I using List<T> in my project, this list contains hundreds of entries. I am using List.Contains method quite a lot and this is hurting performance, I replaced the List with dictionary but it resulted in memory bottleneck, thus made performance even worst. Is there a better solution that one can suggest for searching in List? Is there a replacement of HashSet<T> in C# 2.0 or some other way that is better both memory and speed wise?
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isekaijin
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FIre Panda
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What are you trying to do? What are the list constraints? You have not provided the kind of information that a recommendation needs to be based on. – Oded May 12 '11 at 12:14
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List
Iam using, and List.Contains complexity is O(N), so it is hurting performance. – FIre Panda May 12 '11 at 12:24 -
What kind of data are you holding in this list? Hundreds of entries is not that much normally. Regardless, you don't explain what you are doing with the list. Saying `Contains` means nothing. – Oded May 12 '11 at 12:26
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Only a few hundreds? And this is causing problems? Unless you are nesting multiple loops I can't see how this would be a performance issue. – Oded May 12 '11 at 12:28
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foreach (Entity entity in _lstEntities) { if (entitiesExt.Contains(entity.EntityId)) continue; } Profiler shows this code took 437 sec in all. – FIre Panda May 12 '11 at 12:37
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Try this: `foreach (Entity entity in _lstEntities) { if (entitiesExt.Contains(entity)) continue; }`. You are comparing each entity to an entityId, not to another entity. – Oded May 12 '11 at 12:39
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entitiesExt is of List
not List – FIre Panda May 12 '11 at 12:52. -
3You really should be posting all of this code in your question. – Oded May 12 '11 at 12:52
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@Abdul: If that takes 437 seconds and your list only contains hundreds of elements, there is something very wrong with your computer. – recursive May 12 '11 at 13:13
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Hahaha, nice joke, I am talking about overall it takes 437 seconds, and that method is called thousands time. List.Contains is of O(N) in searching. – FIre Panda May 12 '11 at 14:08
3 Answers
7
A Dictionary<T,bool> can be used in place of a HashSet<T>. Whether you add items with a value of True or False is a coin toss, the value is not relevant.
It's more cumbersome than a HashSet<T>, and not quite a light-weight, but it's certainly better than a List<T>.
Tergiver
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OP says: `I replaced the List with dictionary but it resulted in memory bottleneck` – marsh-wiggle Dec 01 '20 at 15:54
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public class HashCollection <T> : ICollection <T>
{
private Dictionary<T, bool> _innerDictionary;
public HashCollection()
{
_innerDictionary = new Dictionary<T, bool>();
}
void ICollection <T>.Add(T item)
{
AddInternal(item);
}
private void AddInternal(T item)
{
_innerDictionary.Add(item, false);
}
public bool Add(T item)
{
if (_innerDictionary.ContainsKey(item))
return false;
AddInternal(item);
return true;
}
public void Clear()
{
_innerDictionary.Clear();
_innerDictionary = new Dictionary<T, bool>();
}
public bool Contains(T item)
{
return _innerDictionary.ContainsKey(item);
}
public void CopyTo(T[] array, int arrayIndex)
{
_innerDictionary.Keys.CopyTo(array, arrayIndex);
}
public int Count
{
get { return _innerDictionary.Keys.Count; }
}
public bool IsReadOnly
{
get
{
return false;
}
}
public bool Remove(T item)
{
return _innerDictionary.Remove(item);
}
public IEnumerator<T> GetEnumerator()
{
return _innerDictionary.Keys.GetEnumerator();
}
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
}
George Lica
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If you can live withthe requirement that .Net 3.5 framework be installed, you can use the HashSet from .Net 3.5 (System.Core.dll) in a 2.0 project.
See this question: Using HashSet in C# 2.0, compatible with 3.5
If that's a no go, I would use dictionary instead.
