# Array.BinarySearch<T> Method (T[], Int32, Int32, T, IComparer<T>)

July 28, 2014

Searches a range of elements in a one-dimensional sorted Array for a value, using the specified IComparer<T> generic interface.

**Namespace:**System

**Assembly:**mscorlib (in mscorlib.dll)

public static int BinarySearch<T>( T[] array, int index, int length, T value, IComparer<T> comparer )

#### Type Parameters

- T
The type of the elements of the array.

#### Parameters

- array
- Type:
T
[]

The sorted one-dimensional, zero-based Array to search.

- index
- Type: System.Int32

The starting index of the range to search.

- length
- Type: System.Int32

The length of the range to search.

- value
- Type: T

The object to search for.

- comparer
- Type: System.Collections.Generic.IComparer<T>

The IComparer<T> implementation to use when comparing elements.

-or-

null to use the IComparable<T> implementation of each element.

#### Return Value

Type: System.Int32The index of the specified value in the specified array, if value is found. If value is not found and value is less than one or more elements in array, a negative number which is the bitwise complement of the index of the first element that is larger than value. If value is not found and value is greater than any of the elements in array, a negative number which is the bitwise complement of (the index of the last element plus 1).

Exception | Condition |
---|---|

ArgumentNullException | array is null. |

ArgumentOutOfRangeException | index is less than the lower bound of array. -or- length is less than zero. |

ArgumentException | index and length do not specify a valid range in array. -or- comparer is null, and value is of a type that is not compatible with the elements of array. |

InvalidOperationException | comparer is null, value does not implement the IComparable<T> generic interface, and the search encounters an element that does not implement the IComparable<T> generic interface. |

If the Array does not contain the specified value, the method returns a negative integer. You can apply the bitwise complement operator (~) to the negative result (in Visual Basic, Xor the negative result with -1) to produce an index. If this index is greater than or equal to the size 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.

If comparer is not null, the elements of array are compared to the specified value using the specified IComparer<T> generic interface 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<T> generic interface 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<T> implementation; otherwise, the result might be incorrect.

Note: |
---|

If comparer is null and value does not implement the IComparable<T> generic interface, the elements of array are not tested for IComparable<T> before the search begins. An exception is thrown if the search encounters an element that does not implement IComparable<T>. |

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<T>. When sorting, null is considered to be less than any other object.

Note: |
---|

For every element tested, value is passed to the appropriate IComparable<T> implementation, even if value is null. That is, the IComparable<T> implementation determines how a given element compares to null. |

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