Should I be concerned about .NET dictionary speed?

Question

Hello, I will be creating a project that will use dictionary lookups and inserts quite a bit. Is this something to be concerned about?

Also, if I do benchmarking and such and it is really bad, then what is the best way of replacing dictionary with something else? Would using an array with "hashed" keys even be faster? That wouldn't help on insert time though will it?

Also, I don't think I'm micro-optimizing because this really will be a significant part of code on a production server, so if this takes an extra 100ms to complete, then we will be looking for new ways to handle this.

Answer 1

Have a look at C# HybridDictionary Usage

HybridDictionary Class

This class is recommended for cases where the number of elements in a dictionary is unknown. It takes advantage of the improved performance of a ListDictionary with small collections, and offers the flexibility of switching to a Hashtable which handles larger collections better than ListDictionary

Answer 2

I would do a benchmark of the Dictionary, HashTable (HashSet in .NET), and perhaps a home grown class, and see which works out best under your typical usage conditions.

Normally I would say its fine (insert StackOverflow's favorite premature ejaculation quote here), but if this is a core peice of the application, Benchmark, Benchmark, Benchmark.

Answer 3

1. C# Doesn't have any dictionaries. It's a programming language. You mean ".NET Dictionaries".
2. You are micro-optimizing. Do you even have working code yet? You have no idea where the bottlenecks will be, and already you're focused on Dictionary.
3. How do you know how the Dictionary class is implemented? Maybe it already uses an array with hashed keys!

Answer 4

The Dictionary<TKey, TValue> class is actually implemented as a hash table which makes lookups very fast (close to O(1)). See the API documentation for more information. I doubt you could make a better implementation yourself.

Answer 5

You may want to look at the KeyedCollection class in System.ObjectModel. From the MSDN description, "provides the abstract base class for a collection whose keys are embedded in the values."

Answer 6

Wait and see if the performance of your application is below expectations
If it is then use a profiler to determine if the Dictionary lookup is the source of the problem
If it is then do some tests with representative data to see if another choice of list would be quicker.

In short - no, in general you shouldn't worry about the performance of implementation details until after you have a problem.

Answer 7

If your application is multithreaded then the key part of performance is going to be synchronizing this Dictionary correctly.

If it is single-threaded then almost certainly bottleneck will be elsewhere. Such as reading these objects from wherever you are reading them.

Answer 8

The only concern that I can think of is that the speed of the dictionary relies on the key class having a reasonably fast GetHashCode method. Lookups and inserts are really fast, so you shouldn't have any problem there.

Regarding using an array, that's what the Dictionary class does already. Actually it uses two arrays, one for the keys and one for the values.

If you would have any performance problems with a Dictionary, it would be quite easy to make a wrapper for any kind of storage, that has the same methods and behaviour as a Dictionary so that you can replace it seamlessly.

Answer 9

Hello, I will be creating a project that will use dictionary lookups and inserts quite a bit. Is this something to be concerned about?

Yes. It is always wise to consider performance factors up front.

The form that your concern should take is as follows: your concern should be encouraging you to write realistic, user-focused performance specifications. It should be encouraging you to start writing performance tests early, and running them often, so that you can see how every single change to the product affects performance. That way you will be informed immediately when a code change causes a user-affecting change in performance. And it should be encouraging you to run profiles often, so that you are reasoning about performance based on empirical measurements, rather than random guesses and hunches.

Also, if I do benchmarking and such and it is really bad, then what is the best way of replacing dictionary with something else?

The best way to do this is to build a reasonable abstraction layer. If you have a class (or interface) which represents the "insert" and "lookup" abstract data type, then you can replace its internals without changing any of the callers.

Note that adding a layer of abstraction itself has a performance cost. If your profiling shows that the abstraction layer is too expensive, if the extra couple nanoseconds per call is too much, then you might have to get rid of the abstraction layer. Again, this decision will be driven by real-world performance data.

Would using an array with "hashed" keys even be faster? That wouldn't help on insert time though will it?

Neither you nor anyone reading this can possibly know which one is faster until you write it both ways and then benchmark it both ways under real-world conditions. Doing it under "lab" conditions will skew your results; you'll need to understand how things work when the GC is under realistic memory pressure, and so on. You might as well ask us which horse will run faster in next year's Kentucky Derby. If we knew the answer just by looking at the racing form, we'd all be rich already. You can't possibly expect anyone to know which of two entirely hypothetical, unwritten pieces of code will be faster under unspecified conditions!

Answer 10

You may consider using the C5 library. I've found it to be very fast and thoughtfully designed. Others on stackoverflow have found the same. With C5 you have the option of using general type interfaces (with a captial I), or directly the data structures underneath. Naturally the interfaces allow you to swap out different implementations, but I have found in performance testing that the interfaces will cost you.

Paul

Answer 11

I'm not sure that anyone has really answered this part yet:

Also, if I do benchmarking and such and it is really bad, then what is the best way of replacing dictionary with something else?

For this, wherever possible, declare your variables as IDictionary<TKey, TValue>. That's the main interface that Dictionary derives from. (I'm assuming that if you care that much about performance, then you aren't considering non-generic collections.) Then, in the future, you can change the underlying implementation class without having to change any of the code that uses that dictionary. For example:

IDictionary<string, int> myDict = new Dictionary<string, int>();