Efficient way to search object from list of object in c#

Question

I have a list which contains more than 75 thousand object. To search item from list currently I am using following code.

from nd in this.m_ListNodes
where
   nd.Label == SearchValue.ToString()
   select
   nd;

Is this code is efficient?

@Marco: the OP has asked a total of 5 questions (this one included) with quite few answers on them. It could be that there have not been good enough answers yet. — Fredrik Mörk
– Fredrik Mörk, Commented Apr 15, 2011 at 7:47

Jon Skeet · Accepted Answer · 2011-04-15 07:53:12Z

12

How often do you need to search the same list? If you're only searching once, you might as well do a straight linear search - although you can make your current code slightly more efficient by calling SearchValue.ToString() once before the query.

If you're going to perform this search on the same list multiple times, you should either build a Lookup or a Dictionary:

var lookup = m_ListNodes.ToLookup(nd => nd.Label);

or

var dictionary = m_ListNodes.ToDictionary(nd => nd.Label);

Use a dictionary if there's exactly one entry per label; use a lookup if there may be multiple matches.

To use these, for a lookup:

var results = lookup[SearchValue.ToString()];
// results will now contain all the matching results

or for a dictionary:

WhateverType result;
if (dictionary.TryGetValue(SearchValue.ToString(), out result))
{
    // Result found, stored in the result variable
}
else
{
    // No such item
}

edited Apr 15, 2011 at 7:53

answered Apr 15, 2011 at 7:40

Jon Skeet

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Comments

Daniel Hilgarth · Accepted Answer · 2011-04-15 07:50:00Z

4

No. It would be better if you used a Dictionary or a HashSet with the label as the key. In your case a Dictionary is the better choice:

var dictionary = new Dictionary<string, IList<Item>>();

// somehow fill dictionary

IList<Item> result;
if(!dictionary.TryGetValue(SearchValue.ToString(), out result)
{
    // if you need an empty list 
    // instead of null, if the SearchValue isn't in the dictionary
    result = new List<Item>(); 
}

// result contains all items that have the key SearchValue

edited Apr 15, 2011 at 7:50

answered Apr 15, 2011 at 7:38

Daniel Hilgarth

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4 Comments

Jon Skeet Over a year ago

A HashSet wouldn't work, if he's trying to find the item. A dictionary will work if there's only a single match, but fail if multiple items have the same label.

Indrajeet Over a year ago

I try with dictionary still it takes time. The code is : from nd in this.m_NodeDict where nd.Key == SearchValue.ToString() select nd.Value;

Daniel Hilgarth Over a year ago

@JonSkeet: The Dictionary fails only, if you are using it wrong. If there are several items with the same label, use a Dictionary<string, IList<Item>>. Problem solved.

Jon Skeet Over a year ago

@Daniel: At that point, why not just use a Lookup? I'd rather reuse code that's already written than write it myself :) I guess my point was mostly that you ought to distinguish between the two cases (unique label or not).

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