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How do I handle a complex struct return type from a C DLL file within C#?

I've been trying to get a C library (DLL) working with some simple test code in C#. So far I've been able to import and use the simple functions just fine. The issue I'm having right now is that I don't know how to receive the complex struct return type from this imported function.
Here are the two function signatures:
C:
#define HID_API_EXPORT __declspec(dllexport)
#define HID_API_CALL
struct hid_device_info HID_API_EXPORT * HID_API_CALL hid_enumerate(unsigned short vendor_id, unsigned short product_id);
C#:
[DllImport("hidapi.dll")]
public static extern hid_device_info hid_enumerate(ushort vendor_id, ushort product_id);
And here are the two structs:
C:
struct hid_device_info {
char *path;
unsigned short vendor_id;
unsigned short product_id;
wchar_t *serial_number;
unsigned short release_number;
wchar_t *manufacturer_string;
wchar_t *product_string;
unsigned short usage_page;
unsigned short usage;
int interface_number;
struct hid_device_info *next;
};
C#:
[StructLayout(LayoutKind.Sequential)]
public struct hid_device_info
{
public IntPtr path;
public ushort vendorid;
public ushort productid;
public IntPtr serialnumber;
public ushort releasenumber;
public IntPtr manufacturer;
public IntPtr product;
public ushort usagepage;
public ushort usage;
public int interfacenumber;
public IntPtr next;
}
I'm currently getting this error when I run the program:
Managed Debugging Assistant 'PInvokeStackImbalance' has detected a
problem in
'C:\Users\tajensen\Documents\hidapiCS\hidapitest\bin\Debug\hidapitest.vshost.exe'.
Additional information: A call to PInvoke function
'hidapiCS!hidapiCS.hidapi::hid_enumerate' has unbalanced the stack.
This is likely because the managed PInvoke signature does not match
the unmanaged target signature. Check that the calling convention and
parameters of the PInvoke signature match the target unmanaged
signature.
I've done a bit of digging and the only things I've been able to find describe how to receive return types of very simple structs (i.e. no pointers, and only basic types like ints and bools). I would really appreciate some additional insight on the issue, as I know where I want to be, but I don't know enough about this kind of code to dig deeper on my own.
Thanks in advance,
Toms

Your structure looks good, baring any command line flags that change the packing of it.
Likely, it's because of this line
#define HID_API_CALL
This means you're using the default calling convention, which is generally __cdecl. So, change the P/Invoke definition to:
[DllImport("hidapi.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern hid_device_info hid_enumerate(ushort vendor_id, ushort product_id);
So the rules for how to manage the stack on the C side are followed properly.

There is a tool named SWIG, a code generation tool, connects programs written in C and C++ with a variety of high-level programming languages like c#. It generates wrapper code for twenty three different target languages:
It's very powerful for these tasks, specially data type/pointer conversion and many others. It saves manual conversion.
Try to use this tool to generate C# code for your case and finally compiled Dll.
Download the binary from: http://prdownloads.sourceforge.net/swig/swigwin-3.0.10.zip.
For tutorial: http://www.swig.org/tutorial.html

Interfacing with C/C++ routine in C# causes rip

I have a vendor's API call, which I want to use in my C# Windows Forms application. The problem is that no matter what P/Invoke I try, I get a crash on the line stating that the call has unbalanced the stack. I tried working with the vendor, but they do not have Visual Studio 2012 nor have any experience with C# or .NET and after several back and forth emails, they basically washed their hands of the problem.
Here is their official C/C++ declaration
const char *<dll name here>(int Param1,
const char *Param2,
const char *Param3,
unsigned long Param4,
unsigned short Param5,
unsigned short Param6,
unsigned short Param7,
unsigned short Param8,
unsigned short Param9);
Here is a recent P/Invoke DLLImport line that I tried.
[System.Runtime.InteropServices.DllImport(
"<dll name here>", EntryPoint = "<AnsiFunctionName>", ExactSpelling = true,
CharSet = System.Runtime.InteropServices.CharSet.Ansi, SetLastError = true)]
public static extern String <AnsiFunctionName>(int Param1,
String Param2,
String Param3,
ulong Param4,
Int16 Param5,
Int16 Param6,
Int16 Param7,
Int16 Param8,
Int16 Param9);
Here is the actual error message:
Subject Line: PInvokeStackImbalance was detected
A call to P/Invoke function '' has unbalanced the stack. This is likely because the managed PInvoke signature does not match the unmanaged target signature. Check that the calling convention and parameters of the P/Invoke signature match the target unmanaged signature.
I tried various things, even putting a void return type. I tried not using the return type. I tried instead of Strings, StringBuilders and IntPtrs. No matter what I tried, nothing works.
I mentioned to the vendor after reading an article here on Stack Overflow that returning a string is dangerous and should never be done. Their response was:
"The string return problems mentioned on that page don't apply to
our functions. They return a pointer to a statically-allocated string, so you don't need to free it (and must not try to do so). It looks like the "UnmanagedType.LPStr" should work (though the IntPtr thing should work too, with the proper conversions or casting).
Update #1: As mentioned in the comment, I was using uint, not ulong, up until this post. I just thought that I make the comparison look better. Both uint and ulong do not work.
Update #2: Maybe giving the actual function name will help. It is Armadillo CreateCodeShort3A. Going forward, I am using pure .NET, but I do still need to interface to that, just it is not really possible at the moment. More than that, the problem got me curious why I cannot resolve something seemingly so simple as a P/Invoke. After all, how many variations are there? (not a serious question).

