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vb.netvariablesconstructorinitialization
Lou
Lou

Reputation: 2509

VB.Net: Initialising variables with the New constructor

When initialising a list or a queue or a stack or anything similar, which method is preferred?

Dim q as Queue(Of Integer) = New Queue(Of Integer)

or

Dim q as New Queue(Of Integer)

Also, I've started to use New for string and integer variables - is this stupid? Is there any disadvantage to using New rather than just setting the variable to the default setting? E.g.

Dim Num1 As New Integer
Dim Str1 As New String("")
Dim Bool1 As New Boolean

Thank you!

Upvotes: 2

Views: 911

Answers (1)

Hans Passant
Hans Passant

Reputation: 941585

If you'd ask programmers whether they like typing more or less when writing a program then the usual answer is "less!". If you ask them whether they like more or less bugs, the usual answer is "less!" Those are conflicting goals.

The As New syntax has been part of VB.NET for a very long time. It however does come with strings attached, you leave it entirely up to the runtime to figure out whether or not a new object should be created. That does tend to be a bug generator. Sometimes you really do want to create a new object, even though the variable is already assigned. It is also rather ambiguous, in this snippet for example:

For ix As Integer = 0 To 42
   Dim q As New Queue(Of Integer)
   '' etc...
Next

Question is: do you get one instance of the queue, created in the first pass of the loop or do you get 43 of them? What was actually intended by the programmer? It isn't very clear from the syntax.

There is a 3rd alternative that you overlooked and the one I prefer. Available since VB9 (aka VS2008) called "type inference". Where you don't specify the type of the variable but leave it up to the compiler to figure it out. This option needs to be turned on with Option Infer On, it is turned on by default. It combines the advantages of the abbreviated syntax that As New has and still lets you create the object explicitly in your code with the New statement:

Option Infer On
...
For ix As Integer = 0 To 42
   Dim q = New Queue(Of Integer)
   '' etc...
Next

Where q is inferred to be of type Queue by the compiler and it is crystal clear that the code generates 43 instances of the queue. The exact equivalent in the C# language is the var keyword.

Upvotes: 2

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