I'm using a native DLL that is called out of my C# application. One of the DLL-functions is defined like this:
int set_line(const int width,fct_line_callback callback)
fct_line_callback itself is defined as
typedef int (*fct_line_callback)(double power,void *userData);
So how can I use this function out of my C#-application? Is there a way to define a C# method to be used as callback-function for this DLL-call?
Thanks!
You have to declare a delegate type that matches the native function pointer. It probably should look like this:
[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
delegate int fct_line_callback(double power, IntPtr userdata);
Now you can write the pinvoke declaration:
[DllImport("foo.dll", CallingConvention = CallingConvention.Cdecl)]
extern static int set_line(int width, fct_line_callback callback);
If the callback can only be made while set_line() is executing then calling the native function is simple:
public void SetLine(int width) {
set_line(width, MyCallback);
}
private void MyCallback(double power, IntPtr userdata) {
// etc...
}
However, if the callback can be made after set_line() is executed, in other words when the native code stores the function pointer, then you have to make sure that the garbage collector cannot collect the delegate object. Simplest way to do so is by storing the object in a static variable:
static class Wrapper {
private static fct_line_callback callback = MyCallback;
public static void SetLine(int width) {
set_line(width, callback);
}
private static int MyCallback(double power, IntPtr userdata) {
// etc...
}
}
Related
I used to register a delegate from C# to a callback function pointer like this (without using Marshal.GetFunctionPointerForDelegate):
C++ (Test.dll)
typedef void (__stdcall* Callback)(int num);
Callback pCallback = 0;
void __stdcall SetCallback(Callback callback)
{
pCallback = callback;
}
void __stdcall ActionCpp()
{
if(pCallback)
{
pCallback();
}
}
C#
class Program
{
[DllImport("Test.dll", CallingConvention = CallingConvention.StdCall)]
public static extern void SetCallBack(Callback callback);
[DllImport("Test.dll", CallingConvention = CallingConvention.StdCall)]
public static extern void ActionCpp();
public delegate void Callback();
static void Action()
{
Console.WriteLine("Callback succeeded.");
}
static void Main(string[] args)
{
Callback myAction = new Callback(Action);
SetCallback(myAction);
ActionCpp(); //Suppose to do the same thing as Action();
}
}
It seems that this way works fine. However, I found that we can do the same thing by using Marshal.GetFunctionPointerForDelegate and register the IntPtr of the delegate to the function pointer in C++.
I would like to know what the difference is and which practice is better? (And also Why? Thanks in advance.)
Here is the C# code when using Marshal.GetFunctionPointerForDelegate. (Nothing changes in C++ code.)
C# (With Marshal.GetFunctionPointerForDelegate)
class Program
{
[DllImport("Test.dll", CallingConvention = CallingConvention.StdCall)]
public static extern void SetCallBack(IntPtr pCallBack); //change to type "IntPtr" this time.
[DllImport("Test.dll", CallingConvention = CallingConvention.StdCall)]
public static extern void ActionCpp();
public delegate void Callback();
static void Action()
{
Console.WriteLine("Callback succeeded.");
}
static void Main(string[] args)
{
Callback myAction = new Callback(Action);
//GCHandle gcHandle = GCHandle.Alloc(myAction); <-- Is this necessary?
IntPtr pMyAction = Marshal.GetFunctionPointerForDelegate(myAction);
SetCallback(pMyAction);
ActionCpp();
//gcHandle.Free();
}
}
I have another further but related question about whether it is necessary to use "GCHandle.Alloc" (as in my comments in the above code) to avoid any GC action as long as my Callback myAction is still alive?
I am a newbie on C# and C++ callbacks, please let me know if I have naive mistakes made.
A callback requires the start address of the method that handles the event. So your two methods are both getting the same start address. The first method is clear since you are using the name of the method. 2nd method you have more statements that accomplishes the same as first method.
c# is managed language while c++ is un-managed. Which basically means that extra protection was added to c# to prevent PC from going blue screen. So when calling un-managed c++ from c# rules must be followed to prevent blue screen. So any pointer variables passed to a c++ method must be allocated in c# as un-managed using GCHandle.Alloc or by using other Marshal methods. Marshal.StrToPtr will also allocate the un-managed memory.
I'm having trouble converting a C++ .dll function to C#.
The function is this:
void funct(void*(*handler)(void*));
I think this means passing a pointer to function taking a void pointer and returning a void pointer, as explained here:
Passing Function Pointer.
What I'm trying to do is do the same thing in C#, but I have do idea how. I tried to use delegates, but I am both unsure how to and also if they can even do what I am trying to do.
Thanks for the help!
EDIT:
Here are the C++ functions for register_message_handler and message_handler:
void register_message_handler(void*(*handler)(void*));
void *message_handler(void *raw_message);
EDIT:
xanatos has the exact explanation and conversion to C# below. Thanks a ton xanatos!
void funct(void*(*handler)(void*));
is a function that accepts a pointer and returns a pointer.
