I have a large list of values (100-200 character strings) and I need to return a distinct listing of them. What is the most efficient way to do this using .NET? The 2 ways that I can think of are:
If the Dictionary approach is faster in raw terms, consider a trade-off decision around maintainability of code.
I would expect Enumerable.Distinct to be about as fast as using a dictionary if you re only doing it once. If you want to be able to add/remove values and keep the distinct-ness, you could build a HashSet<string> (which is basically what I expect Distinct is doing under the hood, but Distinct() will obviously return new values as it finds them, maintaining order.
In fact, just using:
HashSet<string> distinctItems = new HashSet<string>(list);
will be a pretty good (and simple) solution if you don t mind the ordering being messed up. It s simpler than using a Dictionary, and conceptually cleaner as well (as you don t really want to map keys to values).
(As ever, I would suggest finding the most readable solution first, and benchmark it - if it s "fast enough" then go with that. If you want to use this as part of another query, then Distinct may well be the most readable way. Otherwise, I d suggest HashSet.)
I would personally go with the Distinct() method provided by LINQ. It s far easier to read and maintain. Whilst using LINQ will be slower than using a dictionary the difference will be small (in the case you ve listed) and you d be better spending time optimizing database queries or web service calls.
I would siggest you to use profiling here. Generate a list with sample items, sort it say 1M times using both ways, and measure the time used by each way.
If readability is a concern, create a GetDistinctItems method and put your code inside it: voilà, self-documented code.
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