Nested foreach loop slow

Question

The following piece of code achives the desired results, but performance is extremely slow:

SearchResultCollection absaUsers = ABSAds.FindAll();
SearchResultCollection srcUsers = ds.FindAll();

foreach (SearchResult users in srcUsers)
{
    string cn = users.Properties["cn"][0].ToString();
    string sn = users.Properties["sn"][0].ToString();
    string userID = users.Properties["uid"][0].ToString();
    string description = users.Properties["PersonnelAreaDesc"][0].ToString();
    string jobCodeID = users.Properties["JobcodeID"][0].ToString();
    string CostCentreID = users.Properties["costCentreID"][0].ToString();
    string CostCentreDescription = users.Properties["CostCentreDescription"][0].ToString();
    string givenName = users.Properties["givenName"][0].ToString();
    string employmentStatus = users.Properties["EmploymentStatus"][0].ToString();
    string EmploymentStatusDescription = users.Properties["EmploymentStatusDescription"][0].ToString();

    foreach (SearchResult absaUser in absaUsers)
    {

        string absaUID = absaUser.Properties["uid"][0].ToString();
        string absaEmploymentStatus = absaUser.Properties["EmploymentStatus"][0].ToString();
        string absaEmploymentStatusDescription = absaUser.Properties["EmploymentStatusDescription"][0].ToString();
        string absaEmployeeNumber = absaUser.Properties["employeeNumber"][0].ToString();

        if (absaUID == cn && absaEmployeeNumber==userID)
        {
            Console.WriteLine("User Record Found:" + cn);
            sw.WriteLine("Modify" + "," + cn + "," + description + "," + userID + "," + givenName + "," + sn + "," + jobCodeID + "," + CostCentreID + "," + CostCentreDescription + "," + sn + "," + cn + "," + employmentStatus + "," + EmploymentStatusDescription);
            sw.Flush();
            break;
        }
    }
}

It loops through 2 collections and mtaches the outer loops attributes with the inner's. Any suggestions on how I can optimise the performance?

Any statistics on performance measurements? How many items in the lists absaUsers and srcUsers? — sll
– sll, Commented Oct 6, 2011 at 9:24
If this all come from database, you better do it in the SQL level and Join the tables, fetching only the data you need.. — user447356
– user447356, Commented Oct 6, 2011 at 9:26
Sorry for not being very specific. This program connects 2 different LDAP Stores, and compares user attributes. absaUsers contain about 50 000 users and scrUsers contain about 11 000 — Trishen
– Trishen, Commented Oct 6, 2011 at 9:29

Jon Skeet · Accepted Answer · 2011-10-06 10:05:05Z

5

It would be faster if you extracted all the absaUID values to a lookup first:

var lookup = absaUsers.Cast<SearchResult>()
                      .ToLookup(x => x.Properties["uid"][0].ToString());

Then you can just use:

foreach (SearchResult users in srcUsers)
{
    string cn = users.Properties["cn"][0].ToString();
    foreach (SearchResult matches in lookup[cn])
    {
        ...
    }
}

You haven't shown how absaUsers is defined - if it's a LINQ query expression, then it could be that your existing code will be going to the database on every iteration at the moment - whereas the above won't. On the other hand, if srcUsers is also a LINQ query expression talking to a database, you should consider doing all the matching at the database using a join instead.

edited Oct 6, 2011 at 10:05

answered Oct 6, 2011 at 9:23

Jon Skeet

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1 Comment

Jon Skeet Over a year ago

@Trishen: You'll probably need a call to Cast - will add that.

mtijn · Accepted Answer · 2011-10-06 09:23:42Z

0

you could use LINQ join, some examples are here, I'm assuming whoever built it into .NET found a pretty optimal way of doing that, and then loop through that. on a sidenote: what are your collection types? please add their declaration to the code snippet.

answered Oct 6, 2011 at 9:23

mtijn

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Comments

user416 · Accepted Answer · 2011-10-06 09:49:02Z

0

Use Lamda expressions:

Below is the sample one , You can optizime this to another level.

    List<SearchResult> allResultGroups=new  List<SearchResult>();
    foreach (SearchResult absaUser in absaUsers)
    {
    resultGroups = srcUsers.Where(g => g.cn == absaUser.absaUID && absaUser.absaEmployeeNumber==g.userID ).ToList();
    }

answered Oct 6, 2011 at 9:49

user416

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