與時俱進的語言

一、System Class

1. 泛型(genric)

  • 可更清楚的描述序列中每個元素的類型。

C#1 示例

public class Bookshelf
{
    public IEnumerable Books { get { ... } }
}

C#2 示例：泛型

public class Bookshelf
{
	public IEnumerable<Book> Books { get { ... } }
}

2. 可空值類型(nullable value type)

  • 可有效的表示未定的變量值，以擺脫魔數(用-1當集合索引，用 MinValue 或 MaxValue 做為初始值)。

示例

string? a = null;
Console.WriteLine(a ?? "null");		// null
a = "abc";
Console.WriteLine(a ?? "null");		// abc

3. 匿名類型(anonymous type)、隱式局部變數(var)

  • 兩者皆可解決靜態類型語言的缺陷：程式碼冗長。

示例1: 匿名類型(anonymous type)

var book = new 
{
	Title = "Harry Potter",
	Author = "J.K. Rowling"
}
string title = book.Title;
string author = book.Author;
  • 若已經調用了建構式的話，就無需顯式的宣該告變數的類型了。

示例2: 隱式類型(implicit typing)

Dictionary<string, string> map1 = new Dictionary<string, string>();
var map2 = new Dictionary<string, string>();

4. 簡潔化

示例1:委托

button.Click += new EventHandler(HandleButtonClick);			// C#1
button.Click += HandleButtonClick;								// C#2 方法組轉換+匿名方法
button.Click += delegate { MessageBox.Show("Clicked!") };		// C#2 委托
button.Click += (sender, args) => MessageBox.Show("Clicked!");	// C#3 lamnda 表達式

void HandleButtonClick(object sender, EventArgs args)
{
	MessageBox.Show("Clicked!");
}

示例2:自動實現
下面兩段 code 可以經由自動實現視為相同

private string name;
public string Name
{
	get { return name; }
	set { name = value; }
}

public string Name { get; set; }

示例3:表達式主體成員
下面兩段 code 可以經由表達式主體成員(expression-bodied member)視為相同

public int Count { get { return list.Count; } }
public IEnumerator<string> GetEnumerator()
{
	return list.GetEnumerator();
}

public int Count => list.Count;
public IEnumerator GetEnumerator => list.GetEnumerator();

示例4:字串處理:內插內串字面量(interpolated string literal)

throw new KeyNotFoundException("No calendar system for Id " + id + " exists");				// 字串拼接
throw new KeyNotFoundException(string.Format("No calendar system for Id {0} exists", id));	// 字串格式化
throw new KeyNotFoundException($"No calendar system for Id {id} exists");					// 內插字串字面量

示例5: 利用LINQ進行數據訪問

var offers = 
	from product in db.Products
	where product.SalePrice <= product.Price / 2
	orderby product.SalePrice
	select new {
		producdt.Id, 
		product.Description, 
		product.SalePrice, 
		priduct.Price
	};

示例6: 非同步

private async Task UpdateStatus()
{
	Task<Weather> weatherTask = GetWeatherAsync();
	Task<EmailStatus> emailTask = GetEmailStatusAsync();

	Weather weather = await weatherTask;
	EmailStatus emai = await emailTask;

	weatherLabel.Text = weather.Description;
	inboxLabel.Text = email.InboxCount.ToString();
}