Question

I have a normal GroupBy operation on an enumerable:

e.GroupBy(i => i.Property)

But if i.Property is really a collection, how would I break apart the collection and use the list’s elements as grouping keys?

For example let’s say I have two objects (Z, Y) that each have a list:

Z: { List = { A, B, C }}
Y: { List = { B, C, D }}

Now running the GroupBySubelement(o => o.List) would not group by the list itself, but would iterate over the list and generate the following Groupings.

{A, {Z}}
{B, {Z, Y}}
{C, {Z, Y}}
{D, {Y}

Is this possible?

Thanks!

Answer 1

Here’s some example code that achieves what you want:

//This is just temporary data.  Has the similar structure to what you want.
var parts = new[]
                {
                    new
                        {
                            Name = "X",
                            Property = new[] {'A', 'B', 'C'}
                        },
                    new
                        {
                            Name = "Y",
                            Property = new[] {'B', 'C', 'D'}
                        },
                    new
                        {
                            Name = "Z",
                            Property = new char[] { }
                        }
                    };

var groupedBySub = from part in parts
                   from sub in part.Property
                   group part by sub;

foreach(var group in groupedBySub)
{
    Console.WriteLine("{0} - {1}", group.Key, string.Join(", ", group.Select(x => x.Name)));
}

Which outputs:

A - X
B - X, Y
C - X, Y
D - Y

You can also achieve this in the method chain fashion:

var groupedBySub = parts.SelectMany(part => part.Property, (part, sub) => new {part, sub}).GroupBy(t => t.sub,  t => t.part);

If you want to capture it with the list being empty:

var groupedBySub = from part in parts
                   from sub in part.Property.DefaultIfEmpty()
                   group part by sub;

Which when substituted for the code above, gives output:

A - X
B - X, Y
C - X, Y
D - Y
  - Z