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Call C++ Dll in C#.net using dllImport

Please find the below Examples
void ExitLibrary(); // C++ function
I am able to covert to C# like below
[DllImport("VisualFOD.dll", CharSet = CharSet.Ansi)]
public static extern void ExitLibrary();
This method will be used to start the live capturing
bool StartLive(DShowLib::Grabber* f_Grabber, HDC f_hHdc, RECT f_WindowRect);
This method will be used to start the live capturing
[DllImport("TIS_UDSHL09_vc71.dll", CharSet = CharSet.Ansi)]
public static extern bool StartLive(IntPtr f_Grabber, IntPtr f_hHdc, Rectangle f_WindowRect);
I am not able to access the actual class DShowLib::Grabber as mentioned in the 2nd statement C++ function.
Do you have any idea how to declare and get the library class name?
This dll is not a COM/.Net dll to include as a reference to C# environment.

If I understand your question correctly, you won't be able to access the DShowLib::Grabber class directly in C# if it resides inside the DLL (and is accessed by a C++ header file).
Two solutions are available. Firstly, you can wrap the native DLL class in C++/CLI class, such that you can include the native C++ header files; Secondly, you can write your own DLL, where you can flatten the DLL class and P/Invoke your flatten methods.
By flattening I mean:
class C
{
void M(int i, float j);
}
The method M has to be called like:
__declspec(dllexport) BOOL __stdcall C_M(C * instance, int i, float j)
{
if (instance == nullptr)
return FALSE;
instance->M(i, j);
return TRUE;
}
However, please take into consideration comments for your question. If you can use ready-made wrapper for library you are interested in, you shall do so :)

There were many attempts to make C#/C++ interop easier, but by far managed C++ from Microsoft is probably the best (though it is not part of C++ standard).
Another promising option is CXXI from Mono,
http://tirania.org/blog/archive/2011/Dec-19.html
which provides a standard compliant approach.
It still needs some hacking (Google Summer of Code 2012 maybe), but it should finally support Visual C++ and Windows.

Yes. You can instantiate and access a C++ class object directly from your own C# code via PInvoke, but it is very complicated and involves a lot of work, please go here and read what that C++ PInvoke Interop SDK can do.
http://www.xinterop.com/index.php/2013/04/13/introduction-to-c-pinvoke-interop-sdk/
(It is a commercial software though. I am the author)

.net wrapper for native dll - how to minimize risk of run-time error?

I am developing a C# application. Since I have some algorithms for least-squares fit in C/C++ that would be too cumbersome too translate, I have made the C++ code into a dll and then created a wrapper in C#.
In the C# code, I have defined a struct that is passed to the unmanaged C++ code as a pointer. The struct contains the initial guesstimates for the fitting functions, and it is also used to return the results of the fit.
It appears to me that you must define the struct in both the managed and the unmanaged code. However, someone using my source code in in the future might decide to change the fields of the struct in the C# application, without understanding that they also have to change the struct in the native code. This will result in a run-time error at best (and produce erroneous results at worse), but there will be no error message telling the developer / end user what is wrong.
From my understanding, it impossible to create a test in the unmanaged C++ DLL that checks if the struct contains the correct fields, but is it posible to have the DLL returning a struct of the correct format to the C# wrapper?
Otherwise, what ways are there to reduce the risk that some careless programmer in the future causes run-time errors that are hard to detect?
Sample code:
//C++
struct InputOutputStruct {
double a,b,c;
}
extern "C" __declspec(dllexport) void DoSomethingToStruct(InputOutputStruct* s)
{
// ... algorithm
}
//C#
using System.Runtime.InteropServices;
[StructLayout(LayoutKind.Sequential)]
public struct InputOutputStruct {
public double a,b,c;
}
[DllImport("CpluplusDll.dll")]
public static unsafe extern bool DoSomethingToStruct(InputOutputStruct* s);
class CSharpWrapper {
static void Main(string[] args)
{
InputOutputStruct s = new InputOutputStruct();
unsafe {
InputOutpustruct* sPtr = &s;
DoSomethingToStruct(sPtr);
s = *sPtr;
}
}
}

It appears to me that you must define the struct in both the managed
and the unmanaged code.
Not true. This is what C++/CLI was invented for- facilitate much easier interoperation with C++ and .NET.

but is it posible to have the DLL returning a struct of the correct format to the C# wrapper?
I don't think so, because you always need to define the structs on the C# side.
Here are a solution which may work ( never tested ):
Give each struct which is shared a unique identifier on both sides ( GUID, Macros )
Create a reflection for C++ which contains informations about the the types which are used on C# and C++ side. This could be done by using macros.
Compare the C++ structs and the C# structs on startup by using the GUIDs, reflection and macros. You can also use sizeof to compare sizes first.
That could be a lot of work
When working on C++ side you still can make a lot of things wrong when you not know about the macros

