My c++ function is given below
# define MyFunction _declspec(dllexport)
extern "C" {
MyFunction int SubtractNumbers(int a, int b)
{
return a - b;
}
MyFunction const char* GetStringKey()
{
return "My String";
}
}
Calling c++ function from windows form is given below,
[DllImport(cppFunctionsDll, CallingConvention = CallingConvention.Cdecl)]
private const string DllFilePath = #"D:\\Projects\\December\\17-12-2020\\project-device-
setup\\Debug\\AccurynCPP.dll";
[DllImport(DllFilePath, CallingConvention = CallingConvention.Cdecl)]
public static extern int SubtractNumbers(int a, int b);
[DllImport(DllFilePath, CallingConvention = CallingConvention.Cdecl)]
public static extern string GetStringKey();
public void GetSubtract()
{
lblSubtract.Text= Convert.ToString(SubtractNumbers(1, 2));
}
public void GetStringFunction()
{
lblstringKey.Text= GetStringKey();
}
From the above function, GetSubtract() worked perfectly. But GetStringKey() not worked. when it reach on it's function while debugging, it automatically cancelled running mode in visual studio. How can i fix this issue.
The calling convention describes to the compiler and linker how parameters, the stack and registers are to be handled. So if these don't match things won't work or memory can be corrupted. For instance, if the caller thinks the first parameter should be passed in the register eax, but the callee thinks it's on the stack. Things will obviously not work. For more info on calling conventions Wikipedia has a good description here. The calling convention you choose isn't very important, unless the compiler forces one on you. For instance for 64 bit programs Microsoft only uses one to my knowledge. So, I suggest adding __cdecl in front of the functions you have written. And using CallingConvention.Cdecl on the DLLImport lines.
Also, all the data types must exactly match. For instance:
[DllImport(DllFilePath, CallingConvention = CallingConvention.Cdecl)]
public static extern string GetStringKey();
This says it is returning a string, however despite our using the term string to represent a series of characters followed by a null. In this case string is a formal type. Which is they type std::string. Which is not what is being returned by the C code. The C code is returning a pointer to a char.
Generally passing complex types based on a library is kinda iffy. That's because it requires both languages to use the exact data structure to represent the type. So when crossing language boundaries it's often useful to stay with your own custom structures and very primitive types.
So I recommend your C function be this instead:
__cdecl bool GetStringKey(char *buffer, int maximumLength)
{
// Write code here to copy all the characters from your "My String"
// to the buffer or use a standard library function for copying char * strings (often called C strings) when copying make sure to copy the null character at the end. It's definitely needed.
return true; // Yes it fit
}
You will need to change your DllImport to match. Once that is working you can add code to never go over the maximum buffer length (to prevent memory corruption) and maybe change the bool to the length of the string or -1 for failure.
I suggested this particular function prototype because it doesn't pass memory between languages. Instead the C code fills in a buffer from the C++ code. Which is the safest way to do something. Passing memory between unrelated modules (in this case C and C++ doesn't always work). Though I suspect in this case your C and C++ compiler and libraries are enough alike. That you could just switch to returning a std::string. As long as both sides match perfectly.
Related
If I have a native code that expects a structure with two fields:
[DllImport(LibraryName, CallingConvention = CallingConvention.Cdecl)]
public static extern int my_method(ref MyStruct myStruct);
// this is what native code expects
public struct MyStruct {
IntPtr First;
IntPtr Second;
}
but instead I pass another struct to it, will it work or not - by design or by accident?
[DllImport(LibraryName, CallingConvention = CallingConvention.Cdecl)]
public static extern int my_method(ref MyLongerStruct myLongerStruct);
// this is what I want to pass to it
public struct MyLongerStruct {
IntPtr First;
IntPtr Second;
object ObjectPointer;
}
Will the object reference added to the end of the struct at C# side somehow affect P/Invoke call?
