This method does not support searching arrays that contain negative indexes. array must be sorted before calling this method.

If the Array does not contain the specified value, the method returns a negative integer. You can apply the bitwise complement operator (~ in C#, Not in Visual Basic) to the negative result to produce an index. If this index is one greater than the upper bound of the array, there are no elements larger than value in the array. Otherwise, it is the index of the first element that is larger than value.

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

If comparer is not null, the elements of array are compared to the specified value using the specified IComparer implementation. The elements of array 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 array must already be sorted in increasing value according to the sort order defined by the IComparable implementation; otherwise, the result might be incorrect.

Duplicate elements are allowed. If the Array contains more than one element equal to value, the method returns the index of only one of the occurrences, and not necessarily the first one.

null can always be compared with any other reference type; therefore, comparisons with null do not generate an exception when using IComparable.

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

This example demonstrates the Array.BinarySearch(Array, object) method.

using System;
class BinarySearchExample {
  public static void Main() {    
    int[] intAry = { 0, 2, 4, 6, 8 };
    Console.WriteLine( "The indices and elements of the array are: ");
    for ( int i = 0; i < intAry.Length; i++ )
      Console.Write("[{0}]: {1, -5}", i, intAry[i]);
    Console.WriteLine();
    SearchFor( intAry, 3 );
    SearchFor( intAry, 6 );
    SearchFor( intAry, 9 );
  }
  public static void SearchFor( Array ar, Object value ) {
    int i = Array.BinarySearch( ar, 0, ar.Length, value, null );
    Console.WriteLine();
    if ( i > 0 ) {
      Console.Write( "The object searched for, {0}, was found ", value );
      Console.WriteLine( "at index {1}.", value, i );
    }
    else if ( ~i == ar.Length ) {
      Console.Write( "The object searched for, {0}, was ", value );
      Console.Write( "not found,\nand no object in the array had " );
      Console.WriteLine( "greater value. " );
    }
    else {
      Console.Write( "The object searched for, {0}, was ", value );
      Console.Write( "not found.\nThe next larger object is at " );
      Console.WriteLine( "index {0}.", ~i );
    }
  }
}
   

The output is