Pinvoke a VB6 function from C#.Net 2010

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I want to PInvoke a function written in VB6 from C#. The VB6 code that calls the DLL is declared like this:

C#

Declare Function ApplyNNet Lib " location of the DLL file" (MyNNetType As String, MyAddress As String, MyInput() As Double) As Variant

My C# version looks like this:*

C#

[DllImport("NNetApply.dll")] 
public static extern object ApplyNNet(string type, string add, double[] data);

I call the function like this:

C#

object P_ = ApplyNNet("Back Prop", address,data);

With this declaration I receive a PInvoke error:

C#

PInvoke restriction: cannot return variants.

Posted 28-Jan-13 7:55am

Fiona.fj

Updated 28-Jan-13 8:00am

v2

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Sergey Alexandrovich Kryukov 28-Jan-13 14:08pm

Why? why?! If you are already use VB.NET, why all this trouble?
Why variants, one of the worst "inventions", ever?
—SA

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Dave Kreskowiak · Answer 1 · 2013-01-28T09:35:00

Solution 1

First, if you've got the VB6 code calling this .DLL, why are you calling the VB6 code from C#?? Just bypass the VB6 code and go straight to the .DLL!

Second, You can't call a VB6 function that returns Variant. There is no equivilent type in .NET. You have to declare the actual type being returned.

Posted 28-Jan-13 9:35am

Dave Kreskowiak

Comments

Sergey Alexandrovich Kryukov 28-Jan-13 16:47pm

Of course, a 5. Pointless question and artificial problem, "I cannot use VB6 but I want"...
—SA

Fiona.fj 29-Jan-13 11:56am

I dont know how to program with vb6, so i prefer to write my project in c#.net. and now this problem occurs and i dont know what should i do!

Dave Kreskowiak 29-Jan-13 12:57pm

You said your VB6 code is calling a .DLL. I take it you're trying to call this very same .DLL with your C# code?? It's not very clear what you're doing.

Is this other .DLL also a VB6 .DLL?? Does the function you're trying to call return a Variant?? If so, you're screwed.

Fiona.fj 29-Jan-13 13:03pm

No, I said my code is in C#, and I wrote a declaration of the function in vb6 that was in my dll's tutorial. this function returns Variant, yeah. so you mean I cannot do any thing?

Dave Kreskowiak 29-Jan-13 13:25pm

If the function you're calling returns a Variant, you're screwed. You cannot call that function from managed code (C#, VB.NET, any other managed language).