C# Indexers

An indexer is a special type of property that allows a class or a structure to be accessed like an array for its internal collection. C# allows us to define custom indexers, generic indexers, and also overload indexers.

An indexer can be defined the same way as property with this keyword and square brackets [].

Syntax
<return type> this[<parameter type> index]
{
    get{
        // return the value from the specified index of an internal collection
    }
    set{
        // set values at the specified index in an internal collection
    }
}

The following example defines an indexer in the class.

Example: Indexer
class StringDataStore
{
    private string[] strArr = new string[10]; // internal data storage

    public string this[int index]
    {
        get
        {
            if (index < 0 || index >= strArr.Length)
                throw new IndexOutOfRangeException("Index out of range");

                return strArr[index];
        }

        set
        {
            if (index < 0 ||  index >= strArr.Length)
                throw new IndexOutOfRangeException("Index out of range");

            strArr[index] = value;
        }
    }
}

The above StringDataStore class defines an indexer for its private array strArr. So now, you can use the StringDataStore like an array to add and retrieve string values from strArr, as shown below.

Example: Indexer
StringDataStore strStore = new StringDataStore();

strStore[0] = "One";
strStore[1] = "Two";
strStore[2] = "Three";
strStore[3] = "Four";
        
for(int i = 0; i < 10 ; i++)
    Console.WriteLine(strStore[i]);
Output:
One
Two
Three
Four

You can use expression-bodied syntax for get and set from C# 7 onwards.

Example: Indexer
class StringDataStore
{
    private string[] strArr = new string[10]; // internal data storage

    public string this[int index]
    {
        get => strArr[index];

        set => strArr[index] = value;
    }
}
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Generic Indexer

Indexer can also be generic. The following generic class includes generic indexer.

Example: Generic Indexer
class DataStore<T>
{
    private T[] store; 

    public DataStore()
    {
        store = new T[10];
    }

    public DataStore(int length)
    {
        store = new T[length];
    }

    public T this[int index]
    {
        get
        {
            if (index < 0 &&  index >= store.Length)
                throw new IndexOutOfRangeException("Index out of range");

                return store[index];
        }

        set
        {
            if (index < 0 ||  index >= store.Length)
                throw new IndexOutOfRangeException("Index out of range");

            store[index] = value;
        }
    }

    public int Length
    {
        get
        {
            return store.Length;
        }
    }
}

The above generic indexer can be used with any data type. The following example demonstrates the use of generic indexer.

Example:
DataStore<int> grades = new DataStore<int>();
grades[0] = 100;
grades[1] = 25;
grades[2] = 34;
grades[3] = 42;
grades[4] = 12;
grades[5] = 18;
grades[6] = 2;
grades[7] = 95;
grades[8] = 75;
grades[9] = 53;

for (int i = 0; i < grades.Length;i++)
    Console.WriteLine(grades[i]);

DataStore<string> names = new DataStore<string>(5);
names[0] = "Steve";
names[1] = "Bill";
names[2] = "James";
names[3] = "Ram";
names[4] = "Andy";

for (int i = 0; i < names.Length;i++)
    Console.WriteLine(names[i]);

Overload Indexer

You can be overloaded with the different data types for index. The following example overloads an indexer with int type index as well as string type index.

Example: Override Indexer
class StringDataStore
{
    private string[] strArr = new string[10]; // internal data storage

    // int type indexer
    public string this[int index]
    {
        get
        {
            if (index < 0 || index >= strArr.Length)
                throw new IndexOutOfRangeException("Index out of range");

            return strArr[index];
        }

        set
        {
            if (index < 0 || index >= strArr.Length)
                throw new IndexOutOfRangeException("Index out of range");

            strArr[index] = value;
        }
    }

    // string type indexer
    public string this[string name]
    {
        get
        {
            foreach (string str in strArr){
                if(str.ToLower() == name.ToLower())        
                    return str;
                }
                    
            return null;
        }
    }
}

class Program
{
    static void Main(string[] args)
    {
        StringDataStore strStore = new StringDataStore();

        strStore[0] = "One";
        strStore[1] = "Two";
        strStore[2] = "Three";
        strStore[3] = "Four";
        
        Console.WriteLine(strStore["one"]);
        Console.WriteLine(strStore["two"]);
        Console.WriteLine(strStore["Three"]);
        Console.WriteLine(strStore["Four"]);
    }
}
Note:
Indexer does not allow ref and out paramters.
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