使用 lambda expression pass parameter 時，注意變數是 "參考指標" 時，可能會與預期不同

一般情況下 CallByValue 與 CallByReference

class Class0
    {
        public void CallByValue(int si)
        {
            si *= 2;
        }
        public void CallByReference(Class0_Parm parm)
        {
            parm.Si  *= 2;
        }
        public void CallByReference(ref int si)
        {
           si *= 2;
        }
    }

    class Class0_Parm
    {
        public int Si { get; set; }
    }

        static void Main(string[] args)
        {// 參數為數值型別 是 Call By Value
            int x = 5;
            Class0 c0 = new Class0();
            Console.Write(string.Format("數值 {0} Call by Value", x));

            c0.CallByValue(x);
            Console.WriteLine(string.Format(" = {0}", x)); //

            // 參數為物件型別 是 Call By Reference
            Class0_Parm p = new Class0_Parm() { Si = 5 };
            Console.Write(string.Format("物件 {0} Call By Reference", p.Si));

            c0.CallByReference(p);
            Console.WriteLine(string.Format(" = {0}", p.Si));

            int y = 5;
            Console.Write(string.Format("數值 {0} Call By Reference(使用ref)", y));
            c0.CallByReference(ref y);
            Console.WriteLine(string.Format(" = {0}", y));
            Console.Read();
        }

使用 Lambda Expressions 有問題的程式碼

public void Test(int numSites)
        {
            System.Timers.Timer[] scanLiveTimer = new System.Timers.Timer[numSites];

            for (int si = 0; si < numSites; si++)
            {
                scanLiveTimer[si] = new System.Timers.Timer();

                scanLiveTimer[si].Elapsed +=
                    new System.Timers.ElapsedEventHandler((sender, e) => ScanMarketEvent(si));

                scanLiveTimer[si].Interval = 500;
                scanLiveTimer[si].Start();
                Console.WriteLine(string.Format("Trigger: {0}", si));
            }
        }

        void ScanMarketEvent(int si)
        {
            // 有誤，讀到的值都一樣
            Console.Write(string.Format("{0} ", si));
        }

解法1: 若不使用 Lambda Expressions

        public void Test(int numSites)
        {
            MyTimer[] scanLiveTimer = new MyTimer[numSites];
            for (int si = 0; si < numSites; si++)
            {
                scanLiveTimer[si] = new MyTimer();
                scanLiveTimer[si].Si = si;
                scanLiveTimer[si].Elapsed += new System.Timers.ElapsedEventHandler(MyTimer_Elapsed);
                scanLiveTimer[si].Interval = 500;
                scanLiveTimer[si].Start();
                Console.WriteLine(string.Format("Trigger: {0}", si));
            }
        }

        public class MyTimer : System.Timers.Timer
        {
            public int Si { get; set; }
        }

        void MyTimer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
        {
            MyTimer myTimer = (MyTimer)sender;
            int si = myTimer.Si;
            ScanMarketEvent(si);
        }

        void ScanMarketEvent(int si)
        {
            // 可以讀到正確的值
            Console.Write(string.Format("{0} ", si));
        }

解法2: 先複製變數，再傳入 Lambda Expressions

        public void Test(int numSites)
        {
            System.Timers.Timer[] scanLiveTimer = new System.Timers.Timer[numSites];

            for (int si = 0; si < numSites; si++)
            {
                scanLiveTimer[si] = new System.Timers.Timer();
                // 複製變數，再傳入 lambda expression
                int x = si;

                scanLiveTimer[si].Elapsed +=
                    new System.Timers.ElapsedEventHandler((sender, e) => ScanMarketEvent(x));

                scanLiveTimer[si].Interval = 500;
                scanLiveTimer[si].Start();
                Console.WriteLine(string.Format("Trigger: {0}", si));
            }
        }

        void ScanMarketEvent(int si)
        {
            // 可以讀到正確的值
            Console.Write(string.Format("{0} ", si));
        }