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Posted 25 Oct 2010

# An Example of Strategy Design Pattern for Beginners

, 25 Oct 2010
This example shows how and where to implement the strategy design pattern.

## Introduction

Sometimes, we need to carry out several computations / algorithms depending on certain conditions. We go ahead in implementing those generally by applying either `switch `or ternary operator or `if else`. Though initially somehow we manage to write those programs, but if the program demands are too complex then it is difficult to frame such as well as to maintain.

Moreover, writing all the logic in a single place is not at all advisable as it yields to tight coupling.

## Background

In many situations, we come across the need to write computational logic/algorithms which we generally accomplish by using `if else `/ `switch `or ternary operator. It becomes difficult at times to write such a program and later on adds a lot of cost during maintenance.

## Rescuer

Strategy design pattern. It comes under the category of Behavioral Patterns.

## How

1. Decouples the client and the algorithm/ computation logic in separate classes
2. Helps to switch algorithms at any time
3. Easily allows to plug in a new algorithm

## Pattern Components

The strategy pattern comprises the following components:

1. Strategy Interface - Interface common to all concrete strategies
2. Concrete Strategies / Different algorithm classes - Various concrete classes that implement the strategy interface for the sake of algorithm implementation specific to itself
3. Context Class - Delegates requests to the indicated Concrete Strategies received from the client. It does so as it keeps reference of the concrete strategies.
4. Client - In this case, the Windows form

## Case Study

Let us take the example of MSWord application. The various Font Styles (e.g. Bold, Italic, etc.) can be implemented by using this design pattern. Let us see the same as under.

#### Implementation of Strategy Interface

The `IFontStyle `interface looks as below:

```public interface IFontStyle
{
public Font GetNewFontStyle(Control c);
}```

#### Implementation of Concrete Strategies

There are four concrete strategy classes, viz Bold.cs, Italic.cs, Underline.cs and StrikeThru.cs.

Since all follows the same pattern in this example, so for the sake of simplicity I am describing only for the `Bold `concrete class.

It goes as below:

```public class Bold : IFontStyle
{
#region IFontStyle Members

public Font GetNewFontStyle(System.Windows.Forms.Control c)
{
return new Font(c.Font, FontStyle.Bold);
}

#endregion
}```

The code is self explanatory. The `Bold `class implemented the strategy interface `IFontStyle `where it is returning the font style as `bold`.

#### Implementation of Context

The `FontStyleContext `class is as under:

```public class FontStyleContext
{
//step 1: Create an object of Strategy Interface
IFontStyle _IFontStyle;

//step 2:Set the concrete call called by the client to the
//interface object defined in the step 1
public FontStyleContext(IFontStyle IFontStyle)
{
this._IFontStyle = IFontStyle;
}

//step 3: A method that will return the font style
public Font GetFontStyle(Control c)
{
return _IFontStyle.GetNewFontStyle(c);
}
}```

#### Implementation of Client

The client design is as under:

And the code behind for the `Bold `Button click is as under:

```// Bold Click
private void btnBold_Click(object sender, EventArgs e)
{
objFontStyleContext = new FontStyleContext(new Bold());
richTextArea.Font =  objFontStyleContext.GetFontStyle(richTextArea);
}```

First, we have created an instance of the `FontStyleContext `class and then instantiated the same with the appropriate `StrategyConcrete `class. Next by calling the `GetFontStyle `method, we were able to set the appropriate `Font `style for the `richtextbox`.

The other buttons will have the same implementation with only call to appropriate concrete classes.

## Conclusion

Strategy pattern is of great importance while implementing program logic that will be invoked depending on conditions. This is just a simple example to demonstrate the pattern idea. In real time, we will be adorned with more complex situations but the underlying concept will be the same.

Comments are highly appreciated for the improvement of the topic.

## History

• 26th October, 2010: Initial post

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