问题
HashSet<T> t = new HashSet<T>();
// add 10 million items
Dictionary<K, V> t = new Dictionary<K, V>();
// add 10 million items.
Whose .Contains method will return quicker?
Just to clarify, my requirement is I have 10 million objects (well, strings really) that I need to check if they exist in the data structure. I will NEVER iterate.
回答1:
HashSet vs List vs Dictionary performance test, taken from here.
Add 1000000 objects (without checking duplicates)
Contains check for half the objects of a collection of 10000
Remove half the objects of a collection of 10000
回答2:
I assume you mean Dictionary<TKey, TValue> in the second case? HashTable is a non-generic class.
You should choose the right collection for the job based on your actual requirements. Do you actually want to map each key to a value? If so, use Dictionary<,>. If you only care about it as a set, use HashSet<>.
I would expect HashSet<T>.Contains and Dictionary<TKey, TValue>.ContainsKey (which are the comparable operations, assuming you're using your dictionary sensibly) to basically perform the same - they're using the same algorithm, fundamentally. I guess with the entries in Dictionary<,> being larger you end up with a greater likelihood of blowing the cache with Dictionary<,> than with HashSet<>, but I'd expect that to be insignificant compared with the pain of choosing the wrong data type simply in terms of what you're trying to achieve.
回答3:
From MSDN documentation for Dictionary<TKey,TValue>
"Retrieving a value by using its key is very fast, close to O(1), because the Dictionary class is implemented as a hash table."
With a note:
"The speed of retrieval depends on the quality of the hashing algorithm of the type specified for TKey"
I know your question/post is old - but while looking for an answer to a similar question I stumbled across this.
Hope this helps. Scroll down to the Remarks section for more details. https://msdn.microsoft.com/en-us/library/xfhwa508(v=vs.110).aspx
回答4:
These are different data structures. Also there is no generic version of HashTable.
HashSet contains values of type T which HashTable (or Dictionary) contains key-value pairs. So you should choose collection on what data you need to be stored.
来源：https://stackoverflow.com/questions/2728500/hashsett-versus-dictionaryk-v-w-r-t-searching-time-to-find-if-an-item-exist