C# Lambda運算式－天天向上

「Lambda 運算式」(Lambda Expression) 是一種匿名函式，

它可以包含運算式和陳述式 (Statement)，而且可以用來建立委派 (Delegate)

所有的 Lambda 運算式都會使用 Lambda 運算子 =>，意思為「移至」。
Lambda 運算子的左邊會指定輸入參數 (如果存在)，右邊則包含運算式或陳述式區塊。
Lambda 運算式 x => x * x 的意思是「x 移至 x 乘以 x」。
這個運算式可以指派給委派型別 (Delegate Type)，如下

   1: delegate int power2(int x);   2: delegate int power3(int x);   3: delegate int power5(int x);   4: private void button1_Click(object sender, EventArgs e)   5: {   6:     power2 p2 = x => x * x;                // x to the power of 2   7:     power3 p3 = x => p2(x) * x;            // x to the power of 3   8:     power5 p5 = x => p2(x) * p3(x);        // x to the power of 5    9:     MessageBox.Show("2的2次方 =" + p2(2).ToString());  10:     MessageBox.Show("2的3次方 =" + p3(2).ToString());  11:     MessageBox.Show("2的5次方 =" + p5(2).ToString());  12: }

   1: private void button2_Click(object sender, EventArgs e)   2:        {   3:            // (2) Anonymous Methods匿名方法   4:            power2 p2 = delegate(int x) { return x * x; };   5:            power3 p3 = delegate(int x) { return p2(x) * x; };   6:            power5 p5 = delegate(int x) { return p2(x) * p3(x); };   7:            MessageBox.Show("2的2次方 =" + p2(2).ToString());   8:            MessageBox.Show("2的3次方 =" + p3(2).ToString());   9:            MessageBox.Show("2的5次方 =" + p5(2).ToString());  10:        }

   1: static int powerOf2(int x) { return x * x; }   2: static int powerOf3(int x) { return powerOf2(x) * x; }   3: static int powerOf5(int x) { return powerOf2(x) * powerOf3(x); }   4:     5: private void button3_Click(object sender, EventArgs e)   6: {   7:     power2 p2 = new power2(powerOf2);   8:     power3 p3 = new power3(powerOf3);   9:     power5 p5 = new power5(powerOf5);  10:     MessageBox.Show("2的2次方 =" + p2(2).ToString());  11:     MessageBox.Show("2的3次方 =" + p3(2).ToString());  12:     MessageBox.Show("2的5次方 =" + p5(2).ToString());  13: }

Lambda 兩個輸入引數

   1: // Lambda 兩個輸入引數   2: delegate int maxAbs(int x, int y);   3: private void button4_Click(object sender, EventArgs e)   4: {   5:     maxAbs ma = (x, y) => Math.Abs(x) > Math.Abs(y) ? Math.Abs(x) : Math.Abs(y);   6:     int X  = -3;   7:     int Y = -2;   8:     MessageBox.Show( String.Format("X = {0}, Y= {1}, maxAbs(X,Y) = {2}",X, Y, ma(X, Y)) );   9: }

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有時候編譯器會很難或是無法推斷輸入型別。當這種情形發生時，您就可以明確指定型別

   1: delegate bool CheckStrLen(string str, int len);   2:     3:  private void button5_Click(object sender, EventArgs e)   4:  {   5:      CheckStrLen chkStr = (string str, int x) => str.Length > x;   6:      string s = "Hello World";   7:      int len = 5;   8:      MessageBox.Show( string.Format("{0}長度大於{1}: {2}",s, len, chkStr(s, len)) );   9:  }

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Lambda 零個輸入引數

   1: Thread th;   2:        bool _shouldStop = false;   3:        private void button6_Click(object sender, EventArgs e)   4:        {   5:            th = new Thread(   6:                () =>   7:                {   8:                    while (!_shouldStop)   9:                    {  10:                        Invoke(  11:                              (MethodInvoker)   12:                              (() =>  13:                              {  14:                                  label1.Text = DateTime.Now.ToString("F");  15:                              })  16:                                17:                        );  18:                        Thread.Sleep(1000);  19:                    }  20:                }  21:             );  22:            th.IsBackground = true;  23:            th.Start();  24:        }

Invoke: 在目前執行緒中執行特定匿名方法或輸入個數為零的Lambda Expression, 但該delegate方法包含視窗上的控制元件, 不屬於目前的執行緒
必須將 Lambda 運算式所包含的code block() 轉型為 (MethodInvoker), 其中MethodInvoker 表示可以宣告為 void，而且不接受參數的 managed 程式碼中執行的任何方法的委派

加入Form_Closing

   1: private void Form1_FormClosing(object sender, FormClosingEventArgs e)   2: {   3:     _shouldStop = true;   4:     if (th != null && th.IsAlive)   5:         th.Abort();   6: }

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Invoke(…)嘗試另一種寫法: 上面的範例是呼叫Lambda Expression, 下面這個範例是呼叫匿名方法

   1: delegate void showTime();          // 委派 showTime 輸出個數=0   2:     private void button7_Click(object sender, EventArgs e)   3:     {   4:         // 初始化匿名方法   5:         showTime s = delegate() { label1.Text = DateTime.Now.ToString("F"); };   6:         th = new Thread(   7:             () =>   8:             {   9:                 MethodInvoker method = new MethodInvoker(s);  10:                 while (!_shouldStop)  11:                 {  12:                     Invoke( method );   // 執行匿名方法  13:                     Thread.Sleep(1000);  14:                 }  15:             }  16:             );  17:         th.Start();  18:     }