Here are the possibilities that I can see:
The return value should not be String. Use IntPtr and convert to a string with Marshal.PtrToStringAnsi.
The C++ type unsigned long should be matched with uint. You used ulong in C# which is a 64-bit type. In C++ on Windows, unsigned long is an unsigned 32-bit type.
The C++ type unsigned short should be matched with ushort.
The calling convention used by the C++ DLL is probably cdecl. Add CallingConvention = CallingConvention.Cdecl to your P/Invoke.
Don't use SetLastError for such a function. That's used with Win32 API functions that return error conditions through GetLastError. This DLL file almost certainly does not.
So the P/Invoke should look something like this:
[DllImport(#"mydll.dll", CallingConvention=CallingConvention.Cdecl)]
public static extern IntPtr FunctionName(
int Param1,
string Param2,
string Param3,
uint Param4,
ushort Param5,
ushort Param6,
ushort Param7,
ushort Param8,
ushort Param9
);

C++ int & long marshalling in C#

I'm currently porting a proprietary dll (an API) to C#, and I have some problems when marshalling some of the used data types.
For example the API header file defines the following types:
typedef unsigned int tINT; /* natural unsigned */
typedef signed int tINTs; /* natural signed */
typedef unsigned int tLONG; /* 32 bit unsigned */
typedef unsigned long tPTR; /* 32/64-bit pointer */
Then I for example have the following function definition:
tINTs SomeApiFunction ( const tPTR hdl, tINT param );
I'm usually marshalling the int directly with C#'s int (Int32), or the unsigned version. The API's tLONG and tINT are as such the same (at least for the API). But I'm not sure with the tPTR. I have now made it a uint in C#, because it is a 32bit int, but I'm not sure how the long would behave on a 64bit machine.
What do I have to do to make sure that the API will work on both 32bit and 64bit machines correctly?
This is what I would have done now:
[DllImport( "api.dll" )]
public static extern int SomeApiFunction ( uint hdl, uint param );
Sadly the API is not really well documented, as such I am not sure what the intention was with all those (overlapping) typedefs. Apart from the header comments I have included above there is no information about the actual types.

Try to Marshal it as an IntPtr. IntPtr is 4 bytes on a 32bit system and 8 bytes on a 64bit system in .Net.

It is an implementation detail of the C/C++ compiler. You should go by the comment and declare it IntPtr. If the code is compiled by the MSVC compiler then it isn't going to work in 64-bit mode, it uses 32-bits for a long. Not enough to store a pointer.

Here is good list of type correspondence.
For pointer types is usually IntPtr in C#.

If I remember correctly int uint and other primitive types are the right type for the underlying windows API.
For pointers however, its best to use IntPtr on C# side and void* on C/C++ side.

