LINQ系列
這次要講的是Lambda...
蛤...這個溝系三小...你應該會是以下的表情狀況，我好多好多不懂，我好難理解...
暈倒...
沒關係，我們就從前世今身，開始講起，撰寫LINQ，若用到指定準則或選擇資料的時候
，就會使用到Lambda，舉例我們很常用的例子
products.Where(x => x.ProductId == "1");
在調用Enumerable.where方法時候，就會縮小或篩選出現有的集合內容。
這代表用單一語法，可以表達方法的定義、宣告，執行它的委派過程。EX：匿名方法、投影(後續有時間再談)
以下是很單純的lambda運算式
x => x * 21
上述的意思是：用x做為此函數的參數，x前往(goes to)的結果是 x * 21，那=>代表的解讀是前往
若上述轉換成投影如下
x => new { number = x * 21 };
接者我們再把它寫成C# 1.0
class Program
{
public delegate int IncreaseByAnumber(int x);
static public int MultiplyByANumber(int x)
{
return x * 21;
}
private static void Main(string[] args)
{
IncreaseByAnumber increase = new IncreaseByAnumber(MultiplyByANumber);
Console.WriteLine(increase(10)); //210
}
}
若遇到C# 2.0匿名方法，改寫如下
public delegate int IncreaseByAnumber(int x);
private static void Main(string[] args)
{
IncreaseByAnumber increase = new IncreaseByAnumber(delegate(int i)
{
return i * 21;
});
Console.WriteLine(increase(10));
}
接者改寫成現在的lambda如下
public delegate int IncreaseByAnumber(int x);
private static void Main(string[] args)
{
IncreaseByAnumber increase = x => x * 21;
Console.WriteLine(increase(10));
}
以上這就是Lambda sample簡化過程。
稍微加點變化，加入Func用法，在訂房需求當中如果大人消費
滿一萬則打9折，小孩滿兩千則打8折...這時候我用lambda方式去撰寫
class Program
{
class BookingOrder
{
private Func<int, int> discount;
public BookingOrder(Func<int, int> discount)
=> this.discount = discount;
public int getDiscount(int total) => discount(total);
}
private static void Main(string[] args)
{
//寫法一
//Func<int, int> adoultDiscount = delegate(int amount)
// {
// return (int) (amount >= 10000 ? amount * 0.9 : amount);
// };
//Func<int, int> childDiscount = delegate(int amount)
// {
// return (int)(amount >= 2000 ? amount * 0.8 : amount);
// };
//寫法二
Func<int, int> adoultDiscount = (int amount)
=> (int)(amount >= 10000 ? amount * 0.9 : amount);
Func<int, int> childDiscount = (int amount)
=> (int) (amount >= 2000?amount * 0.8:amount);
BookingOrder bookingOrder1 = new BookingOrder(adoultDiscount);
BookingOrder bookingOrder2 = new BookingOrder(childDiscount);
int total = 5000;
Console.WriteLine(bookingOrder1.getDiscount(total));
Console.WriteLine(bookingOrder2.getDiscount(total));
total = 12000;
Console.WriteLine(bookingOrder1.getDiscount(total));
Console.WriteLine(bookingOrder2.getDiscount(total));
}
}
關於委派參考 http://vito-note.blogspot.com/2012/03/delegate.html
老E隨手寫