fixed array inside a structure

[SleepyTim]

Hi,

 

Im looking into VB.net for a project but I have come against a problem. Structures required in some DLLs have arrays of byte such as

[vB]

Type WSAData

A(100) As Byte

B As Integer

End Type

[/vB]

 

The problem is I cant find out how i can replicate this structure in VB.net since you cant use Type definition but have to use Structure, in which you can only define a reference to a dynamic array.

 

 

the problem is thus:

 

[vB]

Structure MyStruct

Public MyArray() As Integer

Public MyInt As Integer

End Structure

 

Sub UseStruct()

Dim Struct As MyStruct

ReDim Struct.MyArray(9) ' Dimension as 10 elements

Struct.MyArray(2) = 777 ' Use the array.

End Sub

[/vB]

 

would in fact produce something like the following behind the sceens:

Pointer MyArray (this uses up 4 bytes of data)

Integer MyInt (this uses up 4 bytes of data)

 

Somewhere else in memory a block of 10 bytes of memory is allocated and the variable MyArray references it

 

 

In contrast old style VB code

 

[vB]

Type MyStruct

MyArray(9) As Integer

MyInt As Integer

End Type

[/vB]

 

would produce something like the following behind the sceens:

Integer *10 MyArray (this uses up 4*10 bytes of data = 40)

Integer MyInt (this uses up 4 bytes of data)

 

So as you can see there is a big difference in terms of where the data is stored in memory, and the DLL im interfacing with will only accept the data in the format produced by the Type definition.

 

 

Is there any way arround this?

 

thanks

 

Tim

[Volte]

I'm not 100% sure about what your problem is, but you may want

to try declaring your Integers as Int16 in your structure, if it's the

number of bytes you're worried about. I believe that Integers in

VB.NET are the same as Longs in VB6, and Int16 represents the

original Integers from VB6. I could be mistaken though.

[SleepyTim]

Thats not what i need but thanks..

 

The problem I have is that

Structure MyStruct
  Public MyArray() As Integer
  Public MyInt As Integer
End Structure

Sub UseStruct()
  Dim Struct As MyStruct
  ReDim Struct.MyArray(3) ' Dimension as 10 elements
End Sub

Will make the following in RAM :

({MyArray - pointer to an array = 4 Bytes}{MyInt - Integer = 4 Bytes})

And somewhere else in memory allocated dynamically

({MyArray[0] - Integer = 4 Bytes}{MyArray[1] - Integer = 4 Bytes}{MyArray[2] - Integer = 4 Bytes}{MyArray[3] - Integer = 4 Bytes})

 

But what I need is:

({MyArray[0] - Integer = 4 Bytes}{MyArray[1] - Integer = 4 Bytes}{MyArray[2] - Integer = 4 Bytes}{MyArray[3] - Integer = 4 Bytes}{MyInt - Integer = 4 Bytes})

 

In this case it could be simulated by doing

Structure MyStruct
  Public MyArray0 As Integer
  Public MyArray1 As Integer
  Public MyArray2 As Integer
  Public MyArray3 As Integer
  Public MyInt As Integer
End Structure

But this is not a nice work arround for starters and also some of the structures have 250+ ellement and so would be take up a lot of space and time to define like this.

 

I have posted this problem on other VB message boards but no one seems to know of a solution.

 

If anyone knows of one then Please do let me know

 

thanks

Tim

[divil]

You won't find a way to change the way VB.NET handles arrays in structures, but you WILL find a way to make it marshal the structure correctly to your DLL. At this point you may want to tell us more about your DLL and what it's expecting, but you can use attributes to make sure that when your structure is passed, any arrays are passed with all their values sequentially, just like you need.

 

< StructLayout( LayoutKind.Sequential )> _
Public Structure MyArrayStruct 
  Public flag As Boolean
  < MarshalAs( UnmanagedType.ByValArray, SizeConst:=3 )> _
  Public vals As Integer()
End Structure 'MyArrayStruct

 

Can I suggest looking at the MSDN page under Programming .NET -> Interoperating -> Interop Marshaling -> Platform Invoke -> Structs

 

Or, if you have the SDK installed,

 

[mshelp]ms-help://MS.NETFrameworkSDK/cpguidenf/html/cpconstructssample.htm[/mshelp]

Edited December 12, 2002 by Derek Stone