I have a c++ function that look like this :
int Compression::DecompressPacket(const void* inData, int inLength, void* outData, int outLength)
{
int headerLength = ComputeDataHeaderLength(inData);
return DecompressDataContent(static_cast<const unsigned char*>(inData) + headerLength, inLength - headerLength, outData, outLength);
}
The fonction is inside a class which is inside a c++ library.
In the other hand, I need to call this fonction on my c# application. The fonction ask me to enter parameters of types : "void*, int, void*, int".
When I try to make a void* in an unsafe function,
unsafe private void lstbox_packets_SelectedIndexChanged(object sender, EventArgs e)
{
[...]
byte[] value = byteValuePackets[lstbox_packets.SelectedIndices[0]];
void* pValue = &value;
[...]
}
I get the error :
Error 8 Cannot take the address of, get the size of, or declare a pointer to a managed type ('byte[]')
I'm not very familiar with c++ and pointer but how i am suppose to pass a void* type in c# ?
You shouldn't take the address of value, and also you have to use fixed statement:
fixed (void* pValue = value)
{
//...
}
Related
Using SendMessageW function I am passing c# string as a parameter to c++ function. I am typecasting to CString in c++ but it's value is empty. Please check below code and provide solution
-----------------------c# code ------------------
public unsafe IntPtr Testing()
{
string string_aux = "Stringtochange";
void* pt = Marshal.StringToBSTR(string_aux).ToPointer();
IntPtr ab = new IntPtr(pt);
return ab;
}
public void GetValue()
{
SendMessageW(utilityHandle1, TVM_GETITEMHEIGHT, handle,Testing());
}
--------------------- C++ code --------------
CString *st = (CString*)lParam;
MessageBox(NULL,*st,L"stringvalue",NULL);
Here *st value is empty.
You seem to be abusing TVM_GETITEMHEIGHT. Why not use a custom message.
CString is a C++ class. It is not binary compatible with a BSTR.
Personally I would use Marshal.StringToCoTaskMemUni in the C# and cast to wchar_t* in the C++. Remember to destroy the unmanaged memory after you've used it when SendMessageW returns, by calling Marshal.FreeCoTaskMem.
I have a C++ DLL (SimpleDLL.dll), with a exposed function (DllFunctionPoibnterGetName) that has a function pointer (getNameFP). The function pointer takes a char * as a parameter (*char * name*).
// C++
DllExport void DllFunctionPoibnterGetName( void (*getNameFP) (char * name, unsigned short * length ) ) {
char name[1024];
unsigned short length = 0 ;
getNameFP( name, &length );
printf( "length=[%d] name=[%s]\n", length, name );
}
I have a C# application that would like to use this C++ DLL.
// C#
public unsafe delegate void GetName( System.Char* name, System.UInt16* length);
unsafe class Program
{
[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public delegate void delegateGetName(System.Char* name, System.UInt16* length);
[DllImport("SimpleDLL.dll", CharSet = CharSet.Ansi )]
public static extern void DllFunctionPoibnterGetName([MarshalAs(UnmanagedType.FunctionPtr)] delegateGetName getName);
static void Main(string[] args)
{
DllFunctionPoibnterGetName(GetName);
}
static void GetName(System.Char* name, System.UInt16* length)
{
// name = "one two three";
*length = 10;
}
}
Currently I can set the length with out any problems, but I can't seem to find a way to set the name correctly.
My Question is
How do I set the char * name to a value correctly.
You don't need to use unsafe code. You can do it like this:
[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public delegate void delegateGetName(IntPtr name, out ushort length);
....
static void GetName(IntPtr name, out ushort length)
{
byte[] buffