using List(of) inside structure

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am getting an error using

VB

Structure JobItem
       Dim Index As Integer
       Dim Url As String
       Dim Status As String
       Dim WorkData As String
       Dim ListUrls As List(Of Uri)
   End Structure

when i try to add to ListUrls like this

VB

Dim jb As New JobItem
jb.ListUrls.Add(urlNext)

i get an error Object reference not set to an instance of an object.

how do i fix this?

Posted 5-Jun-11 9:28am

Cool Smith

Updated 5-Jun-11 15:15pm

Chris Maunder772.8K

v3

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Sergey Alexandrovich Kryukov · Accepted Answer · 2011-06-05T09:37:00

Solution 1

This is what it is: you did not instantiate one of the reference-type objects, so one of them is null, or some code explicitly assigned null to the variable. In your case, this is uninitialized ListUrls. After you constructed the structure, it's null. Use New to create a list. Add a constructor to the structure and initialize everything. You should use a constructor with parameters Index, Url, Status, and WorkData.

Using string types in inappropriate. Status is likely to be enumeration, Url must be of Url type; I don't know what's WorkData and why, but also unlikely the string type.

—SA

Posted 5-Jun-11 9:37am

Sergey Alexandrovich Kryukov

Updated 5-Jun-11 9:43am

v3

Comments

Cool Smith 5-Jun-11 15:41pm

can you give sample code, i know that is what i need to do, but how?

Sergey Alexandrovich Kryukov 5-Jun-11 15:43pm

There is nothing to code here. What exactly do you expect?
--SA

Sergey Alexandrovich Kryukov 5-Jun-11 15:43pm

See the updated answer.
--SA

Tarun.K.S 5-Jun-11 16:22pm

Well explained SA. 5+

Sergey Alexandrovich Kryukov 5-Jun-11 16:50pm

Thank you, Tarun.
--SA

Sander Rossel 5-Jun-11 17:32pm

That should indeed solve the problem, my 5. However, should this be a Structure at all? Read my 'solution' for additional comments.

Sergey Alexandrovich Kryukov 5-Jun-11 20:50pm

Thank you.
--SA

Cool Smith 10-Jun-11 4:01am

in this case, can you give me sample code on how to initialize, i have tried without success.

Sergey Alexandrovich Kryukov 12-Jun-11 23:11pm

Look at the solution by Tarun. What part did you fail?
--SA

Tarun.K.S · Accepted Answer · 2011-06-05T10:22:00

Solution 2

If you want the code, well here it is but do read SAKryukov's excellent explanation on why there is an error.

VB

Dim jb As New JobItem
Dim newList As New List(Of Uri)
newList.Add(New Uri("some path", UriKind.RelativeOrAbsolute))
jb.ListUrls = newList

It should work now.

Posted 5-Jun-11 10:22am

Tarun.K.S

Comments

Sergey Alexandrovich Kryukov 5-Jun-11 16:52pm

Right, my 5 (than you for translating all this in VB.NET which I never install). Adding constructor to the structure would be more consistent though.
--SA

Tarun.K.S 6-Jun-11 2:59am

Thanks. True, that's why I advised him to have a look at your answer. :)

Sander Rossel · Accepted Answer · 2011-06-05T11:29:00

Solution 3

Not really an answer to your question, but something I want to point out. Using a Structure is usually not the best way to go. Classes are the best way to go in probably 99,99% of the time.
Read here when to use Structures[^].
I think by using the List(Of T) you pretty much break those rules.
I am not saying that using a Structure is plain wrong (you probably have your reasons) I just want to point out that maybe you could go for a Class instead.
Also taken the arguments of SAKryukov you probably need to make Status an Enum, Url an Url Class etc. By creating a Class you will have more flexibility and you will be sure everything is ok in the future.

Posted 5-Jun-11 11:29am

Sander Rossel

Updated 5-Jun-11 11:31am

v2

Comments

Sergey Alexandrovich Kryukov 5-Jun-11 20:54pm

That makes sense, my 5. Not sure about 99.99% of time (probably an exaggeration; also, it depends in many factors), but a choice between structure and class should be taken with responsibility, as well as many other choices on types.
--SA

Sander Rossel 6-Jun-11 2:08am

Personally I have never made a Structure, but lots of Classes. Possibly because my boss thinks Structures are some medieval programming technique coming from VB6 and before and we should not use them even if our lives depended upon it :laugh:
Don't know where he got that idea...

Sergey Alexandrovich Kryukov 6-Jun-11 11:32am

Probably you depend on your boss too much... :-) Structures are certainly good in many cases (and have nothing, absolutely nothing to do with VB!). I do use them, much less then classes though. And they are reasonably used in Microsoft's .NET libraries, including those I consider as very good ones.
--SA