Action是一個泛型的委托，其內部即使用delegate去實現，當普通的delegate定義的參數與Action個數、類型一致時，兩者實現的功能是一樣的。只是Action的方式更加簡潔、規范。如下：
1 public delegate void DoDelegate(object parm);
2 public DoDelegate DoMethod;
3
4 public Action<object> doAction4OneParm;
5 public Action<object, object> doAction4TwoParm;
6
7 private void Form1_Load(object sender, EventArgs e)
8 {
9 DoMethod += DoTestMetohd; //普通委托(由於委托定義時給定一個參數，故此處匹配一個參數的方法)
10 doAction4OneParm += DoTestMetohd; //Action委托(此處匹配一個參數的方法)
11 doAction4TwoParm += DoTestMetohd; //Action委托(此處匹配兩個參數的方法)
12 }
13
14 private void DoTestMetohd(object parm)
15 {
16 MessageBox.Show(Convert.ToString(parm));
17 }
18
19 private void DoTestMetohd(object parm1, object parm2)
20 {
21 MessageBox.Show(Convert.ToString(parm1 + " " + parm2));
22 }
而Action與delegate更重要的一個區別在於泛型，即Action的內部使用了泛型+委托，且泛型的方法的參數個數可擴展到16個，微軟.net corefx中定義的Action內部代碼如下：
1 namespace System
2 {
3 [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Design", "CA1005:AvoidExcessiveParametersOnGenericTypes")]
4 public delegate void Action<in T1, in T2, in T3, in T4, in T5, in T6, in T7, in T8, in T9>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6, T7 arg7, T8 arg8, T9 arg9);
5
6 [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Design", "CA1005:AvoidExcessiveParametersOnGenericTypes")]
7 public delegate void Action<in T1, in T2, in T3, in T4, in T5, in T6, in T7, in T8, in T9, in T10>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6, T7 arg7, T8 arg8, T9 arg9, T10 arg10);
8
9 [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Design", "CA1005:AvoidExcessiveParametersOnGenericTypes")]
10 public delegate void Action<in T1, in T2, in T3, in T4, in T5, in T6, in T7, in T8, in T9, in T10, in T11>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6, T7 arg7, T8 arg8, T9 arg9, T10 arg10, T11 arg11);
11
12 [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Design", "CA1005:AvoidExcessiveParametersOnGenericTypes")]
13 public delegate void Action<in T1, in T2, in T3, in T4, in T5, in T6, in T7, in T8, in T9, in T10, in T11, in T12>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6, T7 arg7, T8 arg8, T9 arg9, T10 arg10, T11 arg11, T12 arg12);
14
15 [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Design", "CA1005:AvoidExcessiveParametersOnGenericTypes")]
16 public delegate void Action<in T1, in T2, in T3, in T4, in T5, in T6, in T7, in T8, in T9, in T10, in T11, in T12, in T13>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6, T7 arg7, T8 arg8, T9 arg9, T10 arg10, T11 arg11, T12 arg12, T13 arg13);
17
18 [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Design", "CA1005:AvoidExcessiveParametersOnGenericTypes")]
19 public delegate void Action<in T1, in T2, in T3, in T4, in T5, in T6, in T7, in T8, in T9, in T10, in T11, in T12, in T13, in T14>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6, T7 arg7, T8 arg8, T9 arg9, T10 arg10, T11 arg11, T12 arg12, T13 arg13, T14 arg14);
20
21 [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Design", "CA1005:AvoidExcessiveParametersOnGenericTypes")]
22 public delegate void Action<in T1, in T2, in T3, in T4, in T5, in T6, in T7, in T8, in T9, in T10, in T11, in T12, in T13, in T14, in T15>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6, T7 arg7, T8 arg8, T9 arg9, T10 arg10, T11 arg11, T12 arg12, T13 arg13, T14 arg14, T15 arg15);
23
24 [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Design", "CA1005:AvoidExcessiveParametersOnGenericTypes")]
25 public delegate void Action<in T1, in T2, in T3, in T4, in T5, in T6, in T7, in T8, in T9, in T10, in T11, in T12, in T13, in T14, in T15, in T16>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6, T7 arg7, T8 arg8, T9 arg9, T10 arg10, T11 arg11, T12 arg12, T13 arg13, T14 arg14, T15 arg15, T16 arg16);
26 }
EventHandler是一個事件委托，即內部也采用委托來實現，但其更多的時候被定義成event，即事件的方式來使用。
以上純個人在使用中的觀點，歡迎指正批評!!!
