ArrayList.IndexOf Method

Definition

Returns the zero-based index of the first occurrence of a value in the ArrayList or in a portion of it.

Overloads

IndexOf(Object)

Searches for the specified Object and returns the zero-based index of the first occurrence within the entire ArrayList.

IndexOf(Object, Int32)

Searches for the specified Object and returns the zero-based index of the first occurrence within the range of elements in the ArrayList that extends from the specified index to the last element.

IndexOf(Object, Int32, Int32)

Searches for the specified Object and returns the zero-based index of the first occurrence within the range of elements in the ArrayList that starts at the specified index and contains the specified number of elements.

IndexOf(Object)

Source:
ArrayList.cs
Source:
ArrayList.cs
Source:
ArrayList.cs

Searches for the specified Object and returns the zero-based index of the first occurrence within the entire ArrayList.

C#
public virtual int IndexOf(object value);
C#
public virtual int IndexOf(object? value);

Parameters

value
Object

The Object to locate in the ArrayList. The value can be null.

Returns

The zero-based index of the first occurrence of value within the entire ArrayList, if found; otherwise, -1.

Implements

Examples

The following code example shows how to determine the index of the first occurrence of a specified element.

C#
using System;
using System.Collections;
public class SamplesArrayList
{

    public static void Main()
    {

        // Creates and initializes a new ArrayList with three elements of the same value.
        ArrayList myAL = new ArrayList();
        myAL.Add( "the" );
        myAL.Add( "quick" );
        myAL.Add( "brown" );
        myAL.Add( "fox" );
        myAL.Add( "jumps" );
        myAL.Add( "over" );
        myAL.Add( "the" );
        myAL.Add( "lazy" );
        myAL.Add( "dog" );
        myAL.Add( "in" );
        myAL.Add( "the" );
        myAL.Add( "barn" );

        // Displays the values of the ArrayList.
        Console.WriteLine( "The ArrayList contains the following values:" );
        PrintIndexAndValues( myAL );

        // Search for the first occurrence of the duplicated value.
        string myString = "the";
        int myIndex = myAL.IndexOf( myString );
        Console.WriteLine( "The first occurrence of \"{0}\" is at index {1}.", myString, myIndex );

        // Search for the first occurrence of the duplicated value in the last section of the ArrayList.
        myIndex = myAL.IndexOf( myString, 4 );
        Console.WriteLine( "The first occurrence of \"{0}\" between index 4 and the end is at index {1}.", myString, myIndex );

        // Search for the first occurrence of the duplicated value in a section of the ArrayList.
        myIndex = myAL.IndexOf( myString, 6, 6 );
        Console.WriteLine( "The first occurrence of \"{0}\" between index 6 and index 11 is at index {1}.", myString, myIndex );

        // Search for the first occurrence of the duplicated value in a small section at the end of the ArrayList.
        myIndex = myAL.IndexOf( myString, 11 );
        Console.WriteLine( "The first occurrence of \"{0}\" between index 11 and the end is at index {1}.", myString, myIndex );
    }

    public static void PrintIndexAndValues(IEnumerable myList)
    {
        int i = 0;
        foreach (Object obj in myList)
            Console.WriteLine("   [{0}]:    {1}", i++, obj);
        Console.WriteLine();
    }
}
/*
This code produces output similar to the following:

The ArrayList contains the following values:
   [0]:    the
   [1]:    quick
   [2]:    brown
   [3]:    fox
   [4]:    jumps
   [5]:    over
   [6]:    the
   [7]:    lazy
   [8]:    dog
   [9]:    in
   [10]:    the
   [11]:    barn

The first occurrence of "the" is at index 0.
The first occurrence of "the" between index 4 and the end is at index 6.
The first occurrence of "the" between index 6 and index 11 is at index 6.
The first occurrence of "the" between index 11 and the end is at index -1.
*/

Remarks

The ArrayList is searched forward starting at the first element and ending at the last element.

This method performs a linear search; therefore, this method is an O(n) operation, where n is Count.

This method determines equality by calling Object.Equals.

Starting with the .NET Framework 2.0, this method uses the collection's objects' Equals and CompareTo methods on item to determine whether item exists. In the earlier versions of the .NET Framework, this determination was made by using the Equals and CompareTo methods of the item parameter on the objects in the collection.

