Question

Not completely sure how I can phrase this question best. How can I compare each element of a list with each elements of the same list only once.

For instance:

var textlist = ["a", "b", "c"];
var intersecting = from string a in textlist
                   from string b in textlist
                   where a != b && a.SomeCondition(b)
                   select new
                   {
                       object1 = a,
                       object2 = b
                   };

Assuming “a” gives ‘True’ to “SomeCondition” with “b”, I want the final result to be:

[["a, b"]]

Whereas now it would be:

[["a, b"], ["b, a"]]

Is this possible with a Linq Query?

Answer 1

One way is to compare each element to just the elements located after it:

string[] textlist = {"a", "b", "c"};
var intersecting = from aIndex in Enumerable.Range(0, textlist.Count())
                   from b in textlist.Skip(aIndex + 1)
                   let a = textlist.ElementAt(aIndex)
                   where a != b && a.SomeCondition(b)
                   select new
                   {
                       object1 = a,
                       object2 = b
                   };

Note: This solution, along with most solutions to this problem that use LINQ, will be horribly inefficient if used on an IEnumerable that is expensive to enumerate.