How to filter generic list based on 2 values

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Hey guys, I need a little help with a little problem. I have a generic list of objects which I want to filter by 2 values an only be let with the filtered list.

My Object Structure:

C#

public class ErrorLog
{
    public int ErrorLogId {get; set; }
    public string ErrorGroupId {get; set; }
    public string ErrorDescription {get; set; }
    public DateTime ErrorDate {get; set; }

}

So in my code I have my list:

C#

List<ErrorLog> _unprocessedErrors = ErrorLogBusinessLogic.GetErrors();

What I have tried :

C#

//Aggregate up the errors
List<ErrorLog> _distinctList = (List<ErrorLog>)_unprocessedErrors.Select(e => e.ErrorGroupId).Select(e => e.ErrorDescription).Distinct();

So I need to filter by ErrorGroup and ErrorSescription to get my distinct list
Does anyone know the best way to achieve this.
Thanks guys

Posted 14-Nov-14 5:49am

frostcox

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Sergey Alexandrovich Kryukov 14-Nov-14 12:01pm

What is the problem (performance, incorrect results, anything else...)?
—SA

3 solutions

Solution 1

Why don't you join them in one?

string someId = "someId"; 
string someDesc = "someDesc";
		
List<ErrorLog> _distinctList = _unprocessedErrors.Where(
               // do the searching...
               e => e.ErrorGroupId == someID && e.ErrorDescription == someDesc
               // get distince and convert to list
               ).Distinct().ToList();

This is a LAMBDA expression to select the objects depending on conditions. You will have to pass the still same comparison exression (variable == value) thing to select the objects.

Also, why don't you change the ErrorGroupID to an int value?

Posted 14-Nov-14 5:59am

Afzaal Ahmad Zeeshan

Updated 14-Nov-14 7:18am

v4

Comments

BillWoodruff 14-Nov-14 12:39pm

My vote of #1: Even if you had written a Linq statement that would compile (yours will not), the result of your Linq statement ... if you assigned to a 'var variable ... would have been at best a List containing two elements: the boolean values 'true and 'false.

Note that I wouldn't bother to write this to you if I didn't think you were a bright person with technical talent, and I don't like writing this kind of message, but I think you've really gotten off-track in CodeProject, probably chasing the reputation-thing, and somebody needs to tell you.

When are you going to stop posting answers which you are obviously making up quickly with no concern for accuracy, or relevance ? Posting answers on CodeProject may be some kind of video-game for you, but, for others of us here it's a place where we sincerely try to help others to the best of our abilities.

Afzaal Ahmad Zeeshan 14-Nov-14 13:20pm

Thanks for bringing this up, you're right, I really need to put a break in this to answer when I'm above fully sure that my answer is right and would compile. I'm sorry for the trouble I've cause.

I've somehow editted the code to make it a little bit fair to send to the OP. Thanks again Bill.

Solution 2

Um... Select doesn't filter - it returns a "different" object for each object in the original collection. So the first Select will result in a list of strings, which you then try to rework with the second list.
If you really want to filter then try Where:

C#

List<errorlog> _distinctList = _unprocessedErrors.Where(e => e.ErrorGroupId == 6 && e.ErrorDescription == "Hello").Distinct().ToList();</errorlog>

But if that isn't what you want, then probably we need a sample input - output set to work out what exactly you are trying to do.

Posted 14-Nov-14 6:01am

OriginalGriff

Comments

BillWoodruff 14-Nov-14 12:46pm

OG, I think in this case, since this is a custom class ... assuming the OP really wanted to use 'Distinct for some reason ... you'd need to implement IEquatable<t> in the ErrorLog class. Otherwise, without that, 'Distinct would just be a no-op.

MSDN: "If you want to return distinct elements from sequences of objects of some custom data type, you have to implement the IEquatable<t> generic interface in the class."

OriginalGriff 14-Nov-14 14:00pm

To be honest Bill, I'm not sure the OP knows exactly what he wants to produce himself - the original code wouldn't even compile, so it's difficult to tell.

frostcox 14-Nov-14 15:18pm

sorry if I was unclear with what I was trying to do do here, I tried to be as clear as I could, but the solution by Richard was exactly what I was looking for. Thanks for your help.

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Richard Deeming · Accepted Answer · 2014-11-14T06:06:00

There are several ways to achieve this.

You could pass a custom IEqualityComparer<T> implementation[^] to the Distinct method[^]:

C#

public sealed class ErrorLogEqualityComparer : IEqualityComparer<ErrorLog>
{
    public int GetHashCode(ErrorLog obj)
    {
        if (obj == null) return 0;
        return StringComparer.Ordinal.GetHashCode(obj.ErrorGroupId)
            ^ StringComparer.Ordinal.GetHashCode(obj.ErrorDescription);
    }
    
    public bool Equals(ErrorLog left, ErrorLog right)
    {
        if (left == null) return right == null;
        if (right == null) return false;
        
        return StringComparer.Ordinal.Equals(left.ErrorGroupId, right.ErrorGroupId)
            && StringComparer.Ordinal.Equals(left.ErrorDescription, right.ErrorDescription);
    }
}

List<ErrorLog> distinctList = _unprocessedErrors
    .Distinct(new ErrorLogEqualityComparer())
    .ToList();

You could use the GroupBy method[^] to group your results by the key values, and then extract the first item from each group:

C#

List<ErrorLog> distinctList =  _unprocessedErrors
    .GroupBy(e => new { e.ErrorGroupId, e.ErrorDescription }, (key, list) => list.First())
    .ToList();

Or, you could use a custom DistinctBy method, such as the one provided by the MoreLinq library[^]:

C#

public static class SetExtensions
{
    private static IEnumerable<TSource> DistinctByIterator<TSource, TKey>(IEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey> keyComparer)
    {
        var knownKeys = new HashSet<TKey>(keyComparer);
        try
        {
            foreach (var item in source)
            {
                if (knownKeys.Add(keySelector(item)))
                {
                    yield return item;
                }
            }
        }
        finally
        {
            // HACK: Avoid iterator memory leak described at:
            // http://codeblog.jonskeet.uk/2011/01/18/gotcha-around-iterator-blocks/
            knownKeys = null;
        }
    }

    public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey> keyComparer = null)
    {
        if (source == null) throw new ArgumentNullException("source");
        if (keySelector == null) throw new ArgumentNullException("keySelector");
        return DistinctByIterator(source, keySelector, keyComparer);
    }
}

List<ErrorLog> distinctList =  _unprocessedErrors
    .DistinctBy(e => new { e.ErrorGroupId, e.ErrorDescription })
    .ToList();

How to filter generic list based on 2 values

3 solutions

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Solution 2

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