See also

Applies to

.NET 9 and other versions
Product Versions
.NET Core 1.0, Core 1.1, Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7, 8, 9
.NET Framework 1.1, 2.0, 3.0, 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8, 4.8.1
.NET Standard 2.0, 2.1
UWP 10.0

IndexOf(Object, Int32)

Source:
ArrayList.cs
Source:
ArrayList.cs
Source:
ArrayList.cs

Searches for the specified Object and returns the zero-based index of the first occurrence within the range of elements in the ArrayList that extends from the specified index to the last element.

C#
public virtual int IndexOf(object value, int startIndex);
C#
public virtual int IndexOf(object? value, int startIndex);

Parameters

value
Object

The Object to locate in the ArrayList. The value can be null.

startIndex
Int32

The zero-based starting index of the search. 0 (zero) is valid in an empty list.

Returns

The zero-based index of the first occurrence of value within the range of elements in the ArrayList that extends from startIndex to the last element, if found; otherwise, -1.

Exceptions

startIndex is outside the range of valid indexes for the ArrayList.

Examples

The following code example shows how to determine the index of the first occurrence of a specified element.

C#
using System;
using System.Collections;
public class SamplesArrayList
{

    public static void Main()
    {

        // Creates and initializes a new ArrayList with three elements of the same value.
        ArrayList myAL = new ArrayList();
        myAL.Add( "the" );
        myAL.Add( "quick" );
        myAL.Add( "brown" );
        myAL.Add( "fox" );
        myAL.Add( "jumps" );
        myAL.Add( "over" );
        myAL.Add( "the" );
        myAL.Add( "lazy" );
        myAL.Add( "dog" );
        myAL.Add( "in" );
        myAL.Add( "the" );
        myAL.Add( "barn" );

        // Displays the values of the ArrayList.
        Console.WriteLine( "The ArrayList contains the following values:" );
        PrintIndexAndValues( myAL );

        // Search for the first occurrence of the duplicated value.
        string myString = "the";
        int myIndex = myAL.IndexOf( myString );
        Console.WriteLine( "The first occurrence of \"{0}\" is at index {1}.", myString, myIndex );

        // Search for the first occurrence of the duplicated value in the last section of the ArrayList.
        myIndex = myAL.IndexOf( myString, 4 );
        Console.WriteLine( "The first occurrence of \"{0}\" between index 4 and the end is at index {1}.", myString, myIndex );

        // Search for the first occurrence of the duplicated value in a section of the ArrayList.
        myIndex = myAL.IndexOf( myString, 6, 6 );
        Console.WriteLine( "The first occurrence of \"{0}\" between index 6 and index 11 is at index {1}.", myString, myIndex );

        // Search for the first occurrence of the duplicated value in a small section at the end of the ArrayList.
        myIndex = myAL.IndexOf( myString, 11 );
        Console.WriteLine( "The first occurrence of \"{0}\" between index 11 and the end is at index {1}.", myString, myIndex );
    }

    public static void PrintIndexAndValues(IEnumerable myList)
    {
        int i = 0;
        foreach (Object obj in myList)
            Console.WriteLine("   [{0}]:    {1}", i++, obj);
        Console.WriteLine();
    }
}
/*
This code produces output similar to the following:

The ArrayList contains the following values:
   [0]:    the
   [1]:    quick
   [2]:    brown
   [3]:    fox
   [4]:    jumps
   [5]:    over
   [6]:    the
   [7]:    lazy
   [8]:    dog
   [9]:    in
   [10]:    the
   [11]:    barn

The first occurrence of "the" is at index 0.
The first occurrence of "the" between index 4 and the end is at index 6.
The first occurrence of "the" between index 6 and index 11 is at index 6.
The first occurrence of "the" between index 11 and the end is at index -1.
*/

Remarks

The ArrayList is searched forward starting at startIndex and ending at the last element.

This method performs a linear search; therefore, this method is an O(n) operation, where n is the number of elements from startIndex to the end of the ArrayList.

This method determines equality by calling Object.Equals.

Starting with the .NET Framework 2.0, this method uses the collection's objects' Equals and CompareTo methods on item to determine whether item exists. In the earlier versions of the .NET Framework, this determination was made by using the Equals and CompareTo methods of the item parameter on the objects in the collection.

See also

Applies to

.NET 9 and other versions
Product Versions
.NET Core 1.0, Core 1.1, Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7, 8, 9
.NET Framework 1.1, 2.0, 3.0, 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8, 4.8.1
.NET Standard 2.0, 2.1
UWP 10.0

IndexOf(Object, Int32, Int32)

Source:
ArrayList.cs
Source:
ArrayList.cs
Source:
ArrayList.cs

Searches for the specified Object and returns the zero-based index of the first occurrence within the range of elements in the ArrayList that starts at the specified index and contains the specified number of elements.

