Dim VersionNum As String

<DllImport("Vertx.dll")> Public Shared Function GetDLLVersion(ByRef VersionNum As String, ByVal DataSize As Integer) As Integer
End Function

GetDLLVersion(VersionNum, 10)

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note: the original structure of the GetDLLVersion is:

int GetDLLVersion(Char *data, int datasize)
data = array to caontain the DLL Version
datasize = size of data

So, the original structure should be a C language, thus i try to store the *data into the variable name VersionNum using the method of ByRef, is this wrong?

I believe it should be the opposite to both parameters:

(Byval VersionNum As String, Byref DataSize As Integer

Why dont you use DllGetVersion which is OS built in function?

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Why it should be opposite?? Not that "Char *data" going to update the variable name "data"?? The "VersionNum" will be update if we use byVal for "VersionNum"?? Actually i am testing out how to store the returned value from the function called, so i wish to know is there any reference that i can look for to solve this problem... since char *data is only one of it... i got another problem is where the function require a parameter of HWND type but VB.NET doesn't support that anymore.

I dont understand the first question. VB and C manages strings differently. You dont store the value, you just pass the string to the API, and set the string to the lenght that can accept the size of the data... YOu can see the example of GetComputerName API, which works similary as I believe this API does. Again, I dont understand why are you using something that you dont know how it works, and if it works correctly, since you have MS API I posted above, that I believe does the same thing.

As for the HWND, in VB6 it is of Long type, in VB.NET it is of Integer type.

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Ya, i know Microsoft API can done that, but i just wanna try to deal with the dll other than the window API, since there are still more function i need to call from that dll, so at this stage, i am trying to make myself familiar with controlling the API. So, what you mean of pass the string to the API?? and how can we set the string to the lenght that can accept the size of the data? that is what i really cant understand, i got read through many example done by other, what i saw is some of them use the IntPtr method to store the returned pointer and later on use the marshal.copy to copy the value to a valiable. some are just use the <marshalas(unmanagedtype....)> to done this. I actually blur with that already, which one should be used?? As the API doc i have for the dll file, it state that the version value will be returned after we call that function, so what i trying to do now is to retrieve that value. Could you guide me on this?

Ok try this code, and see if it works. If there are errors, post the error message and the line that it causing it.

' module declaration
' change the DllName.dll to the actual dll's name

Declare Function GetDLLVersion Lib "DllName.dll" (ByVal VersionNum As String, ByRef DataSize As Integer) As Integer

' form code

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click         Dim VersionNumber As String         Dim RetVal As Integer         VersionNumber = Space(256)         RetVal = GetDLLVersion(VersionNumber, 256)         MessageBox.Show(VersionNumber.Substring(0, RetVal))     End Sub

[Edited by Goran on 16-03-2006 at 01:48 AM GMT]

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quote:

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Goran wrote:
Ok try this code, and see if it works. If there are errors, post the error message and the line that it causing it.

' module declaration
' change the DllName.dll to the actual dll's name

Declare Function GetDLLVersion Lib "DllName.dll" (ByVal VersionNum As String, ByRef DataSize As Integer) As Integer

' form code

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click         Dim VersionNumber As String         Dim RetVal As Integer         VersionNumber = Space(256)         RetVal = GetDLLVersion(VersionNumber, 256)         MessageBox.Show(VersionNumber.Substring(0, RetVal))     End Sub

[Edited by Goran on 16-03-2006 at 01:48 AM GMT]

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YOu need to create a buffer enough to hold a string. Try this:

Versionnum=space(10) RetVal = GetDLLVersion(VersionNum, 10)

just beware that if version has more than 10 chars, rest of it will be truncated.

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Ahhh...... I didnt see that you have postedd c declaration. It is expecting BSTR string to be passed to it, not the LPSTR, so declare a string ByRef..

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So, is that the whole program should be like this?

Declare Function GetDLLVersion Lib "Vertx.dll" (ByRef VersionNum As IntPtr, ByVal DataSize As Integer) As Integer Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load         Dim VersionNumber As String         Dim RetVal As Integer         VersionNumber = Space(10)         RetVal = GetDLLVersion(VersionNumber, 10)         MessageBox.Show(VersionNumber.Substring(0, RetVal))     End Sub

But the VersionNumber still return empty string, what actually wrong with that??

I assumed that this particual function as a result returns the number of characters that is stored in a buffer, like many similar Win32 API's do, but could be it isnt the case with this API. So, to be sure try without the SubString method, I believe it will work

MessageBox.Show(VersionNumber)

If it displays a version number, then you may check for DataSize parameter, which could contain the number of characters that are returned in VersionNum...