In C# it would be:
IntPtr MyFunc(IntPtr ptr);
So you'll need a delegate like:
public IntPtr delegate MessageHandlerDelegate(IntPtr ptr);
[DllImport("mydll.dll")]
public static extern void register_message_handler(MessageHandlerDelegate del);
Note that P/Invoke (calling native methods) is one of the rare cases where Action<...> and Func<...> delegates don't work, and you have to build "specific" delegate.
BUT to call it, it's a little complex, because you must save a "copy" of the delegate so that if the C functions calls this method at any time, this copy is still "alive" and hasn't been GC (see for example https://stackoverflow.com/a/5465074/613130). The most common way to do it is to encapsulate everything in a class, and put the copy in a property/field of the class:
class MyClass
{
public delegate IntPtr MessageHandlerDelegate(IntPtr ptr);
[DllImport("mydll.dll")]
public static extern void register_message_handler(MessageHandlerDelegate del);
[DllImport("mydll.dll")]
public static extern IntPtr message_handler(IntPtr message);
public MessageHandlerDelegate Del { get; set; }
public void Register()
{
// Make a copy of the delegate
Del = Handler;
register_message_handler(Del);
}
public IntPtr Handler(IntPtr ptr)
{
// I don't know what ptr is
Console.WriteLine("Handled");
return IntPtr.Zero; // Return something sensible
}
}
Note that if you use IntPtr then you don't need the unsafe.
If you want to pass message_handler to register_message_handler the safest way is to
// Make a copy of the delegate
Del = message_handler;
register_message_handler(Del);
There is a possibility that you can do directly no there isn't, checked
register_message_handler(message_handler);
and that the CLR will solve this, BUT I'm not sure of this... I can't easily test it, and I wouldn't do it. (if you want to test it, add a GC.Collect() just after the register_message_handler. If after some time you receive a CallbackOnCollectedDelegate error then you know you can't do it :-) )
Mmmh... checked. You can't do the raw register_message_handler(message_handler), you have to use MyClass.
Be very aware of something: it's better to always specify the calling convention C-side and C#-side even in function pointers. C# uses stdcall, while C uses cdecl. In x86 mode you can get very awful silent crashes (in x64 there is a single calling convention)
void __stdcall register_message_handler(void* (__stdcall *handler)(void*));
void * __stdcall message_handler(void *raw_message);
and C# side
[UnmanagedFunctionPointer(CallingConvention.StdCall)]
public delegate IntPtr MessageHandlerDelegate(IntPtr ptr);
[DllImport("Win32Project1.dll", CallingConvention = CallingConvention.StdCall)]
public static extern void register_message_handler(MessageHandlerDelegate del);
[DllImport("Win32Project1.dll", CallingConvention = CallingConvention.StdCall)]
public static extern IntPtr message_handler(IntPtr message);
(or everywhere cdecl)
I'm trying to execute some methods (in this particular case, rdOnAllDone) from a third party DLL, written in C, and looking trough the headers files, I found this:
#ifndef TDECLSDONE
#ifdef STDCALL
#define CCON __stdcall
#else
#define CCON __cdecl
#endif
#define TDECLSDONE
#endif
#define DLLIMP __declspec (dllimport)
DLLIMP int CCON rdOnAllDone (void(CCON *)(int));
After goggling for a way to call this method, I made this:
[DllImport("sb6lib.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern int rdOnAllDone(Delegate d);
public delegate void rdOnAllDoneCallbackDelegate();
private static void rdOnAllDoneCallback()
{
Console.WriteLine("rdOnAllDoneCallback invoked");
}
The method was called correctly except that I couldn't get the int parameter. So I tried adding the input parameter int like this
[DllImport("sb6lib.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern int rdOnAllDone(Delegate d);
public delegate void rdOnAllDoneCallbackDelegate(int number);
private static void rdOnAllDoneCallback(int number)
{
Console.WriteLine("rdOnAllDoneCallback invoked " + number);
}
But now delegate is called twice and and it crashes the program with the following error " vshosts32.exe has stopped working"
What's the correct way to call this DLL method?
EDIT: Forgot to add the Main method:
public static void Main()
{
rdOnAllDoneCallbackDelegate del3 = new rdOnAllDoneCallbackDelegate(rdOnAllDoneCallback);
rdOnAllDone(del3);
while (true)
{
Thread.Sleep(1000);
}
}
Three things you need to do to make this work right:
you need to tell the pinvoke marshaller about the actual delegate type, using Delegate isn't good enough. That will create the wrong thunk that won't properly marshal the argument. Which is what you saw happening.
you need to tell the marshaller about the calling convention if it isn't __stdcall with the [UnmanagedFunctionPointer] attribute. Getting this wrong imbalances the stack with good odds for a hard crash.
you need to store a reference to the delegate object so that the garbage collector won't collect it. It cannot see references held by native code. Getting this wrong makes the native code fail with a hard crash after the next garbage collection.
