Passing Integer Value from .NetCF to eVC4 dll | Tech Off

mohit.chawla
Maverick

Hi,

I am trying to pass an integer value from .NET CF to evc4 ATL Com dll function through <dllImport> attribute. As soon as i Pass this value this value is converted to 0 in evc4. For eg: if i am pass an interger value 7 thorugh .Net CF, then i receive an interger value 0 in eVC4 dll function.
Can anyone help me find the reason for this behaviour. Below is the signature for both functions at both ends:

In eVC4 Dll function defination:

#define EXP extern "C" __declspec(dllexport)

EXP int Sample::intValPassTest(int var)
{
/* //Code for converting the integer value to string and see it through MessageBox in .Net CF

TCHAR arr[5];
wsprintf(arr,TEXT("%d"),var);
MessageBox(NULL,arr,L"emVC4",NULL);
*/

return var;

} ///Class Sample declaration with all function declarations is given in a Header file which is included in the project.

VB.Net CF Function.

<DllImport("Sample_ATL_COM.dll", CallingConvention:=CallingConvention.Winapi, CharSet:=CharSet.Auto, EntryPoint:="intValPassTest", SetLastError:=True)> _
Public Shared Function intValPassTest(ByVal i As Integer) As Integer

End Function

And I am calling this function on a button_click event. And its code is written below:

Private Sub Button10_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button10.Click
        Try

            Dim i As Integer = 55
            Dim ii As Integer
            ii = intValPassTest(i)
            MsgBox(ii)

        Catch ex As Exception
            MsgBox(ex.ToString)
        End Try
    End Sub

Any help from your side is invited, thank you in Advance,

mohit chawla

Mike Dimmick

It looks like you're trying to use a class instance member function. This won't work: P/Invoke only works with 'flat' APIs. You should make your C++ function a global-scoped function - your extern "C" declaration will then work and the entry point name will not be decorated.

Technical detail: the C++ member function is expecting the 'this' pointer to be passed as the first argument (in register R0 on an ARM chip), and 'var' as the second argument (register R1). The P/Invoke is passing your value of 55 as the first argument (R0 = 55), and no second argument (R1 keeps whatever value it had). If you actually referenced any instance members in your C++ function, it would almost certainly crash with an access violation or datatype misalignment exception, since address 55 is not a valid pointer. Since you aren't, it simply doesn't work.

mohit.chawla
Maverick

Hello Mike,
Thanks a lot for you reply, can you please please change the code that i have put in my first post and then send post on the same link. I understood what to do but not how to do it. so if you dont mind then please send me the code snippet also please...

Thank you,

mohit.chawla
Maverick

Thank you Mike, I was able to call the function successfully by making that function global.

Now when i am trying to pass a long datatype value from .net CF to eVC4 global function, it shows a NOT SUPPORTED EXCEPTION. I am very confused about this behaviour since if we can use an interger than why not a long data type value.
I have also tried passing the long value byRef, but it doesnot seem to work aswell.

Any help from your side is appriciated,

Thank you

Sven Groot
You can't have everything; after all, where would you put it?

A long in VB.NET is 64 bits. In regular VC, this is the equivalent of the "long long" or "__int64" data type, NOT "long". In most C++ implementations, both int and long are 32 bits. I assume this applies to eVC4 as well.

Try to use "long long" or "__int64" as the data type on the C++ side, and see if that works.

prateekgarg

Hi Sven,

My name is Prateek and I am working with Mohit on the same project.

In eVC4 __int64 is supported. But even after using this I am getting the same error "System.NotSupportedException"

Any suggestions.

Thanks.

Prateek

AndyC

long can only be marshalled if it's ByRef (ref in C#)

See this MSDN article

prateekgarg

Hi Andy,

I have already tried passing long parameter ByRef in VB .Net but it not working and giving me the same "SystemNotSupportedException" exception.

Thanks.

Prateek

Mike Dimmick

Hacking up the previous example:

C++ bit:

EXP int intValPassTest(__int64* var)
{
    return (int) *var;
}

VB bit:

    <DllImport("mydll.dll")> _
    Private Shared Function intValPassTest(ByRef var As Long) As Integer
    End Function

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        MessageBox.Show(intValPassTest(55).ToString())

    End Sub

prateekgarg

Thanks Mike thanks.
Your comments helped me a lot.

Mike I have another problem in passing a string from .Net CF to eVC4 Dll which I have already posted as a seperate query.

please help me out in that also.

Thanks.

Prateek