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Indexers in C#

Few days back some of my colleagues were discussing about indexers. Although I have heard about indexers but I was thinking of its usage in real time scenario. I have searched on the internet and queried some discussion forums, finally I am ready with the sample code. In this tutorial I will explain about indexers in C#. This topic contains following sections

   1. Introduction.

   2. Syntax.

   3. Creation of XML file.

   4. Creating a class to implement indexer.

   5. Code explanation.

   6. Reading the xml file.

   7. 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

Now 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>

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

Creating a class to implement 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

Now let us explain the above code step by step.

1. First of all an object of type dictionary is created.

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

   2. 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 list of subjects in div tag.

   3. A method GetListOfSubjects() is created. The purpose of this method is to get 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).

   4. A constructor to call GetListOfSubjects() method.

   5. 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 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 is ensuring that you won’t get index out of range run time exception while executing for loop.

Conclusion

An obvious question came in your mind is that the whole logic can be implemented by creating a variable of type dictionary directly, then 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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