How do I pass a const char* to a C function from C#?

Question

I try to call a plain C-function from an external DLL out of my C#-application. This functions is defined as

void set_param(const char *data)

Now I have some problems using this function:

1. How do I specify this "const" in C#-code? public static extern void set_param(sbyte *data) seems to miss the "const" part.

2. How do I hand over a plain, 8 bit C-string when calling this function? A call to set_param("127.0.0.1") results in an error message, "cannot convert from 'string' to 'sbyte'"*.

Answer 1

It looks like you will be using the ANSI char set, so you could declare the P/Invoke like so:

[DllImport("yourdll.dll", CharSet = CharSet.Ansi)]
public static extern void set_param([MarshalAs(UnmanagedType.LPStr)] string lpString);

The .NET marshaller handles making copies of strings and converting the data to the right type for you.

If you have an error with an unbalanced stack, you will need to set the calling convention to match your C DLL, for example:

[DllImport("yourdll.dll", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]

See pinvoke.net for lots of examples using Windows API functions.

Also see Microsoft's documentation on pinvoking strings.