C#
public virtual int IndexOf(object value, int startIndex, int count);
C#
public virtual int IndexOf(object? value, int startIndex, int count);

Parameters

value
Object

The Object to locate in the ArrayList. The value can be null.

startIndex
Int32

The zero-based starting index of the search. 0 (zero) is valid in an empty list.

count
Int32

The number of elements in the section to search.

Returns

The zero-based index of the first occurrence of value within the range of elements in the ArrayList that starts at startIndex and contains count number of elements, if found; otherwise, -1.

Exceptions

startIndex is outside the range of valid indexes for the ArrayList.

-or-

count is less than zero.

-or-

startIndex and count do not specify a valid section in the ArrayList.

Examples

The following code example shows how to determine the index of the first occurrence of a specified element.

C#
using System;
using System.Collections;
public class SamplesArrayList
{

    public static void Main()
    {

        // Creates and initializes a new ArrayList with three elements of the same value.
        ArrayList myAL = new ArrayList();
        myAL.Add( "the" );
        myAL.Add( "quick" );
        myAL.Add( "brown" );
        myAL.Add( "fox" );
        myAL.Add( "jumps" );
        myAL.Add( "over" );
        myAL.Add( "the" );
        myAL.Add( "lazy" );
        myAL.Add( "dog" );
        myAL.Add( "in" );
        myAL.Add( "the" );
        myAL.Add( "barn" );

        // Displays the values of the ArrayList.
        Console.WriteLine( "The ArrayList contains the following values:" );
        PrintIndexAndValues( myAL );

        // Search for the first occurrence of the duplicated value.
        string myString = "the";
        int myIndex = myAL.IndexOf( myString );
        Console.WriteLine( "The first occurrence of \"{0}\" is at index {1}.", myString, myIndex );

        // Search for the first occurrence of the duplicated value in the last section of the ArrayList.
        myIndex = myAL.IndexOf( myString, 4 );
        Console.WriteLine( "The first occurrence of \"{0}\" between index 4 and the end is at index {1}.", myString, myIndex );

        // Search for the first occurrence of the duplicated value in a section of the ArrayList.
        myIndex = myAL.IndexOf( myString, 6, 6 );
        Console.WriteLine( "The first occurrence of \"{0}\" between index 6 and index 11 is at index {1}.", myString, myIndex );

        // Search for the first occurrence of the duplicated value in a small section at the end of the ArrayList.
        myIndex = myAL.IndexOf( myString, 11 );
        Console.WriteLine( "The first occurrence of \"{0}\" between index 11 and the end is at index {1}.", myString, myIndex );
    }

    public static void PrintIndexAndValues(IEnumerable myList)
    {
        int i = 0;
        foreach (Object obj in myList)
            Console.WriteLine("   [{0}]:    {1}", i++, obj);
        Console.WriteLine();
    }
}
/*
This code produces output similar to the following:

The ArrayList contains the following values:
   [0]:    the
   [1]:    quick
   [2]:    brown
   [3]:    fox
   [4]:    jumps
   [5]:    over
   [6]:    the
   [7]:    lazy
   [8]:    dog
   [9]:    in
   [10]:    the
   [11]:    barn

The first occurrence of "the" is at index 0.
The first occurrence of "the" between index 4 and the end is at index 6.
The first occurrence of "the" between index 6 and index 11 is at index 6.
The first occurrence of "the" between index 11 and the end is at index -1.
*/

Remarks

The ArrayList is searched forward starting at startIndex and ending at startIndex plus count minus 1, if count is greater than 0.

This method performs a linear search; therefore, this method is an O(n) operation, where n is count.

This method determines equality by calling Object.Equals.

Starting with the .NET Framework 2.0, this method uses the collection's objects' Equals and CompareTo methods on item to determine whether item exists. In the earlier versions of the .NET Framework, this determination was made by using the Equals and CompareTo methods of the item parameter on the objects in the collection.

See also

Applies to

.NET 9 and other versions
Product Versions
.NET Core 1.0, Core 1.1, Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7, 8, 9
.NET Framework 1.1, 2.0, 3.0, 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8, 4.8.1
.NET Standard 2.0, 2.1
UWP 10.0