C# Action

A delegate in C# represents a reference type that encapsulates a method. When we declare a delegate type we specify some parameters and a return type. We can only store those methods in delegates which has same parameters and same return type. Microsoft introduced some pre-built delegates so that we don't have to declare delegates every time. Action is one of the pre-built delegates.

Action in C# represents a delegate that has void return type and optional parameters. There are two variants of Action delegate.

1. Action
2. Action<in T>

Non-Generic Action Delegate

First variant is non-generic delegate that takes no parameters and has void return type. In this Action delegate, we can store only those methods which has no parameters and void return type.

static void Main(string[] args)
{
    Action doWorkAction = new Action(DoWork);
    doWorkAction(); //Print "Hi, I am doing work."
}

public static void DoWork()
{
    Console.WriteLine("Hi, I am doing work.");
}

We can also store method in the Action delegate with direct initializing with the method. Below is the example.

static void Main(string[] args)
{
    Action doWorkAction = DoWork;
    doWorkAction(); //Print "Hi, I am doing work."
}

public static void DoWork()
{
    Console.WriteLine("Hi, I am doing work.");
}

We call the Action delegate like as method calling.

Generic Action Delegate

The second variant is a family of 16 generic delegates. These delegates can take upto 16 parameters and all have void return type. Below are the syntaxs of these delegates.

public delegate void Action<in T>(T obj);
public delegate void Action<in T1, in T2>(T1 arg1, T2 arg2);
public delegate void Action<in T1, in T2, in T3>(T1 arg1, T2 arg2, T3 arg3);
public delegate void Action<in T1, in T2, in T3, in T4>(T1 arg1, T2 arg2, T3 arg3, T4 arg4);
public delegate void Action<in T1, in T2, in T3, in T4, in T5>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5);
public delegate void Action<in T1, in T2, in T3, in T4, in T5, in T6>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6);
public delegate void Action<in T1, in T2, in T3, in T4, in T5, in T6, in T7>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6, T7 arg7);
public delegate void Action<in T1, in T2, in T3, in T4, in T5, in T6, in T7, in T8>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6, T7 arg7, T8 arg8);
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);
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);
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);
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);
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);
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);
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);
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);

We have to choose Action delegate according to the method, which we want to store. If our method takes two parameters, then we have to choose action delegate which has two parameters Action<in T1, T2>(T1 arg1, T2 arg2). Below are some examples 

static void Main(string[] args)
{
    Action<int> firstAction = DoWorkWithOneParameter;
    Action<int, int> secondAction = DoWorkWithTwoParameters;
    Action<int, int, int> thirdAction = DoWorkWithThreeParameters;

    firstAction(1); // Print 1
    secondAction(1, 2); // Print 1-2
    thirdAction(1, 2, 3); //Print 1-2-3
}

public static void DoWorkWithOneParameter(int arg)
{
    Console.WriteLine(arg);
}

public static void DoWorkWithTwoParameters(int arg1, int arg2)
{
    Console.WriteLine(arg1 + "-" + arg2);
}

public static void DoWorkWithThreeParameters(int arg1, int arg2, int arg3)
{
    Console.WriteLine(arg1 + "-" + arg2 + "-" + arg3);
}

Action with Lambda Expression

We can use Lambda expression with Action delegate. Below is the example.

static void Main(string[] args)
{
    Action act = () =>
    {
        Console.WriteLine("No Parameter");
    };

    Action<int> actWithOneParameter = (arg1) =>
        {
            Console.WriteLine("Par: " + arg1);
        };

    Action<int, int> actWithTwoParameter = (arg1, arg2) =>
        {
            Console.WriteLine("Par1: " + arg1 + ", Par2: " + arg2);
        };

    act();
    actWithOneParameter(1);
    actWithTwoParameter(1, 2);
}