win32 DLL and LPTSTR

[Steven Blair]

Hi there.

Having a problem calling a win32 function from a dll.
Here is the function header:

DWORD thrdGetHcNum (DWORD nPort, DWORD nBaudrate, LPTSTR lpszHcNum)

//Description of the 3rd parameter
LpszHcNum: The head of the buffer area used to store the serial number
of the terminal, 9 bytes at least.

Here is my code:

[DllImport("comdll.dll", SetLastError = true)]
public static extern uint thrdGetHcNum(int nPort, int nBaudrate,
[MarshalAs(UnmanagedType.LPTStr)] ref StringBuilder lpszHcNum);

//The actual call
StringBuilder Buffer = new StringBuilder(10);
result=thrdGetHcNum(Port,Baud,ref Buffer);

Everytime I runt his, I get a Object not set to reference error.
Can anyone tell me what I am doing wrong?
I have tried using a string, a char[] buf = new[10], MarshalAs.LPTstr
and MarshalAs.Array but always hit the same problem.

I am calling other functions from the dll without any problem. The only
difference with this one is I should be getting a string back with some
data in it.

Any help on this would be appreciated.

Regards,

Steven




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[Wiktor Zychla]

[DllImport("comdll.dll", SetLastError = true)]

public static extern uint thrdGetHcNum(int nPort, int nBaudrate,
[MarshalAs(UnmanagedType.LPTStr)] ref StringBuilder lpszHcNum);

I think that "ref" is not necessary here. Reference types are always passed
by reference. With "ref" you pass a reference to the reference.

[DllImport("comdll.dll", SetLastError = true)]
public static extern uint thrdGetHcNum(int nPort, int nBaudrate,
[MarshalAs(UnmanagedType.LPTStr)] StringBuilder lpszHcNum);

I hope this will help
Regards, Wiktor Zychla