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C# calling a function from unmanaged c++ dll

I have this panini scanner api (written in c++ i suppose) which i want to call from c# application.
PS: I dont have the implementation of this DLL, only the header and the dll file.
the function declaration from VApiInterface.h file is
typedef DWORD ERR_CODE;
typedef ERR_CODE VAPI_RET_TYPE;
typedef struct _DeviceListStruct
{
DWORD DeviceType;
char DeviceSerialNumber[MVX_SN_SIZE];
DWORD DeviceID;
DWORD InternalDeviceID;
BOOL Connected;
}DEVICELISTSTRUCT,* PDEVICELISTSTRUCT;
// Tag for VISION API function declaration
#define VISION_API_DECL __declspec(dllexport)
// VISION API function tag
#define VISION_API WINAPI
VISION_API_DECL VAPI_RET_TYPE VISION_API VApiGetDeviceList(PDEVICELISTSTRUCT pDevList, DWORD Length, DWORD *DetectedDevices);
and i'm trying to call this function from c# like this
public class VisionAPI
{
[StructLayout(LayoutKind.Sequential)]
public struct _DeviceListStruct
{
public uint DeviceType;
[MarshalAs(UnmanagedType.LPStr, SizeConst = 11)]
public string DeviceSerialNumber;
public uint DeviceID;
public uint InternalDeviceID;
public bool Connected;
}
[DllImport("VisionAPI.dll", EntryPoint = "VApiGetDeviceList")]
public static extern uint VApiGetDeviceList(ref _DeviceListStruct d,uint len,ref uint devices);
}
////////////
//the call
VisionAPI._DeviceListStruct d = new VisionAPI._DeviceListStruct() ;
VisionAPI.VApiGetDeviceList(ref d, 0, 0);
and I m getting this error
Unable to find an entry point named 'VApiGetDeviceList' in DLL 'VisionAPI.dll'.
as a java backend developer I'm really out of my element here and I have absolutely no Idea what I'm doing wrong.
plz help.

System.AccessViolationException when calling function from unmanaged DLL using DLLImport

I'm trying to call a function from a C++ DLL using DLLImport (P/Invoke) and I keep getting a System.AccessViolationException error when I call it. Note that I other functions to work.
Declaration of the DLLImport function:
[DllImport("DocProc.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
public static extern uint getDPpupTrkBaseConfigPath(DPHandle hdl, StringBuilder str, uint strsize);
Usage:
StringBuilder sb = new StringBuilder(256);
getDPpupTrkBaseConfigPath(handle, sb, (uint)sb.Capacity);
DPHandle: (note that it works in other functions)
public struct DPHandle
{
public uint Size;
public IntPtr UserHandle;
[MarshalAs(UnmanagedType.LPStr)] public string DeviceName;
public uint DeviceTypeId;
public uint DeviceState;
public uint OpenFlags;
public IntPtr Reserved1;
};
C++:
BPS_PROPL getDPpupTrkBaseConfigPath(DPHandle hdl, char *str, unsigned long strsize);
(Note BPS_PROPL = unsigned long)
DPHandle:
struct DocProcHandle {
unsigned long Size;//sizeof(DocProcHandle)
void* UserHandle;
const char* DeviceName;
unsigned long DeviceTypeId;
unsigned long DeviceState;
unsigned long OpenFlags;
void* Reserved1;
};
typedef struct DocProcHandle *DPHandle;
Again, when I try to call the function that exception shows up. I tried looking at other answers but can't figure out what's wrong?

In the C++ code, you have:
typedef struct DocProcHandle *DPHandle;
This means that DPHandle is a pointer to the struct. But in your C# code you pass the the struct by value.
The simplest way to achieve that is to change the declaration of the first argument of your pinvoke code from
DPHandle hdl
to
[In] ref DPHandle hdl

Using cpp code in c# (return custom struct and std:string)

I'm trying to build a wrapper to use cpp code inside c# code, and I want to return custom struct (Class) as output for method2, and std::string for method1, and this is my code in cpp
extern "C" __declspec(dllexport) std::string method1()
{
std::string s;
//Some Code/////////
return s;
}
and this is the method that should return custom struct (or class)
extern "C" __declspec(dllexport) MyStruct method2()
{
MyStruct s;
//Some Code//////
return s;
}
and I tried to write c# code for both of these methods and here is my c# code
[DllImport("<dllPath>", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.StdCall)]
[return: MarshalAs(UnmanagedType.LPStr)]
public static extern string method1(); //Exception
[DllImport("<DllPath>")]
public static extern MyStruct method2(); //Compile Error
Now when I'm trying to run the c# code I get MarshalDirectiveException for method1, and compiling error for method2?

