.NET data structures: ArrayList, List, HashTable, Dictionary, SortedList, SortedDictionary -- Speed, memory, and when to use each?


.NET has a lot of complex data structures. Unfortunately, some of them are quite similar, and I'm not always sure when to use one and when to use another. Most of my C# and Visual Basic books talk about them to a certain extent, but they never really go into any real detail.

What's the difference between Array, ArrayList, List, Hashtable, Dictionary, SortedList, and SortedDictionary?

Which ones are enumerable (IList -- can do 'foreach' loops)? Which ones use key/value pairs (IDict)?

What about memory footprint? Insertion speed? Retrieval speed?

Are there any other data structures worth mentioning?

I'm still searching for more details on memory usage and speed (Big-O notation).

5/18/2015 2:57:32 PM

Accepted Answer

Off the top of my head:

  • Array* - represents an old-school memory array - kind of like a alias for a normal type[] array. Can enumerate. Can't grow automatically. I would assume very fast insert and retrival speed.

  • ArrayList - automatically growing array. Adds more overhead. Can enum., probably slower than a normal array but still pretty fast. These are used a lot in .NET

  • List - one of my favs - can be used with generics, so you can have a strongly typed array, e.g. List<string>. Other than that, acts very much like ArrayList

  • Hashtable - plain old hashtable. O(1) to O(n) worst case. Can enumerate the value and keys properties, and do key/val pairs

  • Dictionary - same as above only strongly typed via generics, such as Dictionary<string, string>

  • SortedList - a sorted generic list. Slowed on insertion since it has to figure out where to put things. Can enum., probably the same on retrieval since it doesn't have to resort, but deletion will be slower than a plain old list.

I tend to use List and Dictionary all the time - once you start using them strongly typed with generics, its really hard to go back to the standard non-generic ones.

There are lots of other data structures too - there's KeyValuePair which you can use to do some interesting things, there's a SortedDictionary which can be useful as well.

2/23/2017 11:59:19 AM

If at all possible, use generics. This includes:

  • List instead of ArrayList
  • Dictionary instead of HashTable

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