C++/CLI wrapper for C dll

So I have this C .dll source code which I want to use in my C# application. Instead of doing bunch of DllImports I've decided to write a wrapper for it in C++/CLI.
My C function takes a pointer to a struct with 4 callback functions in it:
typedef struct
{
PFN_IN readFp;
PFN_OUT writeFp;
}OPEN_ARGS;
C++/CLI shares the same .h file, therefore uses the same typedef.
C# code has it's own definition of this structure and delegates for CBs, because I can't attach .h to C# project.
[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public delegate Int32 PFN_OUT(IntPtr arg, IntPtr pSrc, Int32 len);
[StructLayout(LayoutKind.Sequential)]
public struct OPEN_ARGS
{
public PFN_IN readFp;
public PFN_OUT writeFp;
};
So, when I add my C++/CLI dll explicitly to C# project references, the compliler wouldn't accept calls to C++/CLI functions saying
"Error 2 Argument 2: cannot convert from 'WrapperTest.Program.OPEN_ARGS' to 'SAR_OPEN_ARGS'"
But if I include the C++/CLI dll implicitly like that
[DllImport("Wrapper.dll", CharSet = CharSet.Auto, EntryPoint = "?CLIOpen##YAHHUOPEN_ARGS###Z")]
public static extern int CLIOpen(int a, OPEN_ARGS args);
It will work just fine.
So is there a way to tell C# compiler to ignore this type cast error, or may be other way to wrap C code functions?
EDIT: cleaned up variable names for better readabiltiy

What if you did this another way. Since you have a C++/CLI DLL handling interop duties between the C DLL and the C# assembly, you could expose an equivalent API, only using more .NET-like concepts.
For example, instead of exposing the struct with function pointers, you could expose a class that has three events. The C# assembly would add handlers for those events. Inside the C++ DLL, it would use the function pointers that the C DLL expects, but their implementation would fire the .NET events that the C# assembly is handling.
This would provide a much better experience using the DLL on the C# side, and likely get rid of the interop compiler errors that you're encountering.

Please consider using SWIG to generate the wrapper code for all your pinvoke.
http://www.swig.org/Doc1.3/CSharp.html

So for managed C++, you can use the #pragma managed/unmanaged compiler directives instead of pInvoke, which it looks like you are using. Then you can compile managed and native code together into the same assembly, even the same CPP file.
Then you could do something like:
#pragma managed
// include your native headers here
#include "foo.h" // whatever you call it.
#using <System.dll> // what ever else you need here...
// Have to wrap a function in a class, since .NET doesn't allow free standing functions.
public ref class foo
{
public static int sarCLIOpen(int a, SARWrapperTest::Program::SAR_OPEN_ARGS args)
{
// do something to convert your managed args to native args.
::SAR_OPEN_ARGS native_args = ...
// then call your native function
return sarCLIOpen(a, native_args );
}
};

how do i use the c++ dll in c#

i like import c++ dll in my c# application how can i Do this?, what is concept i need to use for this?

You need to look at different kinds of interop. The easiest is P/Invoke. Then is is COM Interop. And then finally you can use Managed C++

Have access to the C++ library source
The cleanest way is to use C++ Interop to create a wrapper as a mixed mode assembly. This allows you to use the pragmas managed and unmanaged to switch between native and managed code. This way you can nicely wrap your C++ class with a managed C++ wrapper and call it from (of course) managed C#.
Be aware this won't work under Mono.
Don't have access to the C++ library source, but know a bit of C++?
You can write a small managed wrapper in managed C++ to call the unmanaged C++ library due to C++'s unique ability to call COM natively.
This is done using custom runtime callable wrappers, or CRCWs. You can then call your managed wrapper directly from C# using native .Net types and all.
Benefits of this (as stated by MSDN):
The interop code is built into the application, so there is no dependency on a separate assembly. Also, the exposed managed interface is customized to be more .NET-like. For example, the RenderFile method takes a System.String instead of a char*. The managed version of the COM interface is called a Custom Runtime Callable Wrapper (CRCW).
Don't have access to the C++ library source and don't know C++?
Then you are stuck with C++ COM Interop which is a bit more messy. You'll need to generate a wrapper using Tlbimp.exe.
It's messy because:
Types that don't have a direct match in managed code are exposed as IntPtr pointer types (quite difficult to handle in C#)
The resulting assemlblies will be very large as interfaces for everything in the library are generated (not just the stuff you want).
You'll have to deloy an additional assembly with your application and manage versioning etc...

