variable usage

[krollenhagen]

Hello-

Let me start out by saying that I am new to .net and was only a hobby
vb6 programmer.

I am getting the warning:

"Variable 'arrOutputData' is used before it has been assigned a value.
A null reference exception could result at runtime."

I get this warning in the line:

"arrOutputData(i).Days = sinDays"

I also have the following in my code:

Structure Data
Dim Days As Single
Dim Events As Integer
Dim EventsPerWeek As Single
Dim TotalTime As TimeSpan
Dim TimePerWeek As TimeSpan
Dim TimePerEvent As TimeSpan
Dim TimeMin As TimeSpan
Dim TimeMax As TimeSpan
End Structure

Dim arrOutputData() As Data
Dim sinDays As Single

I don't understand why I get a warning saying that the variable is used
before it has been assigned a value when I am trying to assign it a
value at that point in the code.

Any help is appreciated. Thank you.

 

[Tiago Salgado]

Try to put this instead of the line "Dim arrOutputData() As Data":

Dim arrOutputData() As New Data

 

[Theo Verweij]

VB6 types and .Net structures are not the same; a .net structure is a
class (yes, you can add functions and properties), and therefore, you
have to use the new keyword to instantiate the class.

The way you did it, is comparable to the following vb6 code:

dim x as aClass
x.property = 5

The last line will give you a runtime error, because the class x hasn't
been instantiated.
In .Net, the compiler detects these errors, so you get them at compile
time instead of runtime.

 

[Chris Dunaway]

a .net structure is a
class (yes, you can add functions and properties), and therefore, you

A .Net structure is *not* a class, even though you can functions and
properties.

have to use the new keyword to instantiate the class.

And you do *not* have to use new when instantiating them:

Public Structure MyStruct
Public AnInteger As Integer
Public AString As String
End Structure


'Note that New is not required
Dim structInstance As MyStruct

structInstance.AnInteger = 4
structInstance.AString = "Hello"

This is not to say that you cannot have a constructor for your
Structure.

Also, structures are stored differently in memory than classes. See
this article:

http://www.yoda.arachsys.com/csharp/memory.html

Chris

 

[Chris Dunaway]

"Variable 'arrOutputData' is used before it has been assigned a value.
A null reference exception could result at runtime."

I get this warning in the line:

"arrOutputData(i).Days = sinDays"

But where did you assign a value to arrOutputData ? This looks like an
array, perhaps you forgot to initialize the array? Can you show the
code where you declare arrOutputData and how you initialize it?

 

[Branco Medeiros]

I am getting the warning:

"Variable 'arrOutputData' is used before it has been assigned a value.
A null reference exception could result at runtime."

I get this warning in the line:

"arrOutputData(i).Days = sinDays"

I also have the following in my code:

Structure Data
Dim Days As Single
Dim Events As Integer
Dim EventsPerWeek As Single
Dim TotalTime As TimeSpan
Dim TimePerWeek As TimeSpan
Dim TimePerEvent As TimeSpan
Dim TimeMin As TimeSpan
Dim TimeMax As TimeSpan
End Structure

Dim arrOutputData() As Data
Dim sinDays As Single

The Visual Studio IDE performs a data flow analisys in your code to
catch, among other things, when you access members of non-initialized
classes: accessing a member of a class that is still Nothing will
result in a run-time error, as you may know.

Now, arrays, in .Net, are classes -- you can, for instance, verify if
an array is Nothing (well, you can do the same with strings, also).
Therefore, you must instanciate the array before accessing an item of
it.

One way of doing this is like in VB6:

Dim arrOutputData(0 to 99) As Data

If you don't know how many items your array will have, you may declare
an empty array:

'Creates an empty array
Dim arrOutputData() As Data = {}

Or

'Creates an empty array also
Dim arrOutputData(-1) As Data

Afterwards you're free to Redim the array:

Dim Max As Integer = arrOutputData.GetLowerBound(0)
Redim Preserve arrOutputData(0 To Max + 1)

And *then* access items from the array =))

Regards,

Branco.

 

[Michel Posseth [MCP]]

To prevent the nagging warning a lot of programmers have already set the
option off ( cause it warns sometimes without a good reasson )

sometimes the warning is even wrong when using a select case for instance
where you assign in every case a value
what will happen is that it will still display the nag warning that it
hasn`t been assigned a value or that a function doesn`t return a value on
all codepaths while it does

so to prevent this i learned myself to

1. assign a Nothing pointer after delcaration when such a situation occurs

2. use the return statement in functions where i use a slect case and as
last codeline i return a default value ( wich will never get executed )

regards

Michel Posseth [MCP]

 

[krollenhagen]

I declared the array with the statement:

Dim arrOutputData() As Data

I don't know the length of the array when I am declaring it. Is that
why I have to initialize the array? If I were to redim the array,
would I have to initialize it?

Thanks for the help so far.

Keith

 

[Guest]

you could go for a construct like this

Dim arrOutputData() As Data
Dim sindays As Single = 5 'just a dummy value
For i As Integer = 0 To 50
ReDim Preserve arrOutputData(i)
arrOutputData(i).Days = sindays
Next

regards

Michel Posseth [MCP]

 

[Branco Medeiros]

I declared the array with the statement:

Dim arrOutputData() As Data

I don't know the length of the array when I am declaring it. Is that
why I have to initialize the array? If I were to redim the array,
would I have to initialize it?

<snip>

Redim and Redim Preserve both 'understand' non-initialized arrays,
therefore you may pass your aray to them and they'll do the right
thing, *unless*... remember that you can only access members of your
array after it's already initialized. So, a typical use of Redim
Preserve, like the following code, may fail if you forget the array
initialization:

'adding a new element to the array:
Redim Preserve arrOutputData(arrOutputData.GetLowerBound(0) + 1)

The code above will fail because GetLowerBound(0) wil be accessing
Nothing. If you use this pattern, the safest approach is to initialize
the array on the declaration, as suggested, or use something like the
following:

'adding a new element to the array:
If arrOutputData Is Nothing Then
Redim arrOutputData(0)
Else
Redim Preserve arrOutputData(arrOutputData.GetLowerBound(0) + 1)
End If

Personally, I prefer the initialization upon declaration, so I don't
need to test:

Dim arrOutputData(-1) As Data

Notice that in the current version of the framework you can use generic
lists, which are more versatile than arrays.

Dim OutputData As New List(Of Data)

Since using generics I don't remember ever declaring an array
anymore... =))

HTH

Regards,

Branco.