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'"*.

Solution 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.

Solution 2

A const char* is simply a string in .NET - the managed side does not (yet) understand the notion of read-only parameters.

If you are using this in a P/Invoke context, you should declare a MarshalAs attribute and marshal it as a LPStr. The resulting signature would look something along the lines of

[DllImport("SomeModule.dll")]
public static extern void set_param([MarshalAs(UnmanagedType.LPStr)]string lpString);

There's also this article on MSDN with more information on how to marshal native strings to a managed environment.

Comments

[Marius Bancila]

I think public static extern void set_param(string data) should work.

[kenny]

I think you'll find a lot of examples you'll find here pinvoke.net

[Elmi]

This causes an exception: A call to PInvoke function 'yourdll.dll::set_param' has unbalanced the stack. This is likely because the managed PInvoke signature does not match the unmanaged target signature.

[Matthew Watson]

@Elmi So you need to know the calling convention of the C DLL so you can specify it in the P/Invoke. What calling convention does it use?

[Elmi]

Matthew Watson: The DLL has plain undecorated C-functions, all ANSI-C, __declspec(dllexport)

[Matthew Watson]

@Elmi I think therefore that you want CallingConvention = CallingConvention.Cdecl but if that doesn't work, try the other ones.

[Peter Mortensen]

Isn't a string in .NET Unicode? Even with UTF-8 encoding, how would it work reliably for ASCII 128-255 (locale-specific, etc.)?

[aevitas]

@PeterMortensen For ASCII 128-255 you would use LPWStr, which represents a 2-byte char string. As of .NET 4.7, LPUTF8Str was added to "natively" support UTF8 strings rather than returning an IntPtr and handling them manually as well. On a side note, strings in .NET are UTF-16, which is what Microsoft refers to as Unicode, but is a different flavour of Unicode iirc, but you would be correct in saying that strings in .NET are Unicode.