In the first PInvoke, the C++ method should return const char* (s.c_str()). You should remove "[return [...]]" and replace string by IntPtr. Then, you can convert the IntPtr to string with Marshal.PtrToStringAnsi(ptr). http://msdn.microsoft.com/en-US/library/s9ts558h(v=vs.110).aspx can help you.
In the second PInvoke, you should define in C# the MyStruct (but I can't be more precise because I have no information about MyStruct). This link can help you : http://www.codeproject.com/Articles/66243/Marshaling-with-C-Chapter-Marshaling-Compound-Ty
EDIT : Thanks to hvd's comment, it is better sometimes to use BSTR, because the use of char* can be dangerous !

Calling C function with struct in C# - what am I doing wrong?

I thought this one was fairly straight forward but still trying to understand all of this and having some issues.
I don't know much about the C function b/c i've been given limited information.
Here is the function call in C:
int GetCard(CardInfo card);
Here is the request structure:
typedef struct _tCardInfo
{
char CardNumber[80];
char isExist;
} TCardInfo, *pTCardInfo;
I want to pass the card number to see if it exists.
So in C# I did the following:
public struct CardInfo
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 80)]
public string cardNumber;
public byte isExist;
}
[DllImport("card.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Auto)]
public static extern int GetCardInfo(ref CardInfo cardInfo);
Then in the c# method:
CardInfo cardInfo = new CardInfo();
cardInfo.cardNumber = "1234567890";
int success = GetCardInfo (ref cardInfo);
The good thing about the DLL that I'm calling is it generates a log file.
When I execute the call, the log tells me that I'm hitting the DLL but it is not passing the card number which then sets a message saying the card number was not passed.
Any help is greatly appreciated.
Thanks!

The problem is that you're requesting TChar marshaling, but the DLL requires 8-byte characters. Change the C struct to wchar_t.
Also, use Visual Studio to set a breakpoint in your DLL, and actually inspect the data when it comes in! Visual Studio can debug across .NET/native boundaries, which is super cool!

Try to add attribute StructLayout for struct
[StructLayout(LayoutKind.Sequential)]
public struct CardInfo
{
...

Try to create the .Net struct like this:
[StructLayout(LayoutKind.Sequential)]
public struct CardInfo
{
[MarshalAs(UnmanagedType.AnsiBStr, SizeConst = 80)]
public string cardNumber;
[MarshalAs(UnmanagedType.I1)]
public sbyte isExist;
}
And for the function declaration: try not to use the CallingConvention and CharSet in the DLL import, and use the [In, Out] attributes before the parameter. Like this:
[DllImport("card.dll")]
public static extern int GetCardInfo([In, Out] CardInfo cardInfo);

P/Invoke syntax - Am I translating my C++ function correctly in C#?

A quick disclaimer: I'm very new to P/Invoke, so I apologize in advance if this is a silly question.
Here's my function signature in C++:
HRESULT SomeFunction(
_Out_ unsigned long *count,
_Outptr_result_buffer_(*count) GUID **ids,
_In_ const PCWSTR filter
)
And I'm trying to P/Invoke it as such in C#:
[StructLayout(LayoutKind.Sequential)]
struct GUID
{
public int a;
public short b;
public short c;
[MarshalAs(UnmanagedType.ByValArray, SizeConst=8)]
public byte[] d;
}
[DllImport("MyDll.dll", EntryPoint="SomeFunction")]
[return: MarshalAs(UnmanagedType.I8)]
private static extern Int64 SomeFunction
(
out ulong count,
[MarshalAs(UnmanagedType.LPArray)]
out GUID[] ids,
string filter
);
I know my code gets reaches the C++ function (I can see this in windbg) and there is no crash, but from what I can tell, the parameters aren't being passed correctly. My guess is that I've messed up my P/Invoke translation in C#, but I have no idea how to fix this. Any help would be appreciated!

Looks like I found my solution...
[DllImport("MyDll.dll", EntryPoint="SomeFunction")]
[return: MarshalAs(UnmanagedType.I4)]
private static extern int SomeFunction
(
out uint count,
[MarshalAs(UnmanagedType.LPArray)]
out GUID[] ids,
[InAttribute()]
[MarshalAsAttribute(UnmanagedType.LPWStr)]
string filter
);