IJW C# to MC++ to C++ transfer array of strings to char** - c++-cli

Question

Does anyone know how to pin the following string array parameter:
function signature int TestMethod(int argc, array<String^>^ data)
{
pin_ptr<char> p1 = &data[0];
char** p2 = &p1[0];
// char** argv = (char**)calloc(argc+1, sizeof(char*));
}
I want to pin an array<String^>^ object and convert it to the following:
char** argv = (char**)calloc(argc+1, sizeof(char*));
so I can pass it to a native c++ function
any ideas?
What about the following?
UPDATE: just curious? what about the following?
char** argv = (char**)calloc(argc+1, sizeof(char*));
for (int i = 0; i < data->Length; i++)
{
argv[i] = (char*)Marshal::StringToHGlobalAnsi(data[i]).ToPointer();
}
// Use argv as needed here...
// Cleanup
for (int i = 0; i < data->Length; i++)
{
Marshal::FreeHGlobal((IntPtr)argv[i]);
}

Answer 1

You can't directly pin a managed String^ into a char*. .NET Strings are actually 2 bytes per character, so you need to marshal the data manually. This will likely require building up the character arrays, copying data into them, then cleaning up after you're done.
In this case, you likely need to copy the data into your char**, use it, then clean up after yourself. This can be done via something like:
// Requires #include <msclr\marshal.h>
marshal_context context;
char** argv = new char*[data->Length];
for (int i=0;i<Length;++i)
{
const char* tmp = context.marshal_as<const char*>(clrString);
int length = strlen(tmp);
argv[i] = new char[length+1]();
strncpy(argv[i],tmp,length);
}
// Use argv as needed here...
// Cleanup
for (int i=0;i<Length;++i)
delete[] argv[i];
delete[] argv;