How to use extern "C" dll function taking char** as an argument in C# application?

Question

I have dll with function:

extern "C"
int
doJob(char** buffer);

Its usage with C++ looks like this:

char* buf;
int status = doJob(&buf);

What definition should I have for this function in C#? How can I use this function in C#?

Google easily brings many answers, like [this](https://msdn.microsoft.com/en-us/library/26thfadc%28v=vs.110%29.aspx)... — pascal, Apr 08 '15 at 07:14
It can have multiple signatures, depending on what you want to do with it. The problem is who will write on `buffer`, the calling function or the `doJob`? — xanatos, Apr 08 '15 at 07:25
Main question is not about how to use C dll in C# - with simple use cases I know how to do this: — Sviatoslav, Apr 08 '15 at 07:25
[DllImport("containsdojob.dll",CharSet=CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)] public static extern Int32 doJob(out string buffer); — Sviatoslav, Apr 08 '15 at 07:27
But what should be instead of "out string buffer"? With such case I have an exception. — Sviatoslav, Apr 08 '15 at 07:28
@Sviatoslav As Yoda you speak... Object does subject you do (*Buffer will create doJob*) — xanatos, Apr 08 '15 at 07:31
See http://stackoverflow.com/questions/5298268/returning-a-string-from-pinvoke — xanatos, Apr 08 '15 at 07:32
`extern "C"` is not a C issue, but a C++. So please remove the C tag. — Luis Colorado, Apr 09 '15 at 06:48

xanatos · Answer 1 · 2015-04-08T08:30:40.053

One of the possible patterns is:

[DllImport("containsdojob.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern Int32 doJob(out IntPtr buffer);

[DllImport("containsdojob.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern void freeMemory(IntPtr buffer);

and

IntPtr buffer = IntPtr.Zero;
string str = null;

try
{
    doJob(out buffer);

    if (buffer != IntPtr.Zero)
    {
        str = Marshal.PtrToStringAnsi(buffer);
    }
}
finally
{
    if (buffer != IntPtr.Zero)
    {
        freeMemory(buffer);
    }
}

Note that you'll need a freeMemory method to free the memory allocated by doJob.

There are other possible patterns, for example based on BSTR and SysAllocString that are easier to implement C#-side (but more difficult to implement C-side)

The "pattern" for using BSTR:

C-side:

char *str = "Foo"; // your string
int len = strlen(str);
int wslen = MultiByteToWideChar(CP_ACP, 0, str, len, 0, 0);
BSTR bstr = SysAllocStringLen(NULL, wslen);
MultiByteToWideChar(CP_ACP, 0, str, len, bstr, wslen);
// bstr is the returned string

C#-side:

[DllImport("containsdojob.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern Int32 doJob([MarshalAs(UnmanagedType.BStr)] out string buffer);

string str;
doJob(out str);

The memory is automatically handled (freed) by the CLR.

If you are using Visual C++ you can even

char *str = "Foo"; // your string
_bstr_t bstrt(str);
BSTR bstr = bstrt.Detach(); 
// bstr is the returned string

Or C-side you could use one of the two allocators that can be freed C#-side: LocalAlloc or CoTaskMemAlloc:

char *str = "Foo"; // your string
char *buf = (char*)LocalAlloc(LMEM_FIXED, strlen(str) + 1);
// or char *buf = (char*)CoTaskMemAlloc(strlen(str) + 1);
strcpy(buf, str);
// buf is the returned string

Then you use the first example, but instead of calling

freeMemory(buffer);

you call:

Marshal.FreeHGlobal(buffer); // for LocalAlloc

or

Marshal.FreeCoTaskMem(buffer); // for CoTaskMemAlloc

Thank you very much! Your answer helped me - it works now! By the way, "freeMemory" also exists in the same dll. — Sviatoslav, Apr 08 '15 at 08:16

How to use extern "C" dll function taking char** as an argument in C# application?

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