Calling C++ dll from C# using a function with char** as param with DllImport

[Elikhom]

Hi,

I'm trying to call a C++ function that has a char** as a parameter from C# code using DllImport. After googling around and finding some threads I haven't been able to solve my problem. This is what I have...

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.

The C++ team is trying to pass an array of C-strings, but I don't seem to be able to get that. Other methods work fine (ones returning primitive types or char*) but the method with char** doesn't want to work.

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?

Any help would be greatly appreaciated.

Thx

[weaknessforcats]

The C++ function cannot return an array of strings. What it can do is return the address of the first string.

If this is an array of char* strings, then the C++ function can return the address as of element 0 as a char**. However, it also needs to return the number or strings in the array. Otherwise, the caller has no idea how many strings there are.

Possible solutions:
1)The last string in the array is guraranteed empy. The caller processes until it encounters an empty string
2) Return the number of strings in ther array.

I recommend the second choice and return a struct:

struct StringArray
{
    char** str;    //address of first string
    unsigned int;  //number of strings.
};
 

In this case str[3] is the 4th string.

This does not solve problems of ASCII/UNICODE where you may need to use TCHAR mappings in C++. Now that I think of it, I don't believe you are supposed to use char* strings in C++ or C# where Microsoft code is concerned. I remember reading something about that.

An access violation exception is almost certainly an uninitialized pointer onthe C++ side unless this:

IntPtr ptr;

requires some kind of initial value before being used. Not being a C# person, this is definitely a bug in C++.

Also:

string[] categories = new string[num];

is on the C# side. How do you know the C++ folks have num strings??

If they have 4 and num is 10, you die since your load loop runs to num.

[Elikhom]

Thanks for the suggestions I'll give it a try, there's another method that return the number of categories, I don't have a problem reading that one and I'm using it to initialize the array.

I wonder if it would be easier to create a managed C++ dll that wraps the unmanaged one and then use the managed C++ dll to talk to C#. Do you know if the manipulation of char*, char** and structs in managed C++ can make it easier for me and just pass to C# the equivalents like for example instead of having to deal with char* in C#, the managed C++ dll will take care of it and pass to C# a wstring that C# can use directly?

It seems that PInvoke is usefull for simple things but it seems it gets much more cumbersome when you need to pass strucs and arrays of c-string.

[weaknessforcats]

You are probably better off using Managed C++ and inside that code have sections of unmanaged C++ where you need them.