How do I do a linq query to find fields in a dataset that are present in every record of a set?

Question

I have an XML data set with 10K records, each containing a set of fields.

I'd like to know which fields need to be null and which can be non-null in the database schema that matches the dataset.

Does linq offer way to produce a big intersection?

Example:

<set>
 <item>
  <a/>
  <foo />
  <b/>
  <c/>
 </item>
 <item>
   <a/>
   <foo />
   <b/>
   <c/>
  </item>
 <item>
   <a/>
   <b/>
 </item>
 <item>
   <a/>
   <foo />
   <b/>
  </item>
</set>

Prototype:

string[] CommonFieldNames(XElement[] elements)
{
  // ...
}

Desired Result:

{ "a", "b" }

Answer 1

In bellow code, selectedValue is your not null columns.

XDocument doc = XDocument.Parse("<set><item><a/><foo /><b/><c/></item><item><a/><foo /><b/><c/></item></set>");
var items = 
          doc.Descendants("item")
          .Select(x=>x.Descendants().Select(y=>y.Name).ToList()).ToList();
 var selectValue = items[0];
 foreach (var item in items)
 {
    selectValue = selectValue.Intersect(item).ToList();
 }

Answer 2

You could use a GroupBy and compare the group size to the total number of elements:

XDocument doc = XDocument.Parse("<set><item><a/><foo /><b/><c/></item><item><a/><foo /><b/><c/></item><item><a/><b/></item><item><a/><foo /><b/></item></set>");
var items = doc.Document.Element("set").Elements("item");
var commonElementNames = items.SelectMany(x => x.Elements()) // Get all immediate children
                              .GroupBy(x => x.Name) // Group by name
                              .Where(g => g.Count() == items.Count()) // Filter for only those which show in every group.
                              .Select(g => g.Key.LocalName) // Select just the element names
                              ;