LINQ Group By using multiple criteria

Question

How would I go about grouping a list of objects by two different fields?

e.g.

struct Item
{
    public string Name;
    public int Age;
    public int Score;
}

List<Item> items = new List<Item>();
items.Add(new Item(){ "Jeff", 35, 50};
items.Add(new Item(){ "Annie", 22, 45};
items.Add(new Item(){ "Pierce", 60, 90};
items.Add(new Item(){ "Pierce", 60, 10};
items.Add(new Item(){ "Abed", 28, 22};
items.Add(new Item(){ "Troy", 22, 45};
items.Add(new Item(){ "Troy", 22, 65};
items.Add(new Item(){ "Troy", 22, 80};
items.Add(new Item(){ "Troy", 21, 2};
items.Add(new Item(){ "Troy", 21, 5};

var grouped = items.GroupBy(/*Group by age then name*/);

grouped would then be:

"Troy", 21, 2
"Troy", 21, 5

"Annie", 22, 45

"Troy", 22, 80
"Troy", 22, 65
"Troy", 22, 45

"Abed", 28, 22

"Jeff", 35, 50

"Pierce", 60, 90
"Pierce", 60, 10

Most solutions I've found deal with creating a new anonymous type, but that would surely lose the "Score" field.

How do I create a grouping of the original objects where the objects are grouped by multiple criteria?

Answer 1

var grouped = items.GroupBy(item => new { item.Name, item.Age });

This will produce an IGrouping<TKey, TElement> where TKey is the anonymous type and the TElement is Item. Basically, this is an IEnumerable of IEnumerable.

You can access them with a double for-loop:

foreach (var group in grouped) {
    foreach(Item item in group) {
        //...
    }
}

Answer 2

GroupBy method returns a collection of IGrouping<(Of <(TKey, TElement>)>) objects, one for each distinct key that was encountered. The key represents the attribute that is common to each value in the IGrouping<(Of <(TKey, TElement>)>) and can be accessed using a ForEach loop.

  var qry = items.GroupBy(item => new { item .Age, item .Name },
           (key, group) => new
           {
               Key1 = key.Age,
               Key2 = key.Name,
               All = group.ToList()
           });

You can check the result with this:

qry.ToList().ForEach(x => x.All.ForEach(y=>Console.WriteLine("{0}, {1}, {2}",y.Name, y.Age, y.Score)));

Answer 3

Use this:

var grouped = items.GroupBy(item => item new{ item.Name, item.Age });

You will not loose the Score field. sublists of grouped list will still be of type Item and objects of anonymous type will be in Key property of each group.