Using a C DLL in Visual Basic.net

[Dave]

I have a DLL which is compiled from C which I need to use in vb.net.

 

So far I have this:

 

Declare Auto Function UnCompress Lib "lzss.dll" Alias "_uncompress_data" (ByVal Data As String, ByVal Length As Integer) As String

 

But when trying to run this:

 

dim out as String = UnCompress(indata, len(indata))

 

I get the following error message:

 

An unhandled exception of type 'System.NullReferenceException' occurred

 

Additional information: Object reference not set to an instance of an object.

 

The C declaration of this function is as follows:

 

unsigned char __declspec(dllexport) *uncompress_data(unsigned char *inbuf,int *buflen)

 

What am I doing wrong?

[vsnt]

Try This

 

Declare Auto Function UnCompress Lib "lzss.dll" Alias "_uncompress_data" (ByVal Data As String, ByVal Length As Integer) As String

 

UnCompress(indata, len(indata))

 

dim out as String = indata

[Dave]

Still gets the same error!

[vsnt]

OK then try this

 

Declare Function UnCompress Lib "lzss.dll" Alias "_uncompress_data" (ByRef Data As String, ByRef Length As Integer) As String

 

UnCompress(indata, len(indata))

 

dim out as String = indata

[Dave]

still the same error

[PlausiblyDamp]

What is the contents of the string you are passing in?

[Dave]

well, it could be any string but it will actually be a file

[vsnt]

unsigned char ???? Could be that the dll function is looking for a char when you are passing it a string.

[Dave]

it's an array of chars, but changing the type to Char() makes no diffrence and if you change the returned type to Char() you get a 'Could not marshal the return data type' error