Question

Given the following two strings:

Dim str1() As String = {"123", "456", "789", "0"}
Dim str2() As String = {"123", "456", "1"}

How do I perform a full outer join of str1 and str2 and end up having a structure like this:

{Nothing, "1"}
{"0", Nothing}
{"123", "123"}
{"456", "456"}
{"789", Nothing}

Based on several discussions on SO and other websites, I tried using LINQ:

Dim v = From a In str1 Join b In str2 On a Equals b Group By a Into g = Group
        From o In g.DefaultIfEmpty()

But it does not produce the desired result, which is exactly the same as this (regular INNER JOIN):

Dim v = From a In str1 Join b In str2 On a Equals b

The last example I’ve been looking at is here (C#).

And here is Another article, but it seems too complicated to be the shortest possible solution (I’ve seen much more simple C# examples and hope VB can be as efficient).

Why this question is useful

For example, one can have a file structure, where file path is a key. By doing full outer join of the keys, you can compare folders, find which files are missing on which side and show difference to the user. Any kind of synchronization task could use this approach.

Answer 1

I guess this is not the solution you desire, however it seems to fullfil the task:

string[] a1 = { "123", "456", "1" };
string[] a2 = { "123", "456", "789", "0" };

var intersection = a1.Intersect(a2); //get the intersection
var exceptions1 = a1.Except(a2);     //get items from a1, that are not in a2
var exceptions2 = a2.Except(a1);     //get items from a2, that are not in a1

var result = new List<Tuple<string, string>>(); //the result set
result.AddRange(intersection.Select(s => new Tuple<string, string>(s, s)));
result.AddRange(exceptions1.Select(s => new Tuple<string, string>(s, null)));
result.AddRange(exceptions2.Select(s => new Tuple<string, string>(null, s)));

foreach (var t in result)
{
    Console.WriteLine((t.Item1 ?? "null") + "\t" + (t.Item2 ?? "null"));
}

The output is:

123     123
456     456
1       null
null    789
null    0