Array.BinarySearch<T> Method (T[], Int32, Int32, T)
Searches a range of elements in a onedimensional sorted array for a value, using the IComparable<T> generic interface implemented by each element of the Array and by the specified value.
Assembly: mscorlib (in mscorlib.dll)
Parameters
 array

Type:
T[]
The sorted onedimensional, zerobased 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.
Return Value
Type: System.Int32The index of the specified value in the specified array, if value is found; otherwise, a negative number. If value is not found and value is less than one or more elements in array, the negative number returned 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 all elements in array, the negative number returned is the bitwise complement of (the index of the last element plus 1). If this method is called with a nonsorted array, the return value can be incorrect and a negative number could be returned, even if value is present in array.
Type Parameters
 T
The type of the elements of the array.
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 value is of a type that is not compatible with the elements of array. 
InvalidOperationException  T does not implement the IComparable<T> generic interface. 
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 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.
T must implement the IComparable<T> generic interface, which is used for comparisons. 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.
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.
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.
Available since 8
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1