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Goran,

it still doesn't work, after i change the ByVal to ByRef, when ever i try to call the function, an error occur --> "object reference not set to an instance of an object" it is a system.NullReferenceException . What does that mean??

i had attached the dll file, if you have time, plz do help me to test it out. thanks very much

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Attached:
VertX.zip 47 KB (Downloads: 1)

It seems that your C declaration is not correct... The first advice I gave you was the correct one: string should be passed ByVal, not ByRef. If you have a char variable in C, in Vb it should be array of bytes, as I recall, but I thought it could be done by passing a BSTR stringByRef (the actual buffer), opposite to LPSTR string, which should be passed ByVal - which means you are passing pointer to a buffer.

Anyway, if you have send this dll earlier, we didnt have to go through all this troubles.

Private Declare Function GetDLLVersion Lib "Vertx.dll" (ByVal VersionNum As String, ByRef DataSize As Integer) As Integer Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click         Dim VersionNumber As String         Dim BufferSize As Integer = 255         Dim RetVal As Integer         VersionNumber = Space(BufferSize)         RetVal = GetDLLVersion(VersionNumber, BufferSize)         VersionNumber = VersionNumber.Substring(0, VersionNumber.IndexOf(Chr(0)))         MessageBox.Show(VersionNumber)     End Sub

[Edited by Goran on 17-03-2006 at 02:46 AM GMT]

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woooo, Goran, you are genious!!!
I work with this everyday to figure out the solution but i still cant get, since like you need little time only to solve that!!!


But can i ask you some question which i really cant understand:

1) As i know, ByRef is used with the function to change the variable of the caller right?? Eg:

Function CalTotal(ByRef Total, ByVal A, ByVal B)     Total = A + B End Function Dim Total, A, B as integer A=1 B=1 CalTotal(Total, A, B)

After the function called, Total will be 2 right??
But since like the structure of GetDLLVersion is:

int GetDLLVersion (Char *data, Int datasize)

not that it mean this function going to return the data throught *data?? then why we are using ByVal? Why not ByRef? since ByRef used to return the value.

other than that, why we used ByRef for the int datasize?? why not ByVal?? since we are passing data inside.

2)i remember that you said hWnd in VB = Handle in VB.NET, what actually is hWnd and Handle?? i did read some reference from net, it said that handle in VB.Net is an int32, what that mean?? is the integer pointed to the Handle??
i got another function which is:

int Initialization(int port, HWND hWnd, HINSTANCE hInst)

i understood the int port, but how we going to declare this function??Is this correct??

Private Declare Function Initialization Lib "Vertx.dll" (ByVal Port As Integer, ByVal HWND as HandleRef, HINSTANCE as Int32) As Integer

The API Doc explan that HINSTANCE = instance in program using DLL
What is instance a method??

Sorry for asking so many question, cox i am really new to all this kind of things. Thanks for helping me  

[Edited by eddom on 17-03-2006 at 11:07 PM GMT]

quote:

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then why we are using ByVal? Why not ByRef? since ByRef used to return the value.

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quote:

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other than that, why we used ByRef for the int datasize?? why not ByVal?? since we are passing data inside.

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quote:

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2)i remember that you said hWnd in VB = Handle in VB.NET, what actually is hWnd and Handle?? i did read some reference from net, it said that handle in VB.Net is an int32, what that mean?? is the integer pointed to the Handle??

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So, it will be the same case for the unsigned Char *data and structure type as well??
We just have to create a structure with enough size to store the record??

Ps: i got read some post from other ppl, they solve this kind of problem using the IntPtr, Marshal or StringBuilder. So, is there any different with the method we used here?? It seen like those method are quite troublesome, but did it provide other advantages?

IF depends on a structure.... some of them needs a space to allocate, some dont.... MOst strucutures that need to have xxSize memeber, which will hold the number of bytes that need to be allocated to this strucutre.

I am still talking here how it is dealt with it in VB6, not VB.NET. I didnt have problems with API's when tried them in Vb.NET, so I am not aware of the other methods you are saying. If problems arise in the future, then I will search in that direction.

And I am not a C programmer, so I dont know how to do it will all types, I know only about those I have dealt with it, but once you understand how API works with parameters, it will be easier to make it to work.

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Thanks for you guide, at least now i quite understand bout how all those parameter and variable work. I appreciated your help here. Thanks   . you are really