How do I properly return a char * from an Unmanaged DLL to C#?

Question

Function signature:

char * errMessage(int err);

My code:

[DllImport("api.dll")]       
internal static extern char[] errMessage(int err);
...
char[] message = errMessage(err);

This returns an error:

Cannot marshal 'return value': Invalid managed/unmanaged type combination.

What am I doing wrong? Thanks for any help.

Answer 1

try this:

[DllImport("api.dll")]
[return : MarshalAs(UnmanagedType.LPStr)]
internal static extern string errMessage(int err);
...
string message = errMessage(err);

I believe C# is smart enough to handle the pointer and return you a string.

Edit: Added the MarshalAs attribute

Answer 2

Try using String in the managed side. you might as well set the CharSet to Ansi

Answer 3

See this question. To summary, the function should return an IntPtr and you have to use Marshal.PtrToString* to convert it to a managed String object.

Answer 4

A simple and robust way is to allocate a buffer in C# in the form if a StringBuilder, pass it to the unmanaged code and fill it there.

Example:

C

#include <string.h>

int foo(char *buf, int n) {
   strncpy(buf, "Hello World", n);
   return 0;
}

C#

[DllImport("libfoo", EntryPoint = "foo")]
static extern int Foo(StringBuilder buffer, int capacity);

static void Main()
{
    StringBuilder sb = new StringBuilder(100);
    Foo(sb, sb.Capacity);
    Console.WriteLine(sb.ToString());
}

Test:

Hello World

Answer 5

It is a horrible function signature, there's no way to guess how the string was allocated. Nor can you deallocate the memory for the string. If you declare the return type as "string" in the declaration then the P/Invoke marshaller will call CoTaskMemFree() on the pointer. That is very unlikely to be appropriate. It will silently fail in XP but crash your program in Vista and Win7.

You can't even reliably call the function in an unmanaged program. The odds that you'd use the correct version of free() are pretty slim. All you can do is declare it as IntPtr and marshal the return value yourself with Marshal.PtrToStringAnsi(). Be sure to write a test program that does so a million times while you observe it in Taskmgr.exe. If the VM size for the program grows without bound, you have a memory leak you cannot plug.