Object reference not set to an instance of an object.

[HJo]

Hi, I am trying to create a C# interface for a dll with functions. The C++ interface for one of the functions looks like this: __declspec(dllimport) long _cdecl GetLiveList( long, long*, long*, long[]*, long); I declared my C# equivalent as: public class Ismbus32 { //... [DllImport("ismbus32.dll", CharSet=CharSet.Auto, CallingConvention=CallingConvention.Cdecl)] private static extern long GetLiveList( long, ref long, ref long, ref long[], long); } When I try to call the function, using the code below, I get a NullPointerException and the message "Object reference not set to an instance of an object." public class Form1 : System.Windows.Forms.Form { //... private void btn_Click(object sender, System.EventArgs e) { long in = 2; long out1 = 0; long out2 = 0; long[] items = new long[635]; long itemsMax = 634; long lRes = Ismbus32._CDDLG_GetLiveList(in, ref out1, ref out2, ref items, itemsMax); } } Does anyone have a clue?! Thanks in advance, /Henrik Johansson

[Roger Stewart]

It looks as if you have a class, Ismbus32, that wrappers the ismbus32.dll. If so did you create an instance of this class?? For example:

private void Form1_Load(object sender, System.EventArgs e)
{
    Ismbus32= new Ismbus32();
}

Also, you have GetLiveList() defined in your C++ dll as dllimport. Shouldn't you have it defined as dllexport?? Roger Stewart "I Owe, I Owe, it's off to work I go..."

[HJo]

Thank you for your reply! There is no need to create an instance, since the methods are declared static. I'm sure GetLiveList() is declared dllexport in the dll. The C++ code was a header file used when importing the dll into C++ code, which I have used as a model for my C# interface. The problem turned out to be the difference of the long data type between C++ and C#: C++ long is 4 bytes whereas C# long is 8 bytes and C# int is 4 bytes. So the solution was to replace all occurences of long in the interface with int. :) Regards, /Henrik