suppose this DLL that compile with MinGW.
in C-sharp you can follow this code:
using System.Runtime.InteropServices;
using System;
class call_dll {
[StructLayout(LayoutKind.Sequential, Pack=1)]
private struct STRUCT_DLL {
public Int32 count_int;
public IntPtr ints;
}
[DllImport("mingw_dll.dll")]
private static extern int func_dll(
int an_int,
[MarshalAs(UnmanagedType.LPArray)] byte[] string_filled_in_dll,
ref STRUCT_DLL s
);
public static void Main() {
byte[] string_filled_in_dll = new byte[21];
STRUCT_DLL struct_dll = new STRUCT_DLL();
struct_dll.count_int = 5;
int[] ia = new int[5];
ia[0] = 2; ia[1] = 3; ia[2] = 5; ia[3] = 8; ia[4] = 13;
GCHandle gch = GCHandle.Alloc(ia);
struct_dll.ints = Marshal.UnsafeAddrOfPinnedArrayElement(ia, 0);
int ret=func_dll(5,string_filled_in_dll, ref struct_dll);
Console.WriteLine("Return Value: " + ret);
Console.WriteLine("String filled in DLL: " + System.Text.Encoding.ASCII.GetString(string_filled_in_dll));
}
}

This is the concept you need to use :)

Writing a managed wrapper for unmanaged (C++) code - custom types/structs

faacEncConfigurationPtr FAACAPI faacEncGetCurrentConfiguration(
faacEncHandle hEncoder);
I'm trying to come up with a simple wrapper for this C++ library; I've never done more than very simple p/invoke interop before - like one function call with primitive arguments.
So, given the above C++ function, for example, what should I do to deal with the return type, and parameter?
FAACAPI is defined as: #define FAACAPI __stdcall
faacEncConfigurationPtr is defined:
typedef struct faacEncConfiguration
{
int version;
char *name;
char *copyright;
unsigned int mpegVersion;
unsigned long bitRate;
unsigned int inputFormat;
int shortctl;
psymodellist_t *psymodellist;
int channel_map[64];
} faacEncConfiguration, *faacEncConfigurationPtr;
AFAIK this means that the return type of the function is a reference to this struct?
And faacEncHandle is:
typedef struct {
unsigned int numChannels;
unsigned long sampleRate;
...
SR_INFO *srInfo;
double *sampleBuff[MAX_CHANNELS];
...
double *freqBuff[MAX_CHANNELS];
double *overlapBuff[MAX_CHANNELS];
double *msSpectrum[MAX_CHANNELS];
CoderInfo coderInfo[MAX_CHANNELS];
ChannelInfo channelInfo[MAX_CHANNELS];
PsyInfo psyInfo[MAX_CHANNELS];
GlobalPsyInfo gpsyInfo;
faacEncConfiguration config;
psymodel_t *psymodel;
/* quantizer specific config */
AACQuantCfg aacquantCfg;
/* FFT Tables */
FFT_Tables fft_tables;
int bitDiff;
} faacEncStruct, *faacEncHandle;
So within that struct we see a lot of other types... hmm.
Essentially, I'm trying to figure out how to deal with these types in my managed wrapper?
Do I need to create versions of these types/structs, in C#? Something like this:
[StructLayout(LayoutKind.Sequential)]
struct faacEncConfiguration
{
uint useTns;
ulong bitRate;
...
}
If so then can the runtime automatically "map" these objects onto eachother?
And, would I have to create these "mapped" types for all the types in these return types/parameter type hierarchies, all the way down until I get to all primitives?
I know this is a broad topic, any advice on getting up-to-speed quickly on what I need to learn to make this happen would be very much appreciated! Thanks!

Your are on the right track with how you would need to create managed structures that represent unamanged structures for use with P/Invoke.
It is however not the best strategy for interop with unmanaged libraries, because using this API from C# would still feel like using a C API - create and initialise a structure, pass it to a function and get some other structure in return.
It is OK to use P/Invoke for an odd function call that is not otherwise exposed as .NET API, but for a full blown API wrapper I would highly recommend using managed C++(C++/CLI). It is absolutely unparalleled in creating unmanaged interop layers for .NET.
The biggest challenge would be to convert this essentially C interface into an object orientated interface where you call methods off objects, as opposed to calling global functions with structures of all-public members.
As you start writing elaborate structure graphs for P/Invoke you would yourself having to deal with quite a bit of "magic" that governs how managed primitive types convert to unmanaged types. Often, using incorrect types will cause a run-time error.
With managed C++ (IJW - It Just Works) you define managed structures, classes, interfaces in C++, which allows more natural use of the underlying library and a more native interface to your C# application.
This is a great overview of C++/CLI. Also MSDN has extensive documentation for all managed C++ features.

Yes, you would need to declare all these structures in c#. Be careful to declare members with correct sizes. For example, 'long' is 32-bit in C++, but 64-bit in C#. For a pointer or void* in C++, use IntPtr in C#, etc.