I shouldn't work. And even more, you need to add and properly set StructLayoutAttribute to the structure, as it explained here
I think, the result should be like this:
[StructLayout(LayoutKind.Sequential)]
public struct MyStruct {
IntPtr First;
IntPtr Second;
}
If the total difference in structure is fields added to the end, and you use StructLayout to prevent the compiler from optimizing the memory layout of your struct (as Alex Butenko suggests), then it's unlikely that there will be any negative side effects apart from a slight speed hit.
When you pass a managed struct to an external function via P/Invoke (using the DllImport attribute) there is a marshaling phase that converts your struct to a compatible format for the target. For ref and out parameters the temporary is converted back when the invoked function returns, copying the values back to your struct instance. All of this is abstracted away, although exactly how the marshaling is performed for each member can be tweaked with the right attributes.
This is how the .NET framework handles strings in P/Invoke. Since it can't just send a string instance pointer to an API function that is expecting a char * (the two are nothing alike) there has to be some translation.
The fun part is that the marshaling code doesn't know anything about what the target is expecting other than what you tell it at the C# end, so if you are sending an extended version of the structure it will do the whole thing. At the other end the native code will get a pointer to a memory block containing the information it's expecting, and it won't have any way to tell that there is more after the end of the structure.
Apart from that, no problem... as long as you're passing by reference and not by value. Passing structs by value is something that should raise big red stop signs all over your brain. Don't do it, it's evil.
The exception:
Managed Debugging Assistant 'PInvokeStackImbalance' has detected a problem in 'D:\UI_still_in_progress\user_interface\bin\Debug\WindowsFormsApplication1.vshost.exe'.
Basically my application has a background real time process in c++ which communicates with a micro-processor. Ive set up a c++ dll file which allows me to pass data from the c++ application and c# UI in real time.
My problem is that I have to create a finite state machine to make sure the physical device and c++ app are all doing what they should be based on whats going on in the UI. so my goal is to pass simple integers representing the states from c# to c++.
The real time data is passing from c++ to c# with no issues using the following implementation:
c++ .h file snippet:
extern "C" __declspec (dllexport) int grabx();
extern "C" __declspec (dllexport) void setFSMstate(int s);
C++ .cpp file snippet:
int grabx()
{
return x0;
}
void setFSMstate(int s)
{
state = s;
}
c# class
public class MyDllMethods {
[DllImport("dllTESTER.dll")]
public static extern int grabx();
[DllImport("dllTESTER.dll")]
public static extern void setFSMstate(int s);
}
c# method calls
xx = MyDllMethods.grabx();
MyDllMethods.setFSMstate(2);
The grabx() function works fine and i can read the x value from the device in real time. When my UI moves to a new screen i attempt to change the state in every application by passing a 2 into setFSMstate(). When this method is called the exception is thrown.
I am quite new to P/Invoke so if there is anything I missed please help out. I may have marshaled it wrong but im confident that type int is the same in c# and c++?
The problem you're running into is a calling convention mismatch. Calling conventions specify how parameters are passed to functions, and how their return values are passed back to the caller. Your C++ functions are being declared with an implicit calling convention of __cdecl, but P/Invoke's default calling convention is __stdcall.
To fix this, you just have to change your P/Invoke configuration:
[DllImport("dllTESTER.dll", CallingConvention = CallingConvention.Cdecl)]
(The reason you didn't have any problems with your first method is that it doesn't have any parameters—__cdecl and __stdcall pass parameters differently, but they both specify that integer return values are stored in the EAX register.)
I am using PInvoke in C# to call a function in a C++ DLL. The returned object contains a bool, but the value is never correct. I've read several articles on this that indicate a C++ bool is 1 byte while in C# a bool is 4 bytes. I've also seen examples on how to overcome this problem when a function returns a bool, but my function is returning an object that contains a bool. I'm not finding any articles that show how to handle this situation.
