How can I use VB.Net extension methods in a C# project

Question

I have a legacy VB.Net class library that includes some extension methods, one of which is roughly this:

Namespace Extensions
    Public Module OrmExtensions

        <Extension()>
        Public Function ToDomainObjectCollection(ByRef objects As OrmCollection(Of OrmObject)) As DomainObjectCollection
            Return objects.AsQueryable().ToDomainObjectCollection()
        End Function

        <Extension()>
        Public Function ToDomainObjectCollection(ByRef objects As IQueryable(Of OrmObject)) As DomainObjectCollection
            Dim doc As New DomainObjectCollection()
            For Each o In objects
                doc.Add(o.ToDomainObject())
            Next
            Return doc
        End Function
    End Module
End Namespace

To use these extensions in VB I just have to import Extensions.OrmExtensions. I have another project, which is in C# that depends on the VB one with the extensions and I can't get them to work. OrmExtensions isn't available and simply using the namespace VBProject.Extensions doesn't make the extensions available.

There are multiple projects that depend on the VB project and ideally the extensions should be available to all of them.

I've done a fair bit of googling but haven't been able to turn up anything about actually using VB extension methods in C#. I assume the issue is that they're required to be in a Module, but I haven't been able to confirm that.

I'd rather not duplicate the extension methods everywhere they're required(especially in our unit test project, which is in C#).

What happens if, in C# with the correct using, you type OrmExtensions. and look for the methods? How do they appear? Do they look like valid extension methods? — Tim S.
compile those extension method as an external library and then add a reference to that library from your c# projet — Rémi
@SLaks i would disagree. If you need to extend IQueryable then that would be acceptable. Perhaps you need to explain further? — Simon Halsey

SLaks SLaks · Accepted Answer · 2013-05-08T17:21:27

You should not be using ByRef.

VB.Net (apparently) supports extension methods on ref parameters; C# (rightfully) does not.

A ByRef (ref in C#) parameter allows the method to assign a new instance to the variable or field passed by the caller.
They should rarely be used.

How can I use VB.Net extension methods in a C# project

1 Answers