Passing string from C++ to C#

Question

I am using the PInvoke, reverse PInvoke scheme as described by Thottam R. Sriram http://blogs.msdn.com/b/thottams/archive/2007/06/02/pinvoke-reverse-pinvoke-and-stdcall-cdecl.aspx

Everything seems to work well, except for passing a string from C++ to C.

( In Sriram the string is constructed in c# and passed untouched through c++, so the issue is avoided. )

The c# code looks like this

class Program
{
    public delegate void callback(string str);
    public static void callee(string str)
    {
        System.Console.WriteLine("Managed: " + str);
    }

    static void Main(string[] args)
    {
        gpscom_start(new callback(Program.callee));

        Console.WriteLine("NMEA COM Monitor is running");
        System.Threading.Thread.Sleep(50000);
    }

    [DllImport("gpscomdll.dll", CallingConvention = CallingConvention.StdCall)]
    public static extern void gpscom_start(callback call);

}

The C++ code looks like this

extern "C" dllapi void __stdcall gpscom_start(void (__stdcall *callback) (BSTR str))
{

BSTR bstr = SysAllocString(L"starting monitor");
(*callback)(bstr);
SysFreeString(bstr);

When run, everything looks good except the callback string

Managed: m

It looks like a UNICODE string being printed out by an ANSI routine, but surely c# strings are unicode?

TIA

Gideon Engelberth · Accepted Answer · 2014-04-16 13:30:22Z

4

When marshalling strings across a P/Invoke boundary, it is always good practice to use a MarshalAs attribute with the appropriate string type. I think putting a [MarshalAs(UnmanagedType.BStr)] on the parameter should take care of the problem.

public delegate void callback([MarshalAs(UnmanagedType.BStr)]string str);

This article has an similar example where someone was passing BSTRs between managed and unmanaged code, but he used IntPtr and some methods on the Marshal class. Marshal.PtrToStringBSTR seems most useful here.

edited Apr 16, 2014 at 13:30

answered Dec 15, 2010 at 22:11

Gideon Engelberth

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2 Comments

ravenspoint Over a year ago

Can you please expand on this? My c# is not expert enough to know how to implement your answer.

Gideon Engelberth Over a year ago

@ravenspoint try changing the delegate to: public delegate void callback([MarshalAs(UnmanagedType.BStr)] string str); importing the System.Runtime.InteropServices namespace if needed. This tells the runtime that unmanaged code expects this to be a BSTR (or perhaps BSTR *, I am not an interop expert)

Puppy · Accepted Answer · 2010-12-15 21:58:00Z

1

C# strings are UTF-16. I would suggest that C# could be expecting an LPWSTR or something similar rather than a BSTR. If you look at the first example posted, he puts the call type as a wchar_t*, not a BSTR.

answered Dec 15, 2010 at 21:58

Puppy

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3 Comments

ravenspoint Over a year ago

Yes, and so are BSTRs I believe.

Puppy Over a year ago

BSTRs are not plain wchar_t*s, they are a different binary format. That's why SysAllocString exists.

ravenspoint Over a year ago

Yes. They are preceded by the string length. As I understand it, BSTR(c++) == string(c#)

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