Array.BinarySearch Method (Array, Object, IComparer)
Searches an entire onedimensional sorted array for a value using the specified IComparer interface.
Assembly: mscorlib (in mscorlib.dll)
Parameters
 array

Type:
System.Array
The sorted onedimensional Array to search.
 value

Type:
System.Object
The object to search for.
 comparer

Type:
System.Collections.IComparer
The IComparer implementation to use when comparing elements.
or
null to use the IComparable implementation of each element.
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.
Exception  Condition 

ArgumentNullException  array is null. 
RankException  array is multidimensional. 
ArgumentException  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 interface, and the search encounters an element that does not implement the IComparable 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 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 System.Collections.CaseInsensitiveComparer as the comparer to perform caseinsensitive 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.
Ifcomparer 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.
Note 

If comparer is null and value does not implement the IComparable interface, the elements of array are not tested for IComparable before the search begins. An exception is thrown if the search encounters an element that does not implement IComparable. 
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 implementation, even if value is null. That is, the IComparable implementation determines how a given element compares to null. 
This method is an O(log n) operation, where n is the Length of array.
Available since 8
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1