How do I group these XDocuments?

Question

Problem

I have a collection of XDocument instances. Each document has a repeated element that can take a different value. I want to group them by this value but each element can specify a different value.

<sampledoc>
  <value>a</value>
  <value>b</value>
  <value>c</value>
</sampledoc>

Example

• Document A has values a, b, c
• Document B has values b, c, d
• Document C has values a, b

I want a grouping that is:

• group a
  • Document A
  • Document C
• group b
  • Document A
  • Document B
  • Document C
• group c
  • Document A
  • Document B
• group d
  • Document B

Question

I'm sure I must be able to do this but I can't see the wood for the trees right now.

docs.GroupBy... won't work on it's own (as far as I can tell) because the expression it takes should return a single value, and each document can contain multiple values. My head says a single LINQ query should be possible, but it can't fathom what it would be.

Can this be done using the GroupBy or AsLookup LINQ methods? Is there a way to do this?

I'd prefer examples in C# if anyone would be willing to provide one.

Update

Thanks to the answer from Pavel Minaev and a little inspiration, I solved this as follows:

// Collate all the different values
docs.SelectMany(doc => doc.Elements("Value")
                          .Select(el => el.Value))
    // Remove duplicate values
    .Distinct()
    // Generate a lookup of specific value to all
    // documents that contain that value
    .ToLookup(v => v, v => docs.Where(doc => doc.Elements("Value")
                                                .Any(el=>el.Value == v)));

Answer 1

GroupBy won't help you here anyway, because it assigns every element in the sequence to only one group.

var docs = new XDocument[] { docA, docB, docC } ;
var result = docs
   .SelectMany(doc => doc.Root.Elements("Value"))
   .Select(el => el.Value)
   .Distinct()
   .Select(key => new {
        Key = key,
        Documents = docs.Where(doc =>
            doc.Root.Elements("Value").Any(el => el.Value == key))
   });