknown: {value.GetType().Name}"
};
```

### Property Patterns (C# 8.0)

```csharp
// Match on properties of an object
string GetShippingCost(Order order) => order switch
{
    { Total: > 100, IsPremium: true } => "Free",
    { Total: > 100 } => "$5.00",
    { Weight: > 50 } => "$25.00",
    _ => "$10.00"
};

// Nested property patterns
bool IsLocalCustomer(Order order) => order is
{
    Customer.Address.Country: "US",
    Customer.Address.State: "CA"
};
```

### Positional Patterns (C# 8.0)

```csharp
// Works with types that have Deconstruct or are records/tuples
public record Point(int X, int Y);

string Classify(Point point) => point switch
{
    (0, 0) => "Origin",
    (var x, 0) => $"On X-axis at {x}",
    (0, var y) => $"On Y-axis at {y}",
    (var x, var y) when x == y => $"On diagonal at ({x}, {y})",
    _ => "Elsewhere"
};
```

### Relational & Logical Patterns (C# 9.0)

```csharp
// Relational: <, >, <=, >=
string GetTemperatureCategory(double temp) => temp switch
{
    < 0 => "Freezing",
    >= 0 and < 15 => "Cold",
    >= 15 and < 25 => "Pleasant",
    >= 25 and < 35 => "Warm",
    >= 35 => "Hot"
};

// Logical: and, or, not
bool IsValidAge(int age) => age is >= 0 and <= 150;
bool IsNotNull(object? obj) => obj is not null;
bool IsWeekend(DayOfWeek day) => day is DayOfWeek.Saturday or DayOfWeek.Sunday;

// Combined
string Classify(int value) => value switch
{
    > 0 and < 10 => "Small positive",
    >= 10 and < 100 => "Medium positive",
    >= 100 => "Large positive",
    0 => "Zero",
    < 0 => "Negative"
};
```

### List Patterns (C# 11)

```csharp
// Match against array/list structure
int[] numbers = [1, 2, 3, 4, 5];

var result = numbers switch
{
    [] => "Empty",
    [var single] => $"Single: {single}",
    [var first, .., var last] => $"First: {first}, Last: {last}",
    [1, 2, ..] => "Starts with 1, 2",
    [.., 4, 5] => "Ends with 4, 5",
};

// Slice pattern with discard
bool IsValid(int[] data) => data is [> 0, .., > 0]; // first and last are positive

// With ReadOnlySpan<char> for string parsing
bool IsCsvHeader(ReadOnlySpan<char> line) => line switch
{
    ['#', ..] => false,        // comment line
    ['"', .., '"'] => true,    // quoted header
    _ => true
};
```

---

## Records (C# 9.0 / 10.0)

**Records** are reference types (or value types with `record struct`) that provide **value-based equality**, **immutability**, and **deconstruction** with minimal boilerplate.

```csharp
// Record class (reference type, C# 9.0)
public record OrderDto(int Id, string CustomerName, decimal Total);

// Record struct (value type, C# 10.0)
public readonly record struct Money(decimal Amount, string Currency);

// What you get for free:
// • Value-based Equals and GetHashCode
// • ToString (e.g., "OrderDto { Id = 1, CustomerName = Alice, Total = 99.99 }")
// • Deconstruction
// • with-expression for non-destructive mutation

var order = new OrderDto(1, "Alice", 99.99m);
var updated = order with { Total = 150m }; // new instance, only Total changed

var (id, name, total) = order; // deconstruction
```

### Record vs Class vs Struct

| Feature | `record` | `record struct` | `class` | `struct` |
|---|---|---|---|---|
| Equality | Value-based | Value-based | Reference-based | Value-based (slow) |
| Heap/Stack | Heap | Stack | Heap | Stack |
| `with` expression | ✓ | ✓ | ✗ | ✗ |
| Inheritance | ✓ | ✗ | ✓ | ✗ |
| `ToString` | Auto-generated | Auto-generated | `GetType().Name` | `GetType().Name` |
| Best for | DTOs, events, value objects | Small value objects | Complex entities | Low-level perf |

---

## `required` Members (C# 11)

Forces callers to set specific properties at construction time — compile-time enforcement.