PInvokeStackImbalance C# call to unmanaged C++ function

After switching to VS2010, the managed debug assistant is displaying an error about an unbalanced stack from a call to an unmanaged C++ function from a C# application.
The usuals suspects don't seem to be causing the issue. Is there something else I should check? The VS2008 built C++ dll and C# application never had a problem, no weird or mysterious bugs - yeah, I know that doesn't mean much.
Here are the things that were checked:
The dll name is correct.
The entry point name is correct and has been verified with depends.exe - the code has to use the mangled name and it does.
The calling convention is correct.
The sizes and types all seem to be correct.
The character set is correct.
There doesn't seem to be any issues after ignoring the error and there isn't an issue when running outside the debugger.
C#:
[DllImport("Correct.dll", EntryPoint = "SuperSpecialOpenFileFunc", CallingConvention = CallingConvention.StdCall, CharSet = CharSet.Ansi, ExactSpelling = true)]
public static extern short SuperSpecialOpenFileFunc(ref SuperSpecialStruct stuff);
[StructLayout(LayoutKind.Sequential, Pack = 1, CharSet = CharSet.Ansi)]
public struct SuperSpecialStruct
{
public int field1;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 256)]
public string field2;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
public string field3;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 10)]
public string field4;
public ushort field5;
public ushort field6;
public ushort field7;
public short field8;
public short field9;
public uint field10;
public short field11;
};
C++:
short SuperSpecialOpenFileFunc(SuperSpecialStruct * stuff);
struct SuperSpecialStruct
{
int field1;
char field2[256];
char field3[20];
char field4[10];
unsigned short field5;
unsigned short field6;
unsigned short field7;
short field8;
short field9;
unsigned int field10;
short field11;
};
Here is the error:
Managed Debugging Assistant
'PInvokeStackImbalance' has detected a
problem in 'Managed application path'.
Additional Information: A call to
PInvoke function
'SuperSpecialOpenFileFunc' has
unbalanced the stack. This is likely
because the managed PInvoke signature
does not match the unmanaged target
signature. Check that the calling
convention and parameters of the
PInvoke signature match the target
unmanaged signature.

As mentioned in Dane Rose's comment, you can either use __stdcall on your C++ function or declare CallingConvention = CallingConvention.Cdecl on your DllImport.

You specify stdcall in C# but not in C++, a mismatch here will lead to both the function and the caller popping arguments off of the stack.
On the other hand there is a compiler switch that will turn on stdcall as the default calling convention, (-Gz) are you using that?
Or try this in your C++
short __stdcall SuperSpecialOpenFileFunc(SuperSpecialStruct * stuff);

You specify no padding in your C# declaration of the struct, but not in the C++ version. Since you are mixing char arrays that are not all multiples of four and an odd number of 2 byte shorts, the compiler is probably inserting padding within the struct and add the end.
Try wrapping the struct in a #pragma pack to ensure no padding.
#pragma pack(push)
#pragma pack(1)
// The struct
#pragma pack(pop)

Had the same problem as described - unmanaged C++ app that has worked perfectly for years. When we upgraded to VS2010, we started getting PInvokeStackUnbalanced messages.
adding "__stdcall" to the C++ signature as described above made the issue go away.

It's good.I update function define as follow:
[DllImport("mydll.dll", CallingConvention = CallingConvention.Cdecl)]
It works well.

c# p/invoke difficulties marshalling pointers

I am attempting to call into a native .dll from c# using p/invoke. I'm able to make the call (no crash, the function returns a value), but the return code indicates "A pointer parameter does not point to accessible memory." I've resorted to trial and error in order to solve this, but I haven't made any progress so far.
Here's the signature of the native function I'm calling:
LONG extern WINAPI MyFunction ( LPSTR lpszLogicalName, //input
HANDLE hApp, //input
LPSTR lpszAppID, //input
DWORD dwTraceLevel, //input
DWORD dwTimeOut, //input
DWORD dwSrvcVersionsRequired, //input
LPWFSVERSION lpSrvcVersion, //WFSVERSION*, output
LPWFSVERSION lpSPIVersion, //WFSVERSION*, output
LPHSERVICE lphService //unsigned short*, output
);
Here's the imported signature in C#:
[DllImport("my.dll")]
public static extern int MyFunction( [MarshalAs(UnmanagedType.LPStr)]
string logicalName,
IntPtr hApp,
[MarshalAs(UnmanagedType.LPStr)]
string appID,
int traceLevel,
int timeout,
int srvcVersionsRequired,
[Out] WFSVersion srvcVersion,
[Out] WFSVersion spiVersion,
[Out] UInt16 hService
);
Here's the C definition of WFSVERSION:
typedef struct _wfsversion
{
WORD wVersion;
WORD wLowVersion;
WORD wHighVersion;
CHAR szDescription[257];
CHAR szSystemStatus[257];
} WFSVERSION, * LPWFSVERSION;
Here's the C# definition of WFSVersion:
[StructLayout(LayoutKind.Sequential)]
public class WFSVersion
{
public Int16 wVersion;
public Int16 wLowVersion;
public Int16 wHighVersion;
[MarshalAs(UnmanagedType.ByValArray, SizeConst = 257)]
public char[] szDescription;
[MarshalAs(UnmanagedType.ByValArray, SizeConst = 257)]
public char[] szSystemStatus;
}
Here's the call to MyFunction from C#:
WFSVersion srvcVersionInfo = new WFSVersion();
WFSVersion spiVersionInfo = new WFSVersion();
UInt16 hService = 0;
IntPtr hApp = IntPtr.Zero;
string logicalServiceName = tbServiceName.Text;
int openResult = MyFunction(logicalServiceName, hApp, null, 0,
XFSConstants.WFS_INDEFINITE_WAIT,
0x00000004, srvcVersionInfo, spiVersionInfo,
hService);
As I said, this call returns, but the return value is an error code indicating "A pointer parameter does not point to accessible memory." I must be doing something wrong with parameters 1,3,7,8, or 9. However, I've made successful calls to other functions in this .dll which required WFSVERSION* as parameters, so I don't think parameters 7 or 8 are the issue here.
I would appreciate any thoughts you might have about the cause of this issue, or any constructive criticisms of my code. This is my first experience with P/Invoke, so I'm not sure where to begin. Is there any way to narrow down the issue, or is trial an error my only option?

