C# Linq Group By on multiple columns [duplicate]

Question

public class ConsolidatedChild
{
    public string School { get; set; }
    public string Friend { get; set; }
    public string FavoriteColor { get; set; }
    public List<Child> Children { get; set; }
}

public class Child
{
    public string School { get; set; }
    public string Name { get; set; }
    public string Address { get; set; }
    public string Friend { get; set; }
    public string Mother { get; set; }
    public string FavoriteColor { get; set; }
}

Given the two classes above, I would like to use LINQ to create a List from the List, grouped by the School, Friend and FavoriteColor properties. Is this possible with LINQ?

Please ignore the properties, the code has been written just to help with the question.

Yes it is possible. Take a look at the sample provided in this post. — Kamyar
– Kamyar, Commented Mar 8, 2011 at 11:34
Here is another good example stackoverflow.com/questions/15605468/… — NoWar
– NoWar, Commented Aug 29, 2016 at 15:23

Leandro Bardelli · Accepted Answer · 2021-01-10 19:51:04Z

745

var consolidatedChildren =
    from c in children
    group c by new
    {
        c.School,
        c.Friend,
        c.FavoriteColor,
    } into gcs
    select new ConsolidatedChild()
    {
        School = gcs.Key.School,
        Friend = gcs.Key.Friend,
        FavoriteColor = gcs.Key.FavoriteColor,
        Children = gcs.ToList(),
    };

var consolidatedChildren =
    children
        .GroupBy(c => new
        {
            c.School,
            c.Friend,
            c.FavoriteColor,
        })
        .Select(gcs => new ConsolidatedChild()
        {
            School = gcs.Key.School,
            Friend = gcs.Key.Friend,
            FavoriteColor = gcs.Key.FavoriteColor,
            Children = gcs.ToList(),
        });

edited Jan 10, 2021 at 19:51

Leandro Bardelli

11.7k16 gold badges88 silver badges130 bronze badges

answered Mar 8, 2011 at 12:06

Enigmativity

117k12 gold badges101 silver badges185 bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

guiomie Over a year ago

Let's say I have lot's of properties in the group by, is there a cleaner way of doing this then listing all the properties?

Enigmativity Over a year ago

@guiomie - I wouldn't say that there was a cleaner way. You could serialize the object into a single string and group on that, but that would require more code elsewhere. So, in my mind, this answer is the cleanest way regardless how many properties.

Enigmativity Over a year ago

@Doppelganger - I rolled back your edit for two reasons. The choice of using the final comma (after FavoriteColor) is on purpose - it is legal syntax and it enables easier refactoring of code. The choice of using gcs rather than gc for the grouping variable is also on purpose - it shows me that it is a "group of many c's".

Doppelganger Over a year ago

I thought for sure that wouldn't compile, but you're absolutely right. As for renaming your variable, that was just to get past the six character limit. I never like to mess with someone's variable names. Anyway, thanks again for the awesome answer!

Alexander Ryan Baggett Over a year ago

Can you make an alternate example for method syntax?

|

CarenRose · Accepted Answer · 2019-10-11 19:56:28Z

274

Given a list:

var list = new List<Child>()
{
    new Child()
        {School = "School1", FavoriteColor = "blue", Friend = "Bob", Name = "John"},
    new Child()
        {School = "School2", FavoriteColor = "blue", Friend = "Bob", Name = "Pete"},
    new Child()
        {School = "School1", FavoriteColor = "blue", Friend = "Bob", Name = "Fred"},
    new Child()
        {School = "School2", FavoriteColor = "blue", Friend = "Fred", Name = "Bob"},
};

The query would look like:

var newList = list
    .GroupBy(x => new {x.School, x.Friend, x.FavoriteColor})
    .Select(y => new ConsolidatedChild()
        {
            FavoriteColor = y.Key.FavoriteColor,
            Friend = y.Key.Friend,
            School = y.Key.School,
            Children = y.ToList()
        }
    );

Test code:

foreach(var item in newList)
{
    Console.WriteLine("School: {0} FavouriteColor: {1} Friend: {2}", item.School,item.FavoriteColor,item.Friend);
    foreach(var child in item.Children)
    {
        Console.WriteLine("\t Name: {0}", child.Name);
    }
}

Result:

School: School1 FavouriteColor: blue Friend: Bob
    Name: John
    Name: Fred
School: School2 FavouriteColor: blue Friend: Bob
    Name: Pete
School: School2 FavouriteColor: blue Friend: Fred
    Name: Bob

edited Oct 11, 2019 at 19:56

CarenRose

1,3741 gold badge14 silver badges26 bronze badges

answered Mar 8, 2011 at 12:04

Jamiec

136k15 gold badges143 silver badges202 bronze badges

5 Comments

Michael Blackburn Over a year ago

For those of us who prefer the fluent interface. One tiny quibble: using the same variable "x" in both Func definitions isn't very clear for newer users. I'd recommend changing to match the one above, where "c" is the child, and "gcs" is the grouped children.

Jamiec Over a year ago

@jazmatician _ I agree with you on the point that re-using x might confuse some, but not on the choice for variable names. I'll change it to x and y to diferentiate.

Michael Blackburn Over a year ago

The variable names aren't my choice, either. In fact, when writing that I struggled to figure out what "gcs" meant to the author. Best I could come up with was "group (of) Cs." Anyway, I only recommended it to try to make it easy for someone to make the mental map between the two syntaxes. (syntaces? I bet syntax is one of those words which is its own plural. Because this is English, why would "syntax" be subject to syntactic rules about plurals?)

Michael Blackburn Over a year ago

to answer my own tangent: en.wiktionary.org/wiki/syntax

Serj Sagan Over a year ago

I disagree with this opinion Michael. In my opinion, x should be used in both places. The only time it should not is when you use a Func withing another Hunc, then you should go to y. Otherwise it becomes confusing.

Collectives™ on Stack Overflow

C# Linq Group By on multiple columns [duplicate]

2 Answers 2

7 Comments

5 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

7 Comments

5 Comments

Linked

Related