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How to return two different variable in c++?

dll C++
extern "C"
{
__declspec(dllexport) int mainfun()
{
return x;
}
}
In C#
[DllImport("example.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern int mainfun();
I only know how to return and call one variable from C++ to C#. I am writing a program where i need to call two different varables in C# from c++ dll
(like return x,y;). Please i need help.
EDIT1:
In C++
struct Point
{
int count_err;
int statement;
} obj;
extern "C"
{
__declspec(dllexport) Point mainfun()
{
return obj;
}
}
In C#
[StructLayout(LayoutKind.Sequential)]
public struct Point
{
public int errsize;
public int statmnt;
}
[DllImport("example.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern Point mainfun();
errsize = mainfun();
statmnt = mainfun();
Here errsize is giving an error-"the name 'errsize' does not exist in the current context".. What to do?
EDIT2:
In C#
total_errors.Text = p.errsize.ToString();
giving same error-"the name 'p' does not exist in the current." context"

Define new struct or array of data. Something like this:
C++:
struct Point
{
int count_err;
int statement;
} obj;
extern "C"
{
__declspec(dllexport) Point mainfun()
{
return obj;
}
}
C#:
[StructLayout(LayoutKind.Sequential)]
public struct Point
{
public int errsize;
public int statmnt;
}
[DllImport("example.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern Point mainfun();
Point p = mainfun();
var errsize = p.errsize;
var statmnt = p.statmnt;

First understand that if you want to return more than one value from any function, then you will need an object which can hold multiple values like struct, class object, list etc. But in your case you cannot use LIST or KeyValuePairList of C# because you have direct dependency with C++ code.
So use structure which is same in both the platforms. Now first you need to create a suitable data structure and change the return type of mainfun() when you are calling it as follows..
public struct abc {
public int a;
public int b;
}
[DllImport("example.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern abc mainfun();
Now in your C++ library, add the data structure and change the function definition.
typedef struct {
int a;
int b;
} abc;
extern "C"
{
__declspec(dllexport) abc mainfun()
{
abc obj;
obj.x = 1;
obj.y = 2;
return obj;
}
}

PInvoke with a void * versus a struct with an IntPtr

Imagine I have a function called
Myfunction(const void * x);
My C# declaration could be
MyFunction(IntPtr x);
Is this functionally and technically equivalent to
struct MyStruct { IntPtr P; }
MyFunction(MyStruct x);
Or will there be a difference in how they are marshalled.
I'm asking this because the library I'm calling is all void *, typedef'd to other names, and in C# I'd like to get type safety, for what it's worth.

If your StructLayout is Sequential, then it is indeed identical.
Easiest way to verify this for yourself is to try it out, of course:
Make a C++ Win32 DLL project:
extern "C"
{
__declspec(dllexport) void MyFunction(const void* ptr)
{
// put a breakpoint and inspect
}
}
Make a C# project:
public struct Foo
{
public IntPtr x;
}
[DllImport(#"Win32Project1.dll", EntryPoint = "MyFunction", CallingConvention = CallingConvention.Cdecl)]
public static extern void MyFunctionWithIntPtr(IntPtr x);
[DllImport(#"Win32Project1.dll", EntryPoint = "MyFunction", CallingConvention = CallingConvention.Cdecl)]
public static extern void MyFunctionWithStruct(Foo x);
static void Main(string[] args)
{
IntPtr j = new IntPtr(10);
var s = new Foo();
s.x = new IntPtr(10);
MyFunctionWithIntPtr(j);
MyFunctionWithStruct(s);
}
In your debug settings, make sure you select Native debugging is enabled.
You'll see both values to be 0xA.
Note, however, if you use out/ref parameters for your IntPtr vs Struct, they will be different values.

Marshalling PDouble (C# -> Delphi)

I've got an unmanaged DLL file written in Delphi, containing a function with the following definition:
function F(tgran: integer; inputs: PDouble; goutp, outputs, toutp: PDouble): integer; stdcall; external 'mydll.dll';
I've written an Adapter in C# that should help me consume it.
[UnmanagedFunctionPointer(APIAdapter.Convention)]
public delegate int FDelegate(int tgran, IntPtr inputs, IntPtr goutp, IntPtr outputs, IntPtr toutp);
public class APIAdapter : IDisposable
{
public const string DllName = "mydll.dll";
public const CallingConvention Convention = CallingConvention.StdCall;
public FDelegate F;
[DllImport("kernel32")]
private static extern IntPtr LoadLibrary(string lpLibFileName);
[DllImport("kernel32")]
private static extern bool FreeLibrary(IntPtr hModule);
[DllImport("kernel32.dll")]
private static extern IntPtr GetProcAddress(IntPtr hModule, String procname);
private IntPtr _dllHandle;
public APIAdapter()
{
_dllHandle = LoadLibrary(DllName);
F = (FDelegate)GetFunction<CalcCavSpDelegate>("F");
}
private Delegate GetFunction<T>(string procName)
{
IntPtr procAddress = GetProcAddress(_dllHandle, procName);
return Marshal.GetDelegateForFunctionPointer(procAddress, typeof(T));
}
~APIAdapter()
{
Dispose(false);
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
private void Dispose(bool disposing)
{
if (disposing)
{
}
while (FreeLibrary(_dllHandle))
{
}
}
}
The usage is pretty straightforward:
using(var api = new APIAdapter())
{
// Call API functions
}
The problem is an AccessViolationException that happens within the DLL file.
I've tried to pass the PDouble variables as double[], double* (unsafe code), IntPtr. It's the same story regardless of the method I choose. I've tried to substantially increase the size of the arrays passed in to exclude errors with array indexing - AccessViolation exception again.
What is the proper way of passing a PDouble into an unmanaged Delphi DLL file?

