1. Collection Classes Deep Dive
By Gary Short
Head of Gibraltar Labs
Gibraltar Software
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2. Introduction
• Gary Short
• Head of Gibraltar Labs
• C# MVP
• @garyshort
• gary.short@gibraltarsoftare.com
• facebook.com/TheOtherGaryShort
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3. Why do we Care About This Stuff?
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4. 4

5. Let’s Start With Something We Know
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6. List<T> Demo
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7. What we Learned
• Don’t add elements in a loop
• Add causes capacity growths
• Capacity growths uses Array.Copy()
• Array.Copy() is a O(n) operation
• O(n) is sloooooowwwwwww. 
• Use AddRange() instead
• Or set “large enough” initial capacity.
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8. How Slow is Slow?
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9. Performance: Add Versus AddRange
30000
25000
20000
Number of Ticks
15000
Add
AddRange
10000
5000
0
10 100 1000 10000 100000 1000000
Number of Elements Added

10. What About Removing Stuff?
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11. Demo
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12. What we Learned
Prefer RemoveAt() as there’s no IndexOf() step
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13. List<T> - Sorting
• Uses QuickSort under the hood
• Fastest general purpose sort algorithm
• O(n log n) in best case
• O(n log n) in average case
• Though worst case is O(n^2) 
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14. Performance: O(n log n) Vs O(n^2)
120
100
80
Effort
60
O(n log n)
O(n^2)
40
20
0
1 2 3 4 5 6 7 8 9 10
Elements to be Sorted

15. QuickSort Demo
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16. So What is the Worst Case?
• If the list is already sorted
– First partition has lower = 0, upper = n
– Then calls Partition(n-1);
– This happens a further n-2 times
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18. Can we Mitigate the Worst Case?
• Median of Three
– Take an element from the “top” of the array
– Take an element from the “middle” of the array
– Take an element from the “bottom” of the array
– Find the median value of the three
– Pivot on the median
• Let’s see if Microsoft uses this algorithm.
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19. Disadvantage: O(n) Add, Insert, Remove
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20. What if we Need Fast Add, Insert & Remove?
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21. LinkedList<T>
• Double linked
– Each item points to the previous and next items
– This means it’s super fast
• Add, insert and remove are all O(1) operations
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22. Demo
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23. Disadvantage: O(n) lookups
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24. What if we Need Fast Lookups?
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25. Dictionary<TKey, TValue>
• Performance depends on key.GetHashCode()
– Hash codes must be evenly distributed across int
• If two keys return hashes that give the same index
– Dictionary must look for nearest free location to store item
– Must search later to return the item
– This hurts performance
– Use your own type, then this is on you. 
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26. Dictionary<TKey, TValue>
• Objects used as keys must also implement
IEquatable.Equals()
• Or override Equals()
• Why?
– Different keys may return the same hashcode
– Equals() is used by the dictionary comparing keys
– So you must ensure the following
• If A.Equals(B) then A.HashCode() and B.HashCode() return
the same HashCode()
• Override Equals() but not GetHashCode() == compile error.
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27. Disadvantage: one value per key
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28. What if I Need Multiple Values per Key?
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29. Lookup<TKey, TElement> Demo
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30. Concurrent Collections
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31. Types of Concurrent Collections
• ConcurrentBag<T>
• ConcurrentDictionary<T>
• ConcurrentQueue<T>
• ConcurrentStack<T>
• OrderablePartitioner<T>
• BlockingCollection<T>.
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32. Key Characteristics
• New .Net 4.0
• Guards against multi-thread collection conflicts
• Implements IProducerConsumerCollections<T>
– TryAdd()
• Tries to add item to collection returns success bool
– TryTake()
• Tries to remove and return item returns success bool
– Returns the item in an out param.
• Always check the return value before moving on.
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33. Do I Have To Check Every Time?!
• BlockingCollection<T>
– Blocks and waits until task completes
– Uses Add() and Take() methods
• Block the thread and wait until task completes
• Add() has an overload to pass a CancellationToken
• Add() may also block if bounding capacity was used.
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34. But I Don’t Want it to Wait For Ever!
• So we don’t want to wait forever
• Nor do we want to cancel the Add() from
outside
• TryAdd() and TryTake() are offered too
• Where you can specify a timeout.
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35. Summary
• List is a good general purpose collection
– Construct to size if possible
– Construct to upper threshold then trim
– Prefer AddRange() over Add()
– Be aware of “Quicksort Killers”
• Use LinkedList if you need fast insert/remove
• Use Dictionary if you need fast lookup
• Use Lookup if you need multi values
• Use concurrent collections for thread safety. 35

36. Questions
• gary.short@gibraltarsoftware.com
• @garyshort
• Facebook.com/TheOtherGaryShort
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