Convert list of objects in object to csv - vb.net

Question

In vb.net How to convert a list of objects within another List of Objects to csv string.
I tried below didn't work
String.Join(",", Person.Cars.Select(Function(d) d.Colors.Select(Function(o) o.colorid.ToString)))
Output should be colorid in string csv eg: "101,102,103"
If i try for Car ids that works as expected
String.Join(",", Person.Cars.Select(Function(d) d.carid.ToString))
output is "2001,2002,2003"
Here's how data is constructed
Dim objPerson As New Person
With objPerson
.Cars = new Cars (1) {}
.Cars(0) = new Cars
.Cars(1) = new Cars
With Cars(0)
.CarId = "2001"
.Colors = new Colors(1){}
.Colors(0) = new Colors
.Colors(1) = new Colors
With .Colors(0)
.Colorid = "101"
End With
With .Colors(1)
.Colorid = "102"
End With
End With
With Cars(1)
.CarId = "2002"
.Colors = new Colors(1){}
.Colors(0) = new Colors
.Colors(1) = new Colors
With .Colors(0)
.Colorid = "103"
End With
With .Colors(1)
.Colorid = "104"
End With
End With
End With
End With

Answer 1

LINQ can be a very useful tool. It's not that it allows you to do anything that you couldn't do in other ways. It's just that it allows you do some things in a much easier, cleaner, and more readable way. This is not an example of one of those situations. In this case, your use of LINQ is not only likely to be confusing to others, but it's even confusing to you. I'm all for using LINQ in places where it makes the code easier to read, but, since that's it's only real benefit, I see no reason to use it in places where it makes the code harder to read.
Ok, now that I'm off my high-horse, let me explain your problem. You have two LINQ calls nested inside of each other. The inner one returns a list of color ID's for a car. The outer one calls that inner one once for each car in the list. That means that in the end, you don't have a flat list of color ID's for all of the cars. Rather, you have a 2-D list where there is, essentially, one row for each car and one column for each color ID. You need to flatten it into a 1-D list.
You can do it in LINQ with something like this:
String.Join(",", Person.Cars.Select(Function(d) String.Join(",", d.Colors.Select(Function(o) o.colorid.ToString))))
Or by using SelectMany, as recommended by sloth, but I would argue that you are pushing the bounds of readability and should seriously consider trying to bring your cleverness-factor down a notch and just go with a simpler, more readable, For loop. If that makes me sound old-fashioned, so be it. For instance, I would argue that something like this would cause much less head scratching:
Private Function GetAllColorIds(cars As IEnumerable(Of Car)) As IEnumerable(Of Integer)
Dim result As New List(Of Integer)()
For Each i As Car In cars
result.AddRange(i.Colors.Select(Function(x) x.colorid.ToString())
Next
Return result
End Function
' ...
Dim csv As String = String.Join(",", GetAllColorIds(Person.Cars))

Answer 2

Use SelectMany instead of Select to flatten the result that you pass to String.Join:
String.Join(",", Person.Cars.SelectMany(Function(d) d.Colors.Select(Function(o) o.colorid.ToString)))
In response to your comment:
It seems you're looking for something like
String.Join(",", Person.Cars.Select(Function(d) String.Join("", d.Colors.Select(Function(o) o.colorid.ToString))))