C#中Action与delegate、EventHandler的差异

Action是一个泛型的委托，其内部即使用delegate去实现，当普通的delegate定义的参数与Action个数、类型一致时，两者实现的功能是一样的。只是Action的方式更加简洁、规范。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(由于委托定义时给定一个参数，故此处匹配一个参数的方法)
10             doAction4OneParm += DoTestMetohd;  //Action委托(此处匹配一个参数的方法)
11             doAction4TwoParm += DoTestMetohd;  //Action委托(此处匹配两个参数的方法)
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与delegate更重要的一个区别在于泛型，即Action的内部使用了泛型+委托，且泛型的方法的参数个数可扩展到16个，微软.net corefx中定义的Action内部代码如下：

 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是一个事件委托，即内部也采用委托来实现，但其更多的时候被定义成event，即事件的方式来使用。

以上纯个人在使用中的观点，欢迎指正批评!!!