C# Action Object (Lambda That Returns Void)

Understand the Action type. An Action is a delegate that is similar to a void method.


This is a function object. Action objects return no values. The Action type is similar to a void method. This generic type is found in the System namespace.Void

To specify an Action,

we must have no return value. The Action must never return a value onto the evaluation stack. Often we use lambdas to specify Actions.Lambdas

First example.

These Actions point to anonymous functions. These functions cannot return values. An Action instance can receive parameters, but cannot return values.Return

So: An Action instance is similar to a void method. The program shows how to Invoke the Action instances.

Action 1: The first Action instance uses the constructed type Action<int>, and an explicit parameter list in the lambda expression.

And: It receives one argument when invoked. It returns no result—it is a void delegate method.

Next: The second and third Action instances return no value. The second Action receives 2 ints. The third Action receives no parameters.

Int, uint
C# program that uses Action type and Invoke using System; class Program { static void Main() { // Example Action instances. // ... First example uses one parameter. // ... Second example uses two parameters. // ... Third example uses no parameter. // ... None have results. Action<int> example1 = (int x) => Console.WriteLine("Write {0}", x); Action<int, int> example2 = (x, y) => Console.WriteLine("Write {0} and {1}", x, y); Action example3 = () => Console.WriteLine("Done"); // Call the anonymous methods. example1.Invoke(1); example2.Invoke(2, 3); example3.Invoke(); } } Output Write 1 Write 2 and 3 Done

Notes, Invoke.

The Invoke method receives a number of arguments equal to the specific type of Action. We must specify these arguments in the Action type declaration.Generic Class, Method

Tip: This information is determined at compile-time, before runtime. If a program compiles, it uses the correct argument types.

A benchmark.

How much slower are delegate method calls than direct method calls? To test this, we use the Action type with a single parameter.

Overview: This program introduces the Method1 method, which contains some dummy code for testing.

First: In the first loop, Method1 (which uses no Action) is called directly 100 million times.

Second: In the second loop, an Action instance that points to Method1 is invoked the same number of times.

C# program that tests Action invocation using System; using System.Diagnostics; class Program { const int _max = 100000000; static void Main() { // Create Action delegate for Method1. Action<int> action = new Action<int>(Method1); var s1 = Stopwatch.StartNew(); for (int i = 0; i < _max; i++) { // Direct call. Method1(5); } s1.Stop(); var s2 = Stopwatch.StartNew(); for (int i = 0; i < _max; i++) { // Delegate call. action.Invoke(5); } s2.Stop(); Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000 * 1000) / _max).ToString("0.00 ns")); Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000 * 1000) / _max).ToString("0.00 ns")); Console.Read(); } static void Method1(int param) { // Dummy. if (param == -1) { throw new Exception(); } } } Output 0.32 ns 3.52 ns

Notes, above benchmark.

For a method with one parameter and no return value, the Action invocation costs more than 3 ns extra each time. This is unlikely to cause many performance problems.


What is the difference between Action and Func? The Action type receives parameters but does not return a parameter. Func receives parameters and returns a result value.

So: An Action instance never returns anything, while the Func always returns something.



Next, we compare abstract methods. An Action<int> can be an instance that points to any function that receives an int parameter.

However: You can design an abstract class with an abstract method that also can be used in this way.


Next: In the following program, we test such an abstract class against an Action<int>.

C# program that benchmarks abstract using System; using System.Diagnostics; abstract class A { public abstract void MethodA(int y); } class AB : A { public override void MethodA(int y) { } } class Program { static void MethodA(int y) { } static void Main() { A abst = new AB(); abst.MethodA(0); Action<int> action = new Action<int>(MethodA); action.Invoke(0); const int max = 100000000; var s1 = Stopwatch.StartNew(); for (int i = 0; i < max; i++) { abst.MethodA(i); abst.MethodA(i); } s1.Stop(); var s2 = Stopwatch.StartNew(); for (int i = 0; i < max; i++) { action.Invoke(i); action.Invoke(i); } s2.Stop(); Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / max).ToString("0.00 ns")); Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / max).ToString("0.00 ns")); Console.Read(); } } Output 3.54 ns 6.68 ns

Results, above benchmark.

The Action was slower to call than the method from the abstract class reference. If you can use abstract classes instead of Actions, this will be faster.


It is possible to use Action as the value in a Dictionary instance. This makes it possible to call functions by a string key.Dictionary

Here: In this example, we invoke two static void methods based on simple string keys.


Idea: You could use a Dictionary of an abstract class. Then, instantiate each method as an instance of a derived class.

C# program that uses Dictionary with Action using System; using System.Collections.Generic; class Program { static void Main() { Dictionary<string, Action> dict = new Dictionary<string, Action>(); dict["cat"] = new Action(Cat); dict["dog"] = new Action(Dog); dict["cat"].Invoke(); dict["dog"].Invoke(); } static void Cat() { Console.WriteLine("CAT"); } static void Dog() { Console.WriteLine("DOG"); } } Output CAT DOG

A summary.

Programs use Action for many purposes. The Action type specifies a function object that can receive parameters, but never returns a value onto the stack.

Note: Thanks to Turker Tunali for helping clarify the Action descriptive text. All contributions are appreciated.

Void methods.

Action instances are void methods. As higher-order procedures, they can be passed around as objects. This is powerful.
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