Why do linq search has a huge difference in efficiency when I use string and array especially for large data?

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I want to search the name of the students which contains the keywords,at first I pass the keywords separated by commas,but I find the search time is too long.But when I convert these keywords into an array,it is real fast.Why do linq search has a huge difference in efficiency?Is that because of the array or linq?

C#

**Using string to search**

    var keyWord="Lyly,Tom,Jack,Rose"; //and so on,more than 500 names
    var student= Context.Students.Where(i => keyWord.Contains(i.Name));//very slow

**Using array to search**

    var keyWord="Lyly,Tom,Jack,Rose"; //and so on,more than 500 names
     var keyWordArray=keyWord.split(',');
    var student= Context.Students.Where(i => keyWordArray.Contains(i.Name));//fast

Posted 12-Nov-14 3:31am

PEIYANGXINQU

Updated 12-Nov-14 3:34am

v2

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PIEBALDconsult · Answer 1 · 2014-11-12T03:38:00

Solution 1

Because searching a string is inefficient and should be avoided.
Now take that array and populate a HashSet and you should see even better performance over searching the array.

Furthermore, the string search can return false positives -- try searching "Bob,Robert,Mary" for "Rob".

Another benefit of the HashSet is that you can tell it to use a case-insensitive comparer.

Posted 12-Nov-14 3:38am

PIEBALDconsult

Updated 12-Nov-14 5:51am

v3

Comments

BillWoodruff 12-Nov-14 9:47am

As long as there are no duplicate strings to be stored ?

PIEBALDconsult 12-Nov-14 9:54am

Even better then, duplicates "Bob,Bob,Bill,Bob,Mary,Bill,Tom" would cause even worse string and array performance.

BillWoodruff 12-Nov-14 10:50am

... mmm ... I was thinking of the case where there would be a reason to have duplicate values. But, I forget what that was :)

Matt T Heffron 12-Nov-14 12:40pm

In that case, the better data structure would be Dictionary<string, List<something-to-distinguish-actual-values>>
Or a real DB...

PIEBALDconsult 12-Nov-14 12:45pm

Or a class with a custom GetHashCode or comparer that takes that into account.