You have two obvious errors here. In your struct definition you should be using byte[] instead of char[] for szDescription and szSystemStatus.
Also the last parameter in your pInvoke call is not a pointer. When you make your call to MyFunction hService is zero and therefore an invalid pointer as far as the function is concerned. [Out] is a Marshaling directive telling the runtime when and where to copy data not an indicator that the parameter is a pointer. What you need is to change [Out] to out or ref this tells the runtime that hService is a pointer:
[DllImport("my.dll")]
public static extern int MyFunction( [MarshalAs(UnmanagedType.LPStr)]
string logicalName,
IntPtr hApp,
[MarshalAs(UnmanagedType.LPStr)]
string appID,
int traceLevel,
int timeout,
int srvcVersionsRequired,
[Out] WFSVersion srvcVersion,
[Out] WFSVersion spiVersion,
out UInt16 hService);

Some ideas:
The C# WFSVersion class should probably be a struct. I don't know if the P/Invoke marshaller cares, but I've always seen structs used.
Character size might be an issue.
C's CHAR is 8 bits wide (ANSI), and .Net's System.Char is 16 bits wide (Unicode). To give the marshaller as much info as possible so it does the correct conversion, try adding "CharSet = CharSet.Ansi" to the DllImport and StructLayout attributes, and changing the string declarations in WFSVersion:
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 257)]
public string szDescription;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 257)]
public string szSystemStatus;
Another issue could be data alignment in the structs. If no alignment was specified when the C struct was compiled, the data elements in the struct were probably aligned on a one or two byte boundary. Try using Pack in WFSVersion's StructLayout attribute:
[StructLayout(LayoutKind.Sequential, Pack = 1, CharSet = CharSet.Ansi)]
// Try pack values of 2, 4 and 8 if 1 doesn't work.
And some questions:
Was MyFunction intended to be called from non-C code? The original author may have written code that assumes the data passed in is allocated with the C runtime memory manager.
Does code in the C DLL use the pointers passed to it for later processing after MyFunction has returned? If so - and assuming it's possible/wise/sane to go forward with such a situation - it may be necessary to "pin" the structs passed to MyFunction using the fixed keyword. Plus there are probably security issues to deal with.

I'm not sure exactly what the issue with this one is but hopefully this will be a starting point.
Try looking into the options on the DllImport attribute, this could be to do with the marshalling of the strings.
[DllImport("kernel32.dll", SetLastError = true, CharSet = CharSet.Auto)]
The CharSet option is the one I think you may need.

I have found AppVerifier useful for tracking down P/Invoke marshal issues before. It's free and from Microsoft.

My guess would be that one of your pointers (lpSrvcVersion, lpSPIVersion or lphService) isn't accessible from your .NET application. Can you try modifying the DLL (if it's yours) and see if you can get it working without the pointers? (I know you'll have to add them back later, but at least you can narrow down the location of the problem.)

Are you sure that it is not hApp? That looks like an input parameter, the handle to the requesting application. Quick google...yes, there is a function to create an app handle, and a default parameter you can use.