Question

I’m having some trouble marshalling a structure in a call to an external library function. I get no errors from the call itself but the function returns an error code that indicates it hasn’t properly understood the values being passed in.

Here’s the signature of the C function

DLLExport int connect(Client handle, connectOptions* options);

And here is my internal function

[DllImport("some.dll", CharSet = CharSet.Auto, ExactSpelling = false,
        CallingConvention = CallingConvention.Cdecl)]
    private static extern int connect(IntPtr client, connectOptions options);

Here is the specification for the connectOptions struct that I am working with

char    struct_id [4]
int     struct_version
int     keepAliveInterval
int     cleansession
int     reliable
willOptions *   will
char *  username
char *  password
int     connectTimeout
int     retryInterval

And then finally is my class within my application

[StructLayout(LayoutKind.Sequential)]
public class connectOptions
{
    public byte[] struct_id;
    public Int32 struct_version;
    public Int32 keepAliveInterval;
    public Int32 cleansession;
    public Int32 reliable;
    public willOptions will;
    [MarshalAs(UnmanagedType.LPStr, SizeConst = 128)]
    public string username;
    [MarshalAs(UnmanagedType.LPStr, SizeConst = 128)]
    public string password;
    public Int32 connectTimeout;
    public Int32 retryInterval;

    public connectOptions()
    {
        struct_id = Encoding.ASCII.GetBytes("WXYZ");
        struct_version = 0;
        keepAliveInterval = 20;
        cleansession = 1;
        reliable = 0;
        username = string.Empty;
        password = string.Empty;
        connectTimeout = 10;
        retryInterval = 1;
        will = null;
    }
}

[StructLayout(LayoutKind.Sequential)]
public class willOptions
{
    public byte[] struct_id;
    Int32 struct_version;
    [MarshalAs(UnmanagedType.LPStr, SizeConst = 128)]
    public string topicName;
    [MarshalAs(UnmanagedType.LPStr, SizeConst = 128)]
    public string message;
    public Int32 retained;
    public Int32 qos;

    public willOptions()
    {
        struct_id = Encoding.ASCII.GetBytes("VWXY");
        struct_version = 0;
    }
}

Answer 1

I think you have to decorate struct_id like this:

[MarshalAsAttribute(UnmanagedType.ByValArray, SizeConst=4, ArraySubType=UnmanagedType.U1)] 
public byte[] struct_id;

otherwise the C# struct will contain a reference to an array, whereas the C++ one contains the actual 4 byte array.