For example, I have two structs. One for input to the unmanaged DLL's function and one that defines the output:
public struct MyInput
{
public int id;
public string ModelNumber;
}
public struct MyOutput
{
public int SomeValue;
public double AnotherValue;
public bool IsValid;
}
[DllImport("MyUnmanaged.dll", EntryPoint = "?Unit##YA?AUMyOutput##UMyInput###Z")]
public static extern MyOutput Unit(ref MyInput UnitInput);
MyOutput doesn't work as defined above because the signature is invalid. So I changed IsValid to:
public int IsValid;
This took care of the invalid signature error, but the value of IsValid isn't a 0 or 1 like I would expect. Instead it is -858993664. I've also tried changing to byte, but then the returned value is always 0.
I don't understand how to marshal the bool inside of the MyOutput struct or how to define it so that I get a valid bool - a 1 or a 0.
Btw, I don't have any control of the unmanaged DLL...
Can someone provide me with an idea of how to define the bool inside of the MyOutput struct?
EDIT
All I was given is the .DLL and .h for the unmanaged DLL. In the .h, the bool in question is defined as:
bool IsValid;
-858993664 == 0xcccccc00. Sure, this is the kind of value you'd expect to get when you don't use byte or apply the [MarshalAs(UnmanagedType.U1)] attribute on the member. Clearly you are using the debug build of that DLL, it initializes memory to 0xcccccccc.
So you are really getting a false value for IsValid. Of course there's no reasonable guess you could get here why the native code is unhappy. You'll need to talk to the C programmer to learn more about it. You'll also need his help when it is time to deploy your program, you can't ship the debug build of that DLL.
According to this online demangler, your unmanaged function has this signature:
struct MyOutput __cdecl Unit(struct MyInput)
So your C# declaration should be:
[DllImport("MyUnmanaged.dll", EntryPoint = "?Unit##YA?AUMyOutput##UMyInput###Z",
CallingConvention = CallingConvention.Cdecl)]
public static extern MyOutput Unit(MyInput UnitInput);
In other words you were using the wrong calling convention, and passing the parameter by ref rather than by value.
You have also discovered that your return value struct type cannot be marshalled because it contains a bool field. You'll need to change that to byte or int depending on what the unmanaged struct declaration is. It looks like, based on your edits, that the header file declares it as bool, which is a single byte type, so you should use byte.
I'm trying to figure out how to return a complex object from a C++ dll to a calling C# application. I have a simple method which is returning an int that is working fine. Can anyone tell me what I'm doing wrong?
C# Application:
class Program
{
static void Main(string[] args)
{
// Error on this line: "PInvoke: Cannot return variants"
var token = LexerInterop.next_token();
}
}
C# LexerInterop Code:
public class LexerInterop
{
[DllImport("Lexer.dll")]
public static extern object next_token();
[DllImport("Lexer.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern int get_int(int i);
}
C++ Lexer.cpp
extern "C" __declspec(dllexport) Kaleidoscope::Token get_token()
{
int lastChar = ' ';
Kaleidoscope::Token token = Kaleidoscope::Token();
while(isspace(lastChar))
{
lastChar = getchar();
}
... Remainder of method body removed ...
return token;
}
extern "C" __declspec(dllexport) int get_int(int i)
{
return i * 2;
}
C++ Lexer.h
#include "Token.h"
namespace Kaleidoscope
{
class Lexer
{
public:
int get_int();
Token get_token();
};
}
C++ Token.h
#include <string>
namespace Kaleidoscope
{
enum TokenType
{
tok_eof = -1,
tok_def = -2,
tok_extern = -3,
tok_identifier = -4,
tok_number = -5
};
class Token
{
public:
TokenType Type;
std::string IdentifierString;
double NumberValue;
};
}
I'm sure it's something to do with the Token return type but I can't find any decent information what's missing.
Any assistance or direction is appreciated!
You cannot return C++ objects to non-C++ callers. (It doesn't even work when you try to export a C++ object to a caller if it's compiled with a different C++ compiler.) The reason is that the actual in-memory layout of the C++ object is not standardized and different compilers may do it differently.
