Array of strings as return value from C to C#? [closed]

How does one return an array of strings (char**) from a native C function into a managed string type in C#?

char**

string

This works:

C:
char* get_text()
size_t length = ...;
char *text = (char *)CoTaskMemAlloc((length + 1) * sizeof(char));
// populate text...
return text;


C#:
[DllImport(LibName, EntryPoint = "get_text", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
[return: MarshalAs(UnmanagedType.LPStr)]
private static extern string GetText();


This, however, I cannot get to work:

C:
char** get_texts()
size_t count = ...;
char **texts = (char **)CoTaskMemAloc(count * sizeof(char*));
for (size_t i = 0; i < count; ++i)
size_t length = ...;
texts[i] = (char *)CoTaskMemAlloc((length + 1) * sizeof(char));
// populate texts[i]

return texts;


C#:
[DllImport(LibName, EntryPoint = "get_texts", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
[return: MarshalAs(???)]
private static extern string GetTexts();


I've tried setting the return type to marshal as LPArray and SafeArray with SafeArraySubType = VarEnum.VT_LPSTR, but I can't get it to work. Using LPArray seems to suggest I should pass in parameters for the size of the array (along with the subtype, which is LPStr), but I don't know the size at the time of the call.

LPArray

SafeArray

SafeArraySubType = VarEnum.VT_LPSTR

LPArray

LPStr

How can this be achieved?

This question appears to be off-topic. The users who voted to close gave this specific reason:

char **

malloc

void *

1 Answer
1

Try following :

[DllImport("LibName", EntryPoint = "get_texts", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
private static extern IntPtr GetTexts();

static void Main(string args)

List<string> results = new List<string>();

IntPtr data = GetTexts();

IntPtr tmpPtr = IntPtr.Zero;
while(data != null)

string str = Marshal.PtrToStringAnsi(data);
results.Add(str);
data += 1;




The code assumes the array of strings are stored as an array of points. Sometime in c this is not the case and then the code below may work

[DllImport("LibName", EntryPoint = "get_texts", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
private static extern IntPtr GetTexts();

static void Main(string args)

List<string> results = new List<string>();

IntPtr data = GetTexts();

IntPtr tmpPtr = IntPtr.Zero;
Boolean first = true;
string str = "";
do

if (first)

str = Marshal.PtrToStringAnsi(data);
first = false;

results.Add(str);
data += str.Length + 1;
str = Marshal.PtrToStringAnsi(data);

while (str.Length > 0);



data

char**

char*