3分で分かる C# 6 の新機能

What's New in C# 6、New Language Features in C# 6 をベースに、C# 5 と比較してコードがどう変わるのかを、主な機能に絞って記載しています。

1. 自動プロパティの機能強化 (Auto-Property enhancements)

自動プロパティの初期化子 (Auto-Property initializers)

old

public class Customer {
    public Customer() {
        First = "Jane";
        Last = "Doe";
    }

    public string First { get; set; }
    public string Last { get; set; }
}

```csharp:new! public class Customer { public string First { get; set; } = "Jane"; public string Last { get; set; } = "Doe"; } ``` ### 読み取り専用の自動プロパティ (Read-only auto-properties) ```csharp:old public class Customer { private readonly string _name;

public Customer(string name) {
    _name = name;
}

public string Name { get { return _name; } }

}

<div>:arrow_down:</div>
```csharp:new!
public class Customer {
    public Customer(string name) {
        Name = name;
    }

    public string Name { get; }
}

2. 式形式のメンバー (Expression-bodied function members)

式形式のメソッド (Expression bodies on method-like members)

old

public void Print() {
    Console.WriteLine(First + " " + Last);
}

```csharp:new! public void Print() => Console.WriteLine(First + " " + Last); ``` ### 式形式のプロパティ (Expression bodies on property-like function members) ```csharp:old public string Name { get { return First + " " + Last; } } ```

```csharp:new! public string Name => First + " " + Last; ```

3. using static

old

using System;

class Program {
    static void Main() {
        Console.WriteLine(Math.Sqrt(3*3 + 4*4)); 
        Console.WriteLine(DayOfWeek.Friday - DayOfWeek.Monday); 
    }
}

```csharp:new! using static System.Console; using static System.Math; using static System.DayOfWeek;

class Program {
static void Main() {
WriteLine(Sqrt(33 + 44));
WriteLine(Friday - Monday);
}
}

## 4. [Null 条件演算子 (Null-conditional operators)](https://docs.microsoft.com/ja-jp/dotnet/csharp/language-reference/operators/null-conditional-operators)
```csharp:old
var handler = PropertyChanged;
if (handler != null) {
    handler(this, args);
}

```csharp:new! PropertyChanged?.Invoke(this, args); ```

5. 文字列補間 (String interpolation)

old

var s = String.Format("{0,20} is {1:D3} year{2} old", p.Name, p.Age, (p.Age == 1 ? "" : "s"));

```csharp:new! var s = $"{p.Name,20} is {p.Age:D3} year{(p.Age == 1 ? "" : "s")} old"; ```

6. nameof 式 (nameof Expressions)

old

if (x == null) throw new ArgumentNullException("x");

```csharp:new! if (x == null) throw new ArgumentNullException(nameof(x)); ```

7. インデックス初期化子 (Index initializers)

old

var numbers = new Dictionary<int, string> {
    {7, "seven"},
    {9, "nine"},
    {13, "thirteen"},
};

```csharp:new! var numbers = new Dictionary { [7] = "seven", [9] = "nine", [13] = "thirteen", }; ```

8. 例外フィルター (Exception filters)

old

try { … }
catch (MyException e) {
    if (myfilter(e)) {
        …
    }
}

```csharp:new! try { … } catch (MyException e) when (myfilter(e)) { … } ```

9. catch / finally ブロックでの await (Await in catch and finally blocks)

old

Resource res = null;
try {
    res = await Resource.OpenAsync(…);
    …
} 
catch(ResourceException e) {
    Resource.LogAsync(res, e).Wait();
}
finally {
    if (res != null) {
        res.CloseAsync().Wait();
    }
}

```csharp:new! Resource res = null; try { res = await Resource.OpenAsync(…); … } catch(ResourceException e) { await Resource.LogAsync(res, e); } finally { if (res != null) { await res.CloseAsync(); } } ```

For more information

• New Language Features in C# 6 · dotnet/roslyn Wiki
• What's New in C# 6 - C# Guide | Microsoft Docs
• C# 6 の新機能 - C# によるプログラミング入門 | ++C++; // 未確認飛行 C