So this ought to work better, tweak as necessary:
[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public delegate void rdOnAllDoneCallbackDelegate(int parameter);
[DllImport("sb6lib.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern int rdOnAllDone(rdOnAllDoneCallbackDelegate d);
class Foo {
private static rdOnAllDoneCallbackDelegate callback; // Keeps it referenced
public static void SetupCallback() {
callback = new rdOnAllDoneCallbackDelegate(rdOnAllDoneCallback);
rdOnAllDone(callback);
}
private static void rdOnAllDoneCallback(int parameter) {
Console.WriteLine("rdOnAllDoneCallback invoked, parameter={0}", parameter);
}
}
Your delegates signature has to match that of the native callback, also it has to have the UnmanagedFunctionPointerAttribute set appropriately.
In your case like so:
[UnmanagedFunctionPointerAttribute(CallingConvention.Cdecl)]
public delegate void rdOnAllDoneCallbackDelegate(int parameter);
[DllImport("sb6lib.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern int rdOnAllDone(rdOnAllDoneCallbackDelegate callback);
Usage:
{
rdOnAllDone(rdOnAllDoneCallback);
}
private static void rdOnAllDoneCallback(int parameter)
{
Console.WriteLine("rdOnAllDoneCallback invoked, parameter={0}", parameter);
}
I'm trying to call functions of a C DLL.
But I got a StackOverflowException so I think something is wrong with the function as parameter.
In detail it looks like this.
C DLL (header file):
typedef struct
{
MyType aType; /* message type */
int nItems; /* number of items */
const MyItems *lpItem; /* pointer to array of items */
} MyStruct;
typedef void (__stdcall *MyCbFunc) (HWND, const MyStruct *);
API(BOOL) RegisterCbFunc (ARGS, MyCbFunc);
In C# I tried this:
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
public struct MyStruct
{
MyType aType;
int nItems;
MyItems[] lpItem;
}
[UnmanagedFunctionPointer(CallingConvention.StdCall)]
public delegate void CallbackDelegate(MyStruct mStruct);
[DllImport("MY.dll", CallingConvention=CallingConvention.StdCall)]
private static extern int RegisterCbFunc(IntPtr hWnd, Delegate pCB);
public static void MyCbFunc(MyStruct mStruct)
{
// do something
}
static void Main(string[] args)
{
CallbackDelegate dCbFunc = new CallbackDelegate(MyCbFunc);
int returnValue = RegisterCbFunc(IntPtr.Zero, dCbFunc);
// here, returnValue is 1
}
This runs as long until the DLL calls the callback function. Then I got an error:
An unhandled exception of type 'System.StackOverflowException' occurred in Microsoft.VisualStudio.HostingProcess.Utilities.dll
Thanks for help.
ANSWER:
I don't know why, but there was an answer which is now deleted?!
I try to recover it. The solution was to use call by reference instead of call by value for the function parameter.
public delegate void CallbackDelegate(ref MyStruct mStruct);
Your C function is expecting a pointer to a MyStruct, but you're telling the C# that it wants a pointer to a function. The difference between a function and a struct is... significant. Perhaps try something like
[DllImport("MY.dll", CallingConvention=CallingConvention.StdCall)]
private static extern int RegisterCbFunc(IntPtr hWnd, Delegate pCB);
static void Main(string[] args)
{
MyStruct mStruct;
int returnValue = RegisterCbFunc(IntPtr.Zero, mStruct);
}
If your C function is filling in the lpItem member of the MyStruct with something it allocates itself, I've got no idea what happens next, but at least it's not going to be trying to overwrite your code.
In a winapi function I have to pass the address of the callback function
that function has to use.
So I declared a unsafe class and everything, but bool* addressofmyfunction = &GetHandle;
just won't compile!
Please help me or give me an alternative to passing the address like this.
In .NET delegates are used to store pointers to functions. For example if you have the following function:
public int GetHandle(int arg1)
{
return arg1 + 10;
}
it's address will be defined like so:
Func<int, int> addressOfMyFunction = GetHandle;
and you can invoke it like that:
int result = addressOfMyFunction(50);
bool* addressofmyfunction = &GetHandle won't compile in C or C++ either (the correct syntax would be bool (*addressofmyfunction)() = &GetHandle), but you can't do like that. I'm not an expert in interop, but I think you have to use Marshal.GetFunctionPointerForDelegate.
Function pointers are represented by delegates in C#. Since methods aren't moved by the garbage collector, you don't need to pin them (or use unsafe code) to pass them to a Win32 API function.
Example:
using System;
using System.Runtime.InteropServices;
public delegate bool CallBack(int hwnd, int lParam);
public class EnumReportApp {
[DllImport("user32")]
public static extern int EnumWindows(CallBack x, int y);
public static void Main()
{
CallBack myCallBack = new CallBack(EnumReportApp.Report);
EnumWindows(myCallBack, 0);
}
public static bool Report(int hwnd, int lParam) {
Console.Write("Window handle is ");
Console.WriteLine(hwnd);
return true;
}
}
There is a similar example for EnumThreadWindows on pinvoke.net.