Calling unmanaged C++ dll function with char**

[Elikhom]

Hi,

I'm calling an unmanaged C++ dll from C#, everything was fine until I got stuck in one of the methods that goes like this...

C++ part
void populateFieldCategories(char** categories)

I have tried using something like this in C# to get the array of strings but doesn't work...
private static extern void populateFieldCategories([MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.LPArray)] byte[][] categories)
with an error like "There's no marshaling support for nested arrays"

and I have also tried something like

private static extern void populateFieldCategories([MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.LPStr)] string[] categories)
but fails saying "Attempted to read or write protected memory"

I don't know what else to try, if someone want to pass a char** from C++ how can you get the information in C#?

Thanks

[Elikhom]

One thing that I forgot to pot is that this C++ dll is in-house so if calling a function with a char** parameter is not possible and you can give me suggestions I can tell the other team to use other data types or another construct, but I'd like suggestions.

Thanks

[Plater]

I'm a little rusty but isn't a char** a pointer to an array of characters? Or is it a pointer to an array of array of characters?
I think it would transpose to C# better if you could use a char[] of some sort. You really need to avoid pointers though in managed code, you never know what you'll get.

[Elikhom]

I'm a little rusty but isn't a char** a pointer to an array of characters? Or is it a pointer to an array of array of characters?
I think it would transpose to C# better if you could use a char[] of some sort. You really need to avoid pointers though in managed code, you never know what you'll get.

What they are trying to past is an array of strings, but they definied the dll interface as char**. Hope this helps

[Elikhom]

Now I'm trying with something different, like this but it doesn't work for me either...



C++ side

void populateRequiredIndexNames(char** names);





C# side
[DllImport("QRMAPIDummyImpl.dll", CharSet = CharSet.Auto)]
private static extern void populateFieldCategories(out IntPtr categories);



public string[] FieldCategories
{
get
{
int num = NumberOfFieldCategories;
string[] categories = new string[num];
IntPtr ptr;
populateFieldCategories(out ptr); <---- FAILS HERE

for (int i = 0; i < num; i++ )
{
categories[i] = Marshal.PtrToStringAnsi(Marshal.ReadIntPtr(ptr, i*IntPtr.Size));
}

return categories;
}
}



It gives me an System.AccessViolationException : Attempted to read or write protected memory. This is often an indication that other memory is corrupt.



Is it ok for them to pass a char** in the C++ function or should they pass a char*** if they want to give me an array of C-strings?

[TRScheel]

Try putting it into an unsafe block

[Elikhom]

Try putting it into an unsafe block

Thanks, I'll try that