Parameterless constructor in Structure

Question

Following on from this question on using different Visual Basic versions in Visual Studio 2015, I'm running through the new lanugage features in Visual Basic 14, as documented here and here.

One of those is the ability to have parameterless constructors in structures, like this:

Structure MyStruct1
   Public f As Integer
   Sub New()
      f = 15
   End Sub
End Structure

When I try this in code in Visual Studio 2015, I'm still getting an red error squiggle under the New():

BC30629 Structures cannot declare a non-shared 'Sub New' with no parameters.

I haven't seen anywhere that states that this got pulled before release.

Am I mis-understanding what this new feature does?

I don't have an answer, however I can confirm that it doesn't work in VS 2015 (Community), independent on what framework is targeted. (So it's not related to the specific installation). I'm courious as well. — Jens, Aug 24 '15 at 11:30
Have you tried making the constructor public? I think that might be a requirement. — Chris Dunaway, Aug 24 '15 at 13:42
@ChrisDunaway: Makes no difference. FWIW, the example given above is exactly as it is on one of the linked documentation pages. — Jon Egerton, Aug 24 '15 at 13:53
I wasn't sure. I recall seeing something about C# struct constructor needing to be public and wondered if VB was the same. — Chris Dunaway, Aug 24 '15 at 14:05

score 5 · Accepted Answer · edited Aug 26 '15 at 13:49

As you can see in the quoted text below, Roslyn removed support for structures with parameterless constructors, and therefore it's not listed as a new feature in VB 14.

It has been a long standing requirement that C# and VB struct constructors would always have parameters. We have tried to relax this requirements in C#6.0 to make structs more consistent with classes.

While overall parameterless constructors in structs are valid from IL perspective, without a convenient way to declare them they were virtually nonexistent. As we performed more and more testing, we kept discovering cases where parameterless struct constructors caused inconsistent behavior in libraries or even in some versions of CLR.

A good example is the issue reported in the CodePlex bug http://roslyn.codeplex.com/workitem/465. The issue is basically an optimization introduced in Activator.CreateInstance around CLR 4.0 and present ever since. The optimization assumes that parameterless instantiation of generic T type does not cause sideeffects if T is found to be a struct and therefore instances can be cached. Parameterless struct constructors would violate such assumptions and make optimization observable, thus necessitating servicing of existing code several versions back.

After reconsidering the potential issues arising from breaking long standing assumptions, we decided it was best for our users to restore the requirement on struct constructors to always have formal parameters.
Source: Restore requirement for struct constructors to always have formal parameters. #1029

Its a shame that that particular Roslyn wiki page doesn't appear in search results (for VB 14 or VB 2015 for example) - the one that does is the blog post that links through to the out-of-date list. — Jon Egerton, Aug 24 '15 at 21:13
It only mentions C# (6.0). Was VB.NET also changed? Or was it only tried in C#? — Peter Mortensen, Aug 26 '15 at 13:51

score 0 · Answer 2 · edited Jul 01 '18 at 15:19

You need to use Shared Keyword(acts as static) for a Parameter less Constructor in VB.Net.

Try the code below:

Structure MyStruct1
   Public Shared f As Integer
   Shared Sub New()
      f = 15
   End Sub
End Structure
Class My_Class
    Public Shared Sub Main()
        Dim str1 As New MyStruct1()
    End Sub
End Class

Note: Shared keyword here acts as static. While using a Shared Constructor you need to access Shared variables, that is why I have declared the variable with Shared keyword.

Parameterless constructor in Structure

2 Answers2

Linked