Tutorial :Why is there a List.BinarySearch(…)?


I'm looking at List and I see a BinarySearch method with a few overloads, and I can't help wondering if it makes sense at all to have a method like that in List?

Why would I want to do a binary search unless the list was sorted? And if the list wasn't sorted, calling the method would just be a waste of CPU time. What's the point of having that method on List?


Sorting and searching are two very common operations on lists. It would be unfriendly to limit a developer's options by not offering binary search on a regular list.

Library design requires compromises - the .NET designers chose to offer the binary search function on both arrays an lists in C# because they likely felt (as I do) that these are useful and common operations, and programmers who choose to use them understand their prerequisites (namely that the list is ordered) before calling them.

It's easy enough to sort a List<T> using one of the Sort() overloads. If you feel that you need an invariant that gaurantees sorting, you can always use SortedList<TKey,TValue> or SortedSet<T> instead.


I note in addition to the other correct answers that binary search is surprisingly hard to write correctly. There are lots of corner cases and some tricky integer arithmetic. Since binary search is obviously a common operation on sorted lists, the BCL team did the world a service by writing the binary search algorithm correctly once rather than encouraging customers to all write their own binary search algorithm; a significant number of those customer-authored algorithms would be wrong.


BinarySearch only makes sense on a List<T> that is sorted, just like IList<T>.Add only makes sense for an IList<T> with IsReadOnly = false. It's messy, but it's just something to deal with: sometimes functionality X depends on criterion Y. The fact that Y isn't always true doesn't make X useless.

Now, in my opinion, it's frustrating that .NET doesn't have general Sort and BinarySearch methods for any IList<T> implementation (e.g., as extension methods). If it did, we could easily sort and search for items within any non-read-only collection providing random access.

Then again, you can always write your own (or copy someone else's).


Others have pointed out that BinarySearch is quite useful on a sorted List<T>. It doesn't really belong on List<T>, though, as anyone with C++ STL experience would immediately recognize.

With recent C# language developments, it makes more sense to define the notion of a sorted list (e.g., ISortedList<T> : IList<T>) and define BinarySearch (et. al.) as extension methods of that interface. This is a cleaner, more orthogonal type of design.

I've started doing just that as part of the Nito.Linq library. I expect the first stable release to be in a couple of months.


yes but List has Sort() method as well so you can call it before BinarySearch.


I agree it's completely dumb to Call BinarySearch on an unsorted list, but it's perfect if you know your large list is sorted.

I've used it when checking if items from a stream exist in a (more or less) static list of 100,000 items or more.

Binary Searching the list is ORDERS of magnitude faster than doing a list.Find, which is many orders of magnitude faster than a database look up.

I makes sense, and I'm glad it there (not that it would be rocket science to implement it if it wasn't).


Searching and sorting are algorithmic primitives. It's helpful for the standard library to have fast reliable implementations. Otherwise, developers waste time reinventing the wheel.

However, in the case of the .NET Framework, it's unfortunate that the specific choices of algorithms happens to make them less useful than they might be. In some cases, their behaviour is not defined:

  1. List<T>.BinarySearch If the List 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.

  2. List<T> This implementation performs an unstable sort; that is, if two elements are equal, their order might not be preserved. In contrast, a stable sort preserves the order of elements that are equal.

That's a shame, because there are deterministic algorithms that are just as fast, and these would be much more useful as building blocks. It's noteworthy that the binary search algorithms in Python, Ruby and Go all find the first matching element.


Perhaps another point is that an array could be equally as unsorted. So in theory, having a BinarySearch on an array could be invalid too.

However, as with all features of a higher level language, they need to be applied by someone with reason and understanding of the data, or they will fail. Sure, some cross-checks could be applied, and we could have a flag that said "IsSorted" and it would fail on binary search otherwise, but .....


Some pseudo code:

if List is sorted     use the BinarySearch method  else if List is not sorted and you think sorting it is "waste of CPU time"     use a different algorithm that is more suitable and efficient  

Note:If u also have question or solution just comment us below or mail us on toontricks1994@gmail.com
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