Calling a C++ DLL function from VB.NET having structures pointer (RunTime Error : Specified Array was not of the expected type)

Question

Hi ,

I have function in C++ dll which accept structure pointer I want to call this function using dllimport in VB.NET 
Can anybody help me to do this.

C++ structure
struct MyStruct

{
int i;
float j;
} ;

EXPORT bool func(struct MyStruct * lpSlot)
{
    //some code like this

    lpSlot(0).i =10

    lpSlot(0).j =10.5

    lpSlot(1).i =20

    lpSlot(1).j =20.5

    lpSlot(2).i =30    lpSlot(2).j =10.5

}

VB.Net :

Declare Function ParseRackStatus Lib "TestC.dll" Alias "_ParseRackStatus@12" (<MarshalAs(UnmanagedType.SafeArray, SafeArraySubType:=VarEnum.VT_ARRAY)> ByRef lpSlot() As MyStruct) As Boolean.

Public Function ParseRackStatusTest(ByRef lpSlot() As MyStruct) As Boolean
         Return ParseRackStatus(lpSlot)
 End Function

I am getting Runtime error,when i calling above  function "ParseRackStatusTest".

RunTime Error :  Specified Array was not of the expected type.

Answer 1

This should do it:

Declare Function ParseRackStatus Lib "TestC.dll" Alias "_ParseRackStatus@12" (ByVal lpSlot() As MyStruct) As Boolean

I don't know how you can have two functions with the same signature. The second function calls itself recursively.

Don't forget to initialize the array before passing it to the function. According to the C code, it must have at least three items.

EDIT: Could a moderator please move this to the Interop forum? Thanks.

---

Armin

Answer 2

Addendum: Safe arrays are "COM style": Default Marshaling for Arrays

---

Armin

Answer 3

HI ,

Declare Function ParseRackStatus Lib "TestC.dll" Alias "_ParseRackStatus@12" (ByVal lpSlot() As MyStruct) As Boolean

I tried above function  also .I am getting error message like this "Attempt to read or write protected memory".

Declare Function ParseRackStatus Lib "TestC.dll" Alias "_ParseRackStatus@12" (<MarshalAs(UnmanagedType.SafeArray, SafeArraySubType:=VarEnum.VT_ARRAY)> ByRef lpSlot() As MyStruct) As Boolean.

Public Function ParseRackStatusTest(ByRef lpSlot() As MyStruct) As Boolean
         Return ParseRackStatus(lpSlot)
 End Function

Before calling the ParseRackStatusTest function,Ititilized the array.

Answer 4

IMO, the signatures match:

   bool func(struct MyStruct * lpSlot)

   Declare Function ParseRackStatus Lib "TestC.dll" Alias "_ParseRackStatus@12" (ByVal lpSlot() As MyStruct) As Boolean

If someone sees a difference, please let us know.

What's missing is the Structure declaration. How does it look like? It should be...:

   Imports System.Runtime.InteropServices
   <StructLayout(LayoutKind.Sequential)> _
   Structure MyStruct
      Public i As Integer
      Public float As Single
   End Structure

---

Armin

Answer 5

Hi ,

I declared like this :

<StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Ansi, Pack:=1)> _
   Structure MyStruct
   <MarshalAs(UnmanagedType.I4)> _
   Dim i As Integer
   <MarshalAs(UnmanagedType.R4)> _
  Dim float As Single
 End Structure

Answer 6

And without pack:=1 ?

The type is blittable, i.e you don't need to apply further attributes.

---

Armin