大家好，今天是第十天了！
完成三分之一，繼續加油

用 Lambda 找出集合中特定條件下的數量，可使用 Count 語法。
以下範例可找出 Receiver 類別中，name 以 J 開頭的物件數量：

    protected class Receiver{
    	public string name;
    	public string email;

    	public Receiver(string n, string m){
        	name = n;
        	email = m;
    	}
	}

	public void Run(){
    	List<Receiver> receiver = new List<Receiver>();

    	receiver.Add(new Receiver("Jack", "Jack@gmail.com"));
    	receiver.Add(new Receiver("Justin", "Justin@gmail.com"));
    	receiver.Add(new Receiver("Eddy", "Eddy@hotmail.com"));
    	receiver.Add(new Receiver("Jeff", "Chen.Jeff@gmail.com"));

    	int jCount = receiver.Count(n => n.name.StartsWith("J"));
    	Console.WriteLine("Receiver start with J:" + jCount);

    }

結果為：

Receiver start with J:3

若要使用 Lambda 排序，可以使用 OrderBy()、OrderByDescending() 對欄位分別作遞增、遞減排序。如果需要接著對其它欄位排序，還可以使用 ThenBy()、ThenByDescending() 。

延續上方的例子，修改內部的 Run 方法，要求以 Receiver 類別的 email 欄位遞增、遞減排序：

    public void Run(){
    	List<Receiver> receiver = new List<Receiver>();

    	receiver.Add(new Receiver("Jack", "Jack@gmail.com"));
    	receiver.Add(new Receiver("Justin", "Justin@gmail.com"));
    	receiver.Add(new Receiver("Eddy", "Eddy@hotmail.com"));
    	receiver.Add(new Receiver("Jeff", "Chen.Jeff@gmail.com"));

    	IOrderedEnumerable<Receiver> AscReceiver = receiver.OrderBy(n => n.email);
    	foreach(Receiver r in AscReceiver){
        	Console.WriteLine(r.email + "(" + r.name + ")");
    	}

    	IOrderedEnumerable<Receiver> DescReceiver = receiver.OrderByDescending(n => n.email);
    	foreach(Receiver r in DescReceiver){
        	Console.WriteLine(r.email + "(" + r.name + ")");
    	}
	}

輸出結果為：

Chen.Jeff@gmail.com(Jeff)
Eddy@hotmail.com(Eddy)
Jack@gmail.com(Jack)
Justin@gmail.com(Justin)

Justin@gmail.com(Justin)
Jack@gmail.com(Jack)
Eddy@hotmail.com(Eddy)
Chen.Jeff@gmail.com(Jeff)

參考資料

C# 3.0 極簡風 - Lambda Expression-黑暗執行緒
c# - Multiple Order By with LINQ - Stack Overflow
c# - Get item count of a list<> using Linq - Stack Overflow