```csharp
public class CreateOrderRequest
{
    public required string CustomerId { get; init; }
    public required List<OrderItemRequest> Items { get; init; }
    public string? Notes { get; init; }  // optional
}

// ✓ Must set required properties
var request = new CreateOrderRequest
{
    CustomerId = "cust-1",
    Items = [new("SKU-1", 2)]
};

// ✗ Compile error — CustomerId is required
var bad = new CreateOrderRequest
{
    Items = [new("SKU-1", 2)]
}; // CS9035: Required member 'CustomerId' must be set
```

Use `required` instead of constructor parameters when you want object-initializer syntax with compile-time safety.

---

## Collection Expressions (C# 12)

A concise syntax for creating collections — works with arrays, `List<T>`, `Span<T>`, `ImmutableArray<T>`, and more.

```csharp
// Before
int[] numbers = new int[] { 1, 2, 3 };
List<string> names = new List<string> { "Alice", "Bob" };
Span<int> span = stackalloc int[] { 1, 2, 3 };

// After (C# 12)
int[] numbers = [1, 2, 3];
List<string> names = ["Alice", "Bob"];
Span<int> span = [1, 2, 3];

// Empty collections
List<Order> orders = [];
int[] empty = [];

// Spread operator (..) — combine collections
int[] first = [1, 2, 3];
int[] second = [4, 5, 6];
int[] combined = [..first, ..second]; // [1, 2, 3, 4, 5, 6]

// Works with any target type that supports collection expressions
ImmutableArray<int> immutable = [1, 2, 3];
HashSet<string> set = ["a", "b", "c"];
```

---

## Primary Constructors for Classes (C# 12)

Classes can now have primary constructors — parameters are captured and available throughout the class.

```csharp
// Before
public class OrderService
{
    private readonly IOrderRepository _repo;
    private readonly ILogger<OrderService> _logger;

    public OrderService(IOrderRepository repo, ILogger<OrderService> logger)
    {
        _repo = repo;
        _logger = logger;
    }
}

// After (C# 12)
public class OrderService(IOrderRepository repo, ILogger<OrderService> logger)
{
    public async Task<Order?> GetByIdAsync(int id, CancellationToken ct)
    {
        logger.LogInformation("Fetching order {OrderId}", id);
        return await repo.GetByIdAsync(id, ct);
    }
}
```

**Note:** Primary constructor parameters are **not** `readonly` by default — see your clean-code guidelines for the full details.

---

## `file`-Scoped Types (C# 11)

A type visible **only within the file** where it's declared — perfect for hiding implementation details.

```csharp
// OrderService.cs

public class OrderService(IOrderRepository repo)
{
    public async Task<OrderResult> ProcessAsync(Order order, CancellationToken ct)
    {
        var validator = new OrderValidator();
        if (!validator.IsValid(order))
            return OrderResult.Invalid(validator.Errors);

        await repo.SaveAsync(order, ct);
        return OrderResult.Success(order);
    }
}

// Only visible in this file — won't pollute the namespace
file class OrderValidator
{
    public List<string> Errors { get; } = [];

    public bool IsValid(Order order)
    {
        if (order.Total <= 0)
            Errors.Add("Total must be positive");
        if (string.IsNullOrWhiteSpace(order.CustomerName))
            Errors.Add("Customer name is required");
        return Errors.Count == 0;
    }
}
```

---

## `global using` & Implicit Usings (C# 10)

```csharp
// GlobalUsings.cs — declare once, available everywhere in the project
global using System.Collections.Concurrent;
global using Microsoft.Extensions.Logging;
global using MyApp.Domain.Entities;
global using MyApp.Application.Interfaces;

// Implicit usings (.NET 6+ — enabled by default in .csproj)
// <ImplicitUsings>enable</ImplicitUsings>
// Automatically adds: System, System.Collections.Generic, System.Linq,
// System.Threading.Tasks, System.IO, etc.
```

---

## Raw String Literals (C# 11)

```csharp
// Multi-line strings without escape characters
string json = """
    {
        "name": "Alice",
        "age": 30,
        "orders": [
            { "id": 1, "total": 99.99 }
        ]
    }
    """;

// Interpolated raw strings — use $$ with {{ }}
string name = "Alice";
string jsonTemplate = $$"""
    {
        "name": "{{name}}",
        "timestamp": "{{DateTime.UtcNow:O}}"
    }
    """;
```

---

## `params` Collections (C# 13)

`params` now works with **any collection type**, not just arrays.

```csharp
// Before (C# 1.0 — arrays only)
public void Log(params string[] messages) { }

// After (C# 13 — any collection)
public void Log(params ReadOnlySpan<string> messages)
{
    foreach (var msg in messages)
        Console.WriteLine(msg);
}

public void Process(params IEnumerable<int> values) { }
public void Build(params List<string> items) { }

// Still called the same way
Log("hello", "world");
Process(1, 2, 3);
```

💡 `params ReadOnlySpan<T>` avoids the array allocation of `params T[]`.

---

## Other Notable Features

### `using` Declarations (C# 8.0)

No braces needed — automatically disposed at end of scope.

