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Indexers In C#

A few days back some of my colleagues were discussing indexers. Although I have heard about indexers I was thinking of their usage in real-time scenarios. I have searched on the internet and queried some discussion forums, and finally, I am ready with the sample code. In this article, I will explain about indexers in C#. This topic contains the following sections: -

Introduction
Syntax
Creation of XML file
Creating a class to implement an indexer
Code explanation
Reading the XML file
Conclusion

Introduction

"An indexer enables you to treat a class like an array."

Although this is not a formal definition but will give you a fair idea about indexers.

Syntax

<modifier> <return type > this[argument list] {
	get {
		// Get codes goes here
	}
	set {
		// Set codes goes here
	}
}

Creation Of XML File

Let us start with an example. Consider an XML file: -

<root>
    <subject value = ".Net"></subject>
    <subject value = ".Net Web Service"></subject>
    <subject value = "ADO.NET"></subject>
    <subject value = "IBM Lotus Notes"></subject>
    <subject value = "JavaScript"></subject>
    <subject value = "Silverlight"></subject>
    <subject value = "Software Testing"></subject>
    <subject value = "SQL Server"></subject>
    <subject value = "WPF"></subject>
</root>

Our job is to get the list of subjects and show it in a "div" tag. The next step is to create a "Subject" class.

Creating A Class To Implement An Indexer

class Subject
{
    private Dictionary<string, string> _subjectDictionary = new Dictionary<string, string>();

    /// <summary>
    /// Indexer
    /// </summary>
    /// <param name="index"></param>
    /// <returns></returns>
    public string this[int index]
    {
        get
        {
            string key = _subjectDictionary.Keys.Where((n, i) => i == index).FirstOrDefault();
            return string.Format("{0}", _subjectDictionary[key]);
        }
    }

    /// <summary>
    /// Get List of subjects
    /// </summary>
    private void GetListOfSubjects()
    {
        DataSet ds = new DataSet();
        ds.ReadXml("subject.xml");
        for (int i = 0; i < ds.Tables[0].Rows.Count - 1; i++)
        {
            _subjectDictionary.Add(i.ToString(), ds.Tables[0].Rows[i]["value"].ToString());
        }
    }

    /// <summary>
    /// Constructor
    /// </summary>
    public Subject()
    {
        if (_subjectDictionary.Count == 0)
        {
            GetListOfSubjects();
        }
    }
    /// <summary>
    /// Subject Count to save from 
    /// index out of range
    /// </summary>
    public int SubjectCount
    {
        get
        {
            return _subjectDictionary.Count;
        }
    }
}

Code Explanation

Let us explain the above code step by step.

First of all an object of type dictionary is created

private Dictionary<string,string> _subjectDictionary = new Dictionary<string,string>();

A property "SubjectCount" is created to get the count of items in "_subjectDictionary". This property is optional but it is better to create it, because when you loop through "_subjectDictionary", then this property ensures that you won’t get any index out-of-range run time error. I will come back to this point when I will show the list of subjects in the "div" tag
A method "GetListOfSubjects()" is created. The purpose of this method is to get a list of subjects from XML (It should be noted that I have ignored the validation part deliberately to make the code simple and easy to understand)
A constructor to call the "GetListOfSubjects()" method
Indexer is defined

public string this[int index] {
	get {
		string key = _subjectDictionary.Keys.Where((n, i) = >i == index).FirstOrDefault();
		return string.Format("{0}", _subjectDictionary[key]);
	}
}

If we look at the syntax given above then "public" is an access modifier, "string" is the return type, and the "index" is an argument. In the indexer, we are getting value from "_subjectDictionary" based on "index" value like if "index = 0" then "_subjectDictionary" will return ".Net" and so on.

Reading The Xml File

Now we are ready with the class and it’s time to use it.

Subject oSubject = new Subject();
StringBuilder sbMenu = new StringBuilder(string.Empty);
sbMenu.Append("<ul id = \"MenuLeft\">");
for (int i = 0; i < oSubject.SubjectCount; i++) {
	sbMenu.Append("<li>");
	sbMenu.Append(oSubject[i].ToString());
	sbMenu.Append("</li>");
}
sbMenu.Append("</ul>");
divLeftMenu.InnerHtml = sbMenu.ToString();

As I have mentioned in point 2, the property "SubjectCount" here ensures that you will not get index out-of-range run time exceptions while executing for loop.

Conclusion

An obvious question that came to your mind is that the whole logic can be implemented by creating a variable of type dictionary directly, so why do we need to create a separate class for implementing all this. The answer is we are not exposing any of the property of type dictionary but still getting the luxury of using the dictionary class.

Posted By -	Karan Gupta

Posted On -	Monday, June 21, 2010

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