C# .NET – Collections Comparison


I liked a James Michael Hare’s post “C#/.NET Fundamentals: Choosing the Right Collection Class“, and I like to re-blog the comparison table:

Collection Ordering Contiguous Storage? Direct Access? Lookup Efficiency ManipulateEfficiency Notes
Dictionary Unordered Yes Via Key Key:O(1) O(1) Best for high performance lookups.
SortedDictionary Sorted No Via Key Key: O(log n) O(log n) Compromise of Dictionary speed and ordering, uses binarysearch tree.
SortedList Sorted Yes Via Key Key:O(log n) O(n) Very similar toSortedDictionary, except tree is implemented in an array, so has faster lookup on preloaded data, but slower loads.
List User has precise control over element ordering Yes Via Index Index: O(1)
Value: O(n)
O(n) Best for smaller lists where direct access required and no sorting.
LinkedList User has precise control over element ordering No No Value:O(n) O(1) Best for lists where inserting/deleting in middle is common and no direct access required.
HashSet Unordered Yes Via Key Key:O(1) O(1) Unique unordered collection, like a Dictionary except key and value are same object.
SortedSet Sorted No Via Key Key:O(log n) O(log n) Unique sorted collection, like SortedDictionary except key and value are same object.
Stack LIFO Yes Only Top Top: O(1) O(1)* Essentially same as List except only process as LIFO
Queue FIFO Yes Only Front Front: O(1) O(1) Essentially same as List except only process as FIFO

Linq Take Random – Query to Get a Random Sub Collection – Random Order

While writing a load test today, I had to simulate different kind of customers with different kind of taste, and I had a collection of item ids, and to simulate a random distribution, I had to generate a sub collection taking random ids from the collection source, in that way I can simulate different kind of customers that will end-up querying the server requesting different item details.

Googling a little-bit I found an easy way to do this, and is to Order the collection by a different Guid for each row:

var result = collection.OrderBy(t => Guid.NewGuid());

and to get (take) 10 random elements:

var result = collection.OrderBy(t => Guid.NewGuid()).Take(10);

From the Scott Mitchell’s post, you can see some empirical tests that proves a nice random distribution. Of course this is not to use in production code where you absolutely need an unpredictable results, but for load testing, and other take-it-easy purpose, that’s cool.

This is a LINQPad example:

void Main()
	var collection = Enumerable.Range(1, 100).ToList();
	for (int i = 0; i < 3; i++)
		collection.OrderBy(t => Guid.NewGuid()).Take(10).Dump();