List<'T>.BinarySearch Method ('T)


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Searches the entire sorted List<'T> for an element using the default comparer and returns the zero-based index of the element.

Namespace:   System.Collections.Generic
Assembly:  mscorlib (in mscorlib.dll)

member BinarySearch : 
        item:'T -> int


Type: T

The object to locate. The value can be null for reference types.

Return Value

Type: System.Int32

The zero-based index of item in the sorted List<'T>, if item is found; otherwise, a negative number that is the bitwise complement of the index of the next element that is larger than item or, if there is no larger element, the bitwise complement of Count.

Exception Condition

The default comparer Comparer<'T>.Default cannot find an implementation of the IComparable<'T> generic interface or the IComparable interface for type T.

This method uses the default comparer Comparer<'T>.Default for type T to determine the order of list elements. The Comparer<'T>.Default property checks whether type T implements the IComparable<'T> generic interface and uses that implementation, if available. If not, Comparer<'T>.Default checks whether type T implements the IComparable interface. If type T does not implement either interface, Comparer<'T>.Default throws an InvalidOperationException.

The List<'T> must already be sorted according to the comparer implementation; otherwise, the result is incorrect.

Comparing null with any reference type is allowed and does not generate an exception when using the IComparable<'T> generic interface. When sorting, null is considered to be less than any other object.

If the List<'T> contains more than one element with the same value, the method returns only one of the occurrences, and it might return any one of the occurrences, not necessarily the first one.

If the List<'T> does not contain the specified value, the method returns a negative integer. You can apply the bitwise complement operation (~) to this negative integer to get the index of the first element that is larger than the search value. When inserting the value into the List<'T>, this index should be used as the insertion point to maintain the sort order.

This method is an O(log n) operation, where n is the number of elements in the range.

The following example demonstrates the Sort() method overload and the BinarySearch('T) method overload. A List<'T> of strings is created and populated with four strings, in no particular order. The list is displayed, sorted, and displayed again.

The BinarySearch('T) method overload is then used to search for two strings that are not in the list, and the Insert method is used to insert them. The return value of the BinarySearch('T) method is negative in each case, because the strings are not in the list. Taking the bitwise complement (the ~ operator in C# and Visual C++, Xor -1 in Visual Basic) of this negative number produces the index of the first element in the list that is larger than the search string, and inserting at this location preserves the sort order. The second search string is larger than any element in the list, so the insertion position is at the end of the list.

No code example is currently available or this language may not be supported.

Universal Windows Platform
Available since 8
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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