DLLImport - System.NullReferenceException

[James]

Greetings,

I have a PowerBasic unmanaged library that I wish to call from C#.
Here's the PB code...

FUNCTION Hello(str AS STRING) EXPORT AS STRING
FUNCTION = "Hello World " & str
END FUNCTION

Simple enough. I can successfully call it from a VB6 component but
can't seem to get the C# syntax right. I've tried the following with
no success. Each time, I get this error at the call to HELLO. Any
ideas?

System.NullReferenceException: Object reference not set to an instance
of an object.

**Attempt 1**
[DllImport("Hello.dll", EntryPoint="HELLO")]
public static extern string HELLO(string someText);
Console.WriteLine(HELLO("blah"));

**Attempt 2**
[DllImport("Hello.dll", EntryPoint="HELLO", CharSet=CharSet.Auto)]
public static extern string HELLO(ref string someText);
string str = "blah";
Console.WriteLine(HELLO(ref str));

**Attempt 3**
[DllImport("Hello.dll", EntryPoint="HELLO")]
public static extern string HELLO([MarshalAs(UnmanagedType.LPStr)]
string someText);
Console.WriteLine(HELLO("blah"));

**Attempt 4**
[DllImport("Hello.dll", EntryPoint="HELLO")]
public static extern string HELLO(StringBuilder someText);
StringBuilder sb = new StringBuilder("blah");
Console.WriteLine(HELLO(sb));

-Cliff

 

[Mohamoss]

Hi
where is Hello.dll, since it is not a system file locate it into the exe
directory
Mohamed Mahfouz
MEA Developer Support Center
ITworx on behalf of Microsoft EMEA GTSC

 

[James]

Hi,
Yes, Hello.dll and the exe are in the same directory (..\bin\debug).
I've tried moving it to other folders with the same results.
What else could cause an issue here? I'm stumped.
-Cliff

 

[Mohamoss]

HI
You need to debug your program and step into each statement to get
exactly where this exception occurs , this will help a lot since there is
no obvious reason we can see now
Mohamed Mahfouz
MEA Developer Support Center
ITworx on behalf of Microsoft EMEA GTSC