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ArrayList.BinarySearch Method (Object, IComparer)

Searches the entire sorted ArrayList for an element using the specified comparer and returns the zero-based index of the element.

Namespace:  System.Collections
Assembly:  mscorlib (in mscorlib.dll)

public virtual int BinarySearch(
	Object value,
	IComparer comparer


Type: System.Object
The Object to locate. The value can be null.
Type: System.Collections.IComparer
The IComparer implementation to use when comparing elements.
null to use the default comparer that is the IComparable implementation of each element.

Return Value

Type: System.Int32
The zero-based index of value in the sorted ArrayList, if value is found; otherwise, a negative number, which is the bitwise complement of the index of the next element that is larger than value or, if there is no larger element, the bitwise complement of Count.


comparer is null and neither value nor the elements of ArrayList implement the IComparable interface.


comparer is null and value is not of the same type as the elements of the ArrayList.

The comparer customizes how the elements are compared. For example, you can use a CaseInsensitiveComparer instance as the comparer to perform case-insensitive string searches.

If comparer is provided, the elements of the ArrayList are compared to the specified value using the specified IComparer implementation. The elements of the ArrayList must already be sorted in increasing value according to the sort order defined by comparer; otherwise, the result might be incorrect.

If comparer is null, the comparison is done using the IComparable implementation provided by the element itself or by the specified value. The elements of the ArrayList must already be sorted in increasing value according to the sort order defined by the IComparable implementation; otherwise, the result might be incorrect.

Comparing null with any type is allowed and does not generate an exception when using IComparable. When sorting, null is considered to be less than any other object.

If the ArrayList contains more than one element with the same value, the method returns only one of the occurrences, and it might return any one of the occurrences, not necessarily the first one.

If the ArrayList does not contain the specified value, the method returns a negative integer. You can apply the bitwise complement operation (~) to this negative integer to get the index of the first element that is larger than the search value. When inserting the value into the ArrayList, this index should be used as the insertion point to maintain the sort order.

This method is an O(log n) operation, where n is Count.

The following example creates an ArrayList of colored animals. The provided IComparer performs the string comparison for the binary search. The results of both an iterative search and a binary search are displayed.

using System;
using System.Collections;

public class SimpleStringComparer : IComparer
    int IComparer.Compare(object x, object y)
        string cmpstr = (string)x;
        return cmpstr.CompareTo((string)y);

public class MyArrayList : ArrayList
    public static void Main()
        // Creates and initializes a new ArrayList.
        MyArrayList coloredAnimals = new MyArrayList();

        coloredAnimals.Add("White Tiger");
        coloredAnimals.Add("Pink Bunny");
        coloredAnimals.Add("Red Dragon");
        coloredAnimals.Add("Green Frog");
        coloredAnimals.Add("Blue Whale");
        coloredAnimals.Add("Black Cat");
        coloredAnimals.Add("Yellow Lion");

        // BinarySearch requires a sorted ArrayList.

        // Compare results of an iterative search with a binary search
        int index = coloredAnimals.IterativeSearch("White Tiger");
        Console.WriteLine("Iterative search, item found at index: {0}", index);

        index = coloredAnimals.BinarySearch("White Tiger", new SimpleStringComparer());
        Console.WriteLine("Binary search, item found at index:    {0}", index);

    public int IterativeSearch(object finditem)
        int index = -1;

        for (int i = 0; i < this.Count; i++)
            if (finditem.Equals(this[i]))
                index = i;
        return index;
// This code produces the following output.
// Iterative search, item found at index: 5
// Binary search, item found at index:    5

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.