C# | Getting an enumerator for a range of elements in the ArrayList

ArrayList.GetEnumerator(Int32, Int32) method is used to get an enumerator for a range of elements in the ArrayList.

Syntax:

public virtual System.Collections.IEnumerator GetEnumerator (int index, int count);

Parameters:

index: It is the zero-based starting index of type Int32 of the ArrayList section that the enumerator should refer to.

count: It is the number of elements of the type Int32 in the ArrayList section that the enumerator should refer to.

Return Value: This method returns an IEnumerator for the specified range of elements in the ArrayList.

Exceptions:

• ArgumentOutOfRangeException: If the index or count is less than zero.
• ArgumentException: If the index and count do not specify a valid range in the ArrayList.

Below programs illustrate the use of above-discussed method:

Example 1:

// C# code to get an enumerator for a
// range of elements in the ArrayList
using System;
using System.Collections;

class GFG {

    // Driver code
    public static void Main()
    {

        // Creating an ArrayList
        ArrayList myList = new ArrayList();

        // adding elements in myList
        myList.Add(14);
        myList.Add(45);
        myList.Add(78);
        myList.Add(48);
        myList.Add(49);
        myList.Add(51);
        myList.Add(77);

        // To get an Enumerator
        // for the ArrayList
        IEnumerator enumerator = myList.GetEnumerator();

        // If MoveNext passes the end of the
        // collection, the enumerator is positioned
        // after the last element in the ArrayList
        // and MoveNext returns false.
        while (enumerator.MoveNext()) {

            Console.WriteLine(enumerator.Current);
        }

        Console.WriteLine("After GetEnumerator(Int32, Int32) Method: ");

        // To get an Enumerator for a range
        // of elements of the ArrayList
        // here index is 3 and count is 2
        IEnumerator e = myList.GetEnumerator(3, 2);

        // If MoveNext passes the end of the
        // collection, the enumerator is positioned
        // after the last element in the ArrayList
        // and MoveNext returns false.
        while (e.MoveNext()) {
            Object obj1 = e.Current;
            Console.WriteLine(obj1);
        }
    }
}

Output:

14
45
78
48
49
51
77
After GetEnumerator(Int32, Int32) Method: 
48
49

Example 2:

// C# code to get an enumerator for a
// range of elements in the ArrayList
using System;
using System.Collections;

class GFG {

    // Driver code
    public static void Main()
    {

        // Creating an ArrayList
        ArrayList myList = new ArrayList();

        // adding elements in myList
        myList.Add("C");
        myList.Add("C++");
        myList.Add("Java");
        myList.Add("Python");
        myList.Add("C#");
        myList.Add("HTML");
        myList.Add("CSS");

        Console.WriteLine("After GetEnumerator(Int32, Int32) Method: ");

        // To get an Enumerator for a range
        // of elements of the ArrayList
        // this will give error as index is
        // less than zero
        IEnumerator e = myList.GetEnumerator(-1, 2);

        // If MoveNext passes the end of the
        // collection, the enumerator is positioned
        // after the last element in the ArrayList
        // and MoveNext returns false.
        while (e.MoveNext()) {
            Object obj1 = e.Current;
            Console.WriteLine(obj1);
        }
    }
}

Runtime Error:

Unhandled Exception:
System.ArgumentOutOfRangeException: Non-negative number required.
Parameter name: index

Note:

• The foreach statement of the C# language hides the complexity of the enumerators. Therefore, using foreach is recommended, instead of directly manipulating the enumerator.
• Enumerators can be used to read the data in the collection, but they cannot be used to modify the underlying collection.
• Current returns the same object until either MoveNext or Reset is called. MoveNext sets Current to the next element.
• An enumerator remains valid as long as the collection remains unchanged. If changes are made to the collection, such as adding, modifying, or deleting elements, the enumerator is irrecoverably invalidated and its behavior is undefined.
• This method is an O(1) operation.

Reference:

• https://docs.microsoft.com/en-us/dotnet/api/system.collections.arraylist.getenumerator?view=netframework-4.7.2