I think you can do away with all the LoadLibrary, GetProcAddress and FreeLibrary complexity by simply using DllImport attribute. Though, I can say that I am not aware about any specific reason of why you chose to go this way.
Anyway, you can simply include ref double doubleArg in your declaration to pass PDouble. There should be no need of IntPtr here.

C# Marshalled Callbacks

I am trying to Marshall c call backs that are in a struct. I am pretty sure I have everything correct, but when using my C# example I don't get events, when using c++ I do get events.
Here is the C#
class Program
{
[DllImport("Some.dll", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
public static extern int SetCallbacks(Callbacks callBack);
static Callbacks Callback = new Callbacks { DataArrived = DataArrived, SendFailure = SendFailure };
static void Main(string[] args)
{
SetCallbacks(Callback);
Console.ReadLine();
}
static void DataArrived(uint id, IntPtr data)
{
}
static void SendFailure(uint id, uint id2, IntPtr data)
{
}
}
[StructLayout(LayoutKind.Sequential)]
public struct Callbacks
{
public DataArrived DataArrived;
public SendFailure SendFailure;
}
[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public delegate void DataArrived(uint id, IntPtr data);
[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public delegate void SendFailure(uint id, uint id2, IntPtr ulpData);
This is from the C header file.
struct callBacks
{
void (*dataArriveNotif) (unsigned int, void*);
void (*sendFailureNotif) (unsigned int, unsigned int, void*);
}
int SetCallbacks(callBacks callBacks);
Here is the working c++.
struct callBacks;
callbacks.dataArriveNotif = &dataArriveNotif;
callbacks.sendFailureNotif = &sendFailureNotif;
SetCallbacks(callBacks);

Everything dealing with the delegate was actually correct. I simplified the senario a little bit in the example.
public static extern int SetCallbacks(Callbacks callBack);
was actually
public static extern int SetCallbacks(String[] array, Callbacks callBack);
The string array had lots of trailing 0's at the end. Which made the callback struct all nulls. I gave up trying to marshal the string[] the correct way and just made it a Intptr and everything started working.

so very similar to this question asked yesterday...
PInvoke C#: Function takes pointer to function as argument

C# Error Finding Method in DLLImport

I have a C++ assembly that I am importing using DLLImport.
I am attempting to call its method:
namespace Testing
{
class Test{
int Run(char* filePath, bool bEntry, double duration){//code}
};
}
by
[DllImport(dllName, CharSet = CharSet.Auto)]
public static extern int Run(string filePath, bool bEntry, double duration)
);
When I call its method, I get the error message:
Unable to find an entry point named Run in dll

The "Run" looks to be a non-static class method. Although, it's possible to call such methods from C# this is not the primary use-case. It would be way easier to consume it from .NET if you expose it via COM, or at-least as a plain C interface:
extern "C" __declspec(dllexport) void* Testing_Test_Create();
extern "C" __declspec(dllexport) void Testing_Test_Destroy(void* self);
extern "C" __declspec(dllexport) int Testing_Test_Run(void* self, char* filePath, bool bEntry, double duration);
And here is a sample how to call C++ class methods from C#:
// Test.cpp in NativeLib.dll
namespace Testing
{
class __declspec(dllexport) Test
{
public:
explicit Test(int data)
: data(data)
{
}
int Run(char const * path)
{
return this->data + strlen(path);
}
private:
int data;
};
}
// Program.cs in CSharpClient.exe
class Program
{
[DllImport(
"NativeLib.dll",
EntryPoint = "??0Test#Testing##QAE#H#Z",
CallingConvention = CallingConvention.ThisCall,
CharSet = CharSet.Ansi)]
public static extern void TestingTestCtor(IntPtr self, int data);
[DllImport(
"NativeLib.dll",
EntryPoint = "?Run#Test#Testing##QAEHPBD#Z",
CallingConvention = CallingConvention.ThisCall,
CharSet = CharSet.Ansi)]
public static extern int TestingTestRun(IntPtr self, string path);
static void Main(string[] args)
{
var test = Marshal.AllocCoTaskMem(4);
TestingTestCtor(test, 10);
var result = TestingTestRun(test, "path");
Console.WriteLine(result);
Marshal.FreeCoTaskMem(test);
}
}
Entry point names might be different for your build configuration/compiler, so use dumpbin utility to obtain them. Again, this is just a proof of concept, in real code it would be better to use COM.

See here: http://dotnetperls.com/dllimport

I'm not sure this will help if the function is a member of a class, but to locate the entry point by name, not ordinal, you'll need a .def file in your dll..
LIBRARY mylib
Run #1