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I have the following struct used in a C program

typedef enum { ndberror_st_success = 0, ndberror_st_temporary = 1, ndberror_st_permanent = 2, ndberror_st_unknown = 3 } ndberror_status_enum; typedef enum { ndberror_cl_none = 0, ndberror_cl_application = 1, ndberror_cl_no_data_found = 2, ndberror_cl_constraint_violation = 3, ndberror_cl_schema_error = 4, ndberror_cl_user_defined = 5, ndberror_cl_insufficient_space = 6, ndberror_cl_temporary_resource = 7, ndberror_cl_node_recovery = 8, ndberror_cl_overload = 9, ndberror_cl_timeout_expired = 10, ndberror_cl_unknown_result = 11, ndberror_cl_internal_error = 12, ndberror_cl_function_not_implemented = 13, ndberror_cl_unknown_error_code = 14, ndberror_cl_node_shutdown = 15, ndberror_cl_configuration = 16, ndberror_cl_schema_object_already_exists = 17, ndberror_cl_internal_temporary = 18 } ndberror_classification_enum; typedef struct { ndberror_status_enum status; ndberror_classification_enum classification; int code; int mysql_code; const char * message; char * details; } ndberror_struct;

I need to marshal it in a c# program, the C# side declaration is:

 public enum ndberror_status { ndberror_st_success = 0, ndberror_st_temporary = 1, ndberror_st_permanent = 2, ndberror_st_unknown = 3, } public enum ndberror_classification { ndberror_cl_none = 0, ndberror_cl_application = 1, ndberror_cl_no_data_found = 2, ndberror_cl_constraint_violation = 3, ndberror_cl_schema_error = 4, ndberror_cl_user_defined = 5, ndberror_cl_insufficient_space = 6, ndberror_cl_temporary_resource = 7, ndberror_cl_node_recovery = 8, ndberror_cl_overload = 9, ndberror_cl_timeout_expired = 10, ndberror_cl_unknown_result = 11, ndberror_cl_internal_error = 12, ndberror_cl_function_not_implemented = 13, ndberror_cl_unknown_error_code = 14, ndberror_cl_node_shutdown = 15, ndberror_cl_configuration = 16, ndberror_cl_schema_object_already_exists = 17, ndberror_cl_internal_temporary = 18, } [System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)] public struct ndberror_struct { public ndberror_status status; public ndberror_classification classification; public int code; public int mysql_code; [System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.LPStr)] public string message; [System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.LPStr)] public string details; }

The C-Side signature of the method to be called is:

const ndberror_struct __stdcall Ndb_getNdbError(void* obj);

The C#-Side instead is:

 [System.Runtime.InteropServices.DllImportAttribute("Ndb_CWrapper.dll", EntryPoint = "Ndb_getNdbError", CallingConvention = System.Runtime.InteropServices.CallingConvention.StdCall,CharSet = CharSet.Ansi)] private static extern ndberror_struct Ndb_getNdbError(System.IntPtr obj);

When i try to invoke the method from the managed code a MarshalDirectiveException raises. I think the problem is somewhere in the string marshalling, but i'm not able to solve the problem. The native library is compiled for a x64 system multibyte charset code (with visual studio c++ 2012)

The alternative can be some strange operation inside the native code, where i cast a NdbError (defined in another header) to ndberror_struct, here the code:

Ndb* tmp=(Ndb*)obj; ndberror_struct tmpRes=(ndberror_struct)tmp->getNdbError();

The NdbStruct returned by tmp->getNdbError() overload the casting operator (i suppose), this is the declaration:

operator ndberror_struct() const { ndberror_struct ndberror; ndberror.status = (ndberror_status_enum) status; ndberror.classification = (ndberror_classification_enum) classification; ndberror.code = code; ndberror.mysql_code = mysql_code; ndberror.message = message; ndberror.details = details; return ndberror; }

Any help will be appreciated.

Edit: I tried to change a bit the method's signature in this way: C-Side:

void* __stdcall Ndb_getNdbError(void* obj)

C#-Side:

IntPtr Ndb_getNdbError(IntPtr obj)

the native method return &tmpRes and in the managed code i had marshalled manually the struct calling

var tmp=Ndb_getNdbError(raw); var f = (ndberror_struct)Marshal.PtrToStructure(tmp, typeof(ndberror_struct));
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  • 1
    What message (text) do you get with the MarshalDirectiveException? Commented Nov 6, 2013 at 14:54
  • The message says: Method's type signature is not PInvoke compatible. Commented Nov 6, 2013 at 15:15

1 Answer 1

Reset to default
2

The marshaller won't marshal those char* fields from the native code to the managed. You'll have to do that bit yourself.

One way to do that is to change the C# struct declaration as follows:

public struct ndberror_struct { public ndberror_status status; public ndberror_classification classification; public int code; public int mysql_code; public IntPtr message; public IntPtr details; }

Leave your p/invoke declaration as it is:

[DllImport("Ndb_CWrapper.dll")] private static extern ndberror_struct Ndb_getNdbError(IntPtr obj);

You'll be able to call the function now. In order to convert the two IntPtr fields into strings, call Marshal.PtrToStringAnsi.

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12 Comments

i think i will do as you suggested, in this way i don't need to change the signature of the native code
Out of interest, who deallocates the strings?
Up to now i cannot give you the right answer because i really don't know. I am at the early stage of creating a wrapper from the NDB API provided by Oracle (MYSQL): the library is written in C++, i need to make a double wrapper from C++ to C and after from C to C#. So for this strings and surely for other furter things i need to take care of the deallocating
If the target code is C++ then single C++/CLI wrapper sounds best
Unfortunately I need 2 wrapper, one wrapper for pure C code/compiler and one for C# so in this way i should take 2 birds with one stone...i hope
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