```csharp
// Before
using (var stream = File.OpenRead("data.txt"))
{
    // use stream
}

// After — disposed when the enclosing block (method, if, etc.) exits
using var stream = File.OpenRead("data.txt");
// use stream
```

### Switch Expressions (C# 8.0)

```csharp
string result = status switch
{
    OrderStatus.Pending => "Processing",
    OrderStatus.Shipped => "On the way",
    OrderStatus.Delivered => "Complete",
    _ => "Unknown"
};
```

### Null-Coalescing Assignment `??=` (C# 8.0)

```csharp
_cache ??= new Dictionary<string, int>();
```

### Target-Typed `new` (C# 9.0)

```csharp
Dictionary<string, List<int>> map = new(); // type inferred from left side
```

### File-Scoped Namespaces (C# 10)

```csharp
// Before
namespace MyApp.Services
{
    public class OrderService { }
}

// After — saves one level of indentation
namespace MyApp.Services;

public class OrderService { }
```

### Generic Attributes (C# 11)

```csharp
// Before — typeof required
[Validator(typeof(OrderValidator))]
public class Order { }

// After — generic attribute
[Validator<OrderValidator>]
public class Order { }
```

### `nameof` Scope Improvement (C# 11)

```csharp
// nameof works on method parameters in attributes
[return: NotNullIfNotNull(nameof(input))]
public static string? Process(string? input) => input?.Trim();
```

---

## Feature Version Quick Reference

| Feature | C# Version | .NET Version |
|---|---|---|
| Pattern matching (basic) | 7.0 | .NET Core 2.0 |
| `Span<T>`, `ref struct` | 7.2 | .NET Core 2.1 |
| Nullable reference types | 8.0 | .NET Core 3.0 |
| `using` declarations | 8.0 | .NET Core 3.0 |
| Switch expressions | 8.0 | .NET Core 3.0 |
| Default interface methods | 8.0 | .NET Core 3.0 |
| `IAsyncEnumerable<T>` | 8.0 | .NET Core 3.0 |
| Records | 9.0 | .NET 5 |
| Relational / logical patterns | 9.0 | .NET 5 |
| Target-typed `new` | 9.0 | .NET 5 |
| `global using` | 10.0 | .NET 6 |
| File-scoped namespaces | 10.0 | .NET 6 |
| Record structs | 10.0 | .NET 6 |
| Natural lambda types | 10.0 | .NET 6 |
| Raw string literals | 11.0 | .NET 7 |
| List patterns | 11.0 | .NET 7 |
| `required` members | 11.0 | .NET 7 |
| `file`-scoped types | 11.0 | .NET 7 |
| Generic attributes | 11.0 | .NET 7 |
| Static abstract interface members | 11.0 | .NET 7 |
| Generic math (`INumber<T>`) | 11.0 | .NET 7 |
| Primary constructors (classes) | 12.0 | .NET 8 |
| Collection expressions `[1,2,3]` | 12.0 | .NET 8 |
| `params` collections | 13.0 | .NET 9 |
| `Lock` type | 13.0 | .NET 9 |
| `Task.WhenEach` | 13.0 | .NET 9 |
| Extension members (`extension` blocks) | 14.0 | .NET 10 |
| `field` keyword in properties | 14.0 | .NET 10 |
| Null-conditional assignment `?.=` | 14.0 | .NET 10 |
| Unbound generics in `nameof` | 14.0 | .NET 10 |

---

## Summary — Modern C# Checklist

| Feature | Use For |
|---|---|
| **Pattern matching** | Replace `if/switch` chains with expressive matching |
| **Records** | DTOs, events, value objects — value equality + immutability |
| **`required`** | Enforce property initialization at compile time |
| **Collection expressions** | `[1, 2, 3]` — concise collection creation |
| **Primary constructors** | DI injection in services — less boilerplate |
| **`file` types** | Hide implementation details within a file |
| **`global using`** | Reduce repetitive `using` directives |
| **Raw strings** | JSON, SQL, multi-line text without escaping |
| **`params` spans** | Variable arguments without array allocation |
| **List patterns** | Destructure arrays/lists in `switch` expressions |


Explore modern C# features from C# 7 to 13, including pattern matching, records, collection expressions, and primary constructors for cleaner, more expressive code.

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Modern C# Language Features: Pattern Matching, Records, and More

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