Uses of Action delegate in C#

Question

I was working with the Action Delegates in C# in the hope of learning more about them and thinking where they might be useful.

Has anybody used the Action Delegate, and if so why? or could you give some examples where it might be useful?

Solution 1

MSDN says:

This delegate is used by the Array.ForEach method and the List.ForEach method to perform an action on each element of the array or list.

Except that, you can use it as a generic delegate that takes 1-3 parameters without returning any value.

Solution 2

Here is a small example that shows the usefulness of the Action delegate

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        Action<String> print = new Action<String>(Program.Print);

        List<String> names = new List<String> { "andrew", "nicole" };

        names.ForEach(print);

        Console.Read();
    }

    static void Print(String s)
    {
        Console.WriteLine(s);
    }
}

Notice that the foreach method iterates the collection of names and executes the print method against each member of the collection. This a bit of a paradigm shift for us C# developers as we move towards a more functional style of programming. (For more info on the computer science behind it read this: http://en.wikipedia.org/wiki/Map_(higher-order_function).

Now if you are using C# 3 you can slick this up a bit with a lambda expression like so:

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List<String> names = new List<String> { "andrew", "nicole" };

        names.ForEach(s => Console.WriteLine(s));

        Console.Read();
    }
}

Solution 3

Well one thing you could do is if you have a switch:

switch(SomeEnum)
{
  case SomeEnum.One:
      DoThings(someUser);
      break;
  case SomeEnum.Two:
      DoSomethingElse(someUser);
      break;
}

And with the might power of actions you can turn that switch into a dictionary:

Dictionary<SomeEnum, Action<User>> methodList = 
    new Dictionary<SomeEnum, Action<User>>()

methodList.Add(SomeEnum.One, DoSomething);
methodList.Add(SomeEnum.Two, DoSomethingElse); 

...

methodList[SomeEnum](someUser);

Or you could take this farther:

SomeOtherMethod(Action<User> someMethodToUse, User someUser)
{
    someMethodToUse(someUser);
}  

....

var neededMethod = methodList[SomeEnum];
SomeOtherMethod(neededMethod, someUser);

Just a couple of examples. Of course the more obvious use would be Linq extension methods.

Solution 4

You can use actions for short event handlers:

btnSubmit.Click += (sender, e) => MessageBox.Show("You clicked save!");

Solution 5

I used the action delegate like this in a project once:

private static Dictionary<Type, Action<Control>> controldefaults = new Dictionary<Type, Action<Control>>() { 
            {typeof(TextBox), c => ((TextBox)c).Clear()},
            {typeof(CheckBox), c => ((CheckBox)c).Checked = false},
            {typeof(ListBox), c => ((ListBox)c).Items.Clear()},
            {typeof(RadioButton), c => ((RadioButton)c).Checked = false},
            {typeof(GroupBox), c => ((GroupBox)c).Controls.ClearControls()},
            {typeof(Panel), c => ((Panel)c).Controls.ClearControls()}
    };

which all it does is store a action(method call) against a type of control so that you can clear all the controls on a form back to there defaults.

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Comments

[mackenir]

I never noticed those multi-parameter versions of Action. Thanks.

[Biswanath]

Nice, not a big deal of change but there is something called keyedbyTypeCollection, although I think it wraps around dictioinary(type, Object), may be.

[Biswanath]

Great, I think this could be used as a decision table.

[David Robbins]

Nice - this is a refactoring pattern "Replace Conditional with Polymorphism". refactoring.com/catalog/replaceConditionalWithPolymorphism.h‌​tml

[surfmuggle]

Hi Sorskoot, could you expand how UpdateMethod, MyEventArgs and new BalieEventArgs are playing together. is the string Message passed into UpdateMethod: UpdateMethod("A Message")? Which method uses the object "someDataObject"? Thanks in advance

[tdgtyugdyugdrugdr]

You can use them for long ones too; btnSubmit.Click += (sender, e) => { MessageBox.Show("You clicked save!"); MessageBox.Show("You really did!"); };