The .NET runtime has no idea how to deal with your object and it doesn't know about the various types that make up your token object. Similarly, std::string makes no sense to .NET, since it's a C++ type, relying on unmanaged memory allocation and with a different internal string format and different semantics than C#.
You can try turning your C++ objects into COM objects and then you can return COM interfaces to your C# caller. This will take some work since COM types are, again, incompatible with C++ types (for similar reasons as above).
You can also try to serialize your C++ object into a byte array and then just return a buffer / length to C# that will first deserialize the object into a .NET representation. You'll have to duplicate the classes you're trying to handle this way.
Yet another possibility is that you return all data as independent out parameters from your function calls - that is, you don't try to return token but return each of its properties as a separate out parameter from C++. You'll still have to convert some of your data types to something recognizable by .NET. (std::string cannot be returned, you'll have to copy it to a byte array or allocate it as a BSTR, etc.)
Finally, a fairly involved option: you can return the address of token as a void* from your C++ function and then create a number of exported plain C functions to read the various properties based on an input pointer. You'd also need a function to free up the C++ object when you're done with it. Something like:
__declspec(dllexport) TokenType WINAPI GetTokenType ( Kaleidoscope::Token* pToken )
{
return ( pToken->Type );
}
You'd then declare these functions in C# like so:
[DllImport ( "MyDll.dll" )]
public static extern int GetTokenType ( UIntPtr pObj );
The UIntPtr instance would be initialized from the void* that your next_token function returns. You'd then call each of the exported property reader functions to get the individual properties of token.
I'd probably go with a custom serializer because that's the least amount of work (in my opinion) and I think it's the cleanest solution (in terms of readability and maintainability).
Note: using COM objects could be less work but you'd have to move away from any C++ types (at least in the public interface).
Edit:
I forgot to mention earlier a somewhat obvious option - using mixed code and C++/CLI. This way you can have unmanaged code and managed code at the same time in the same DLL and you can perform all conversions in C++ code. If you define most of your classes in C++/CLI, you can just pass the relevant (managed) instances to your C# application without additional conversion.
(You will, of course, still have to convert between std:string and System.String if you use the former but at least you can have all your conversion logic in the same single project.)
This solution is simple but the resulting DLL now requires the .NET runtime.
In a function in my C++ DLL, I'm returning a std::string to my c# application. It pretty much looks like this:
std::string g_DllName = "MyDLL";
extern "C" THUNDER_API const char* __stdcall GetDLLName()
{
return g_DllName.c_str();
}
But when my C# code calls this function, I get this message in my output window:
Invalid Address specified to RtlFreeHeap( 00150000, 0012D8D8 )
The function declaration in c# looks like this:
[DllImport("MyDll", EntryPoint = "GetDLLName")]
[return: MarshalAs(UnmanagedType.LPStr)]
public static extern string GetDLLName();
From what I've been able to find online, sometimes this message appears when there's an inconsistency between which version of new(debug or release, etc) is being used with delete. But I'm not sure if that is what is going on in my case. So I'm not sure exactly what's causing it. Maybe the MashallAs might have something to do with it?
Any ideas?
Thanks!
I managed to find the issue. It was the way the C# definition was done. From what I can understand, using the MarshallAs(UnmanagedType.LPStr) in combination with the string return type makes it so that it'll attempt to free the string when its done. But because the string comes from the C++ DLL, and most likely a totally different memory manager, it fails. And even if it didn't fail, I don't want it to be freed anyway.
The solution I found was to change the C# declaration to this (the C++ code is unchanged):
[DllImport("MyDll", EntryPoint = "GetDLLName")]
public static extern IntPtr GetDLLName();
So this makes it so that it just returns a pointer to the string data. And then to change it to a string, pass it to Marshal.PtrToStringAnsi()
return Marshal.PtrToStringAnsi(GetDLLName());
And that gets wrapped into another function for cleanliness.
I found the solution from this page:
http://discuss.fogcreek.com/dotnetquestions/default.asp?cmd=show&ixPost=1108