Array.BinarySearch(T) Method (T, T)
Assembly: mscorlib (in mscorlib.dll)
The type of the elements of the array.
- Type: 
The sorted one-dimensional, zero-based Array to search.
The object to search for.
Return ValueType: System.Int32
The 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).
This method does not support searching arrays that contain negative indexes. array must be sorted before calling this method.
If 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.
a null reference (Nothing in Visual Basic) can always be compared with any other reference type; therefore, comparisons with a null reference (Nothing in Visual Basic) do not generate an exception.
For every element tested, value is passed to the appropriate IComparable(T) implementation, even if value is a null reference (Nothing in Visual Basic). That is, the IComparable(T) implementation determines how a given element compares to a null reference (Nothing in Visual Basic).
This method is an O(log n) operation, where n is the Length of array.
The following code example demonstrates the Sort(T)(T) generic method overload and the generic method overload. An array of strings is created, in no particular order.
The array is displayed, sorted, and displayed again. Arrays must be sorted in order to use the BinarySearch method.
The calls to the Sort and BinarySearch generic methods do not look any different from calls to their nongeneric counterparts, because Visual Basic, C#, and C++ infer the type of the generic type parameter from the type of the first argument. If you use the Ildasm.exe (IL Disassembler) to examine the Microsoft intermediate language (MSIL), you can see that the generic methods are being called.
The generic method overload is then used to search for two strings, one that is not in the array and one that is. The array and the return value of the BinarySearch method are passed to the ShowWhere generic method, which displays the index value if the string is found, and otherwise the elements the search string would fall between if it were in the array. The index is negative if the string is not in the array, so the ShowWhere method takes the bitwise complement (the ~ operator in C# and Visual C++, Xor -1 in Visual Basic) to obtain the index of the first element in the list that is larger than the search string.
.NET FrameworkSupported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0
.NET Framework Client ProfileSupported in: 4, 3.5 SP1
XNA FrameworkSupported in: 3.0, 2.0, 1.0
Portable Class LibrarySupported in: Portable Class Library
Supported in: Windows Phone 8.1
Supported in: Windows Phone Silverlight 8.1
Supported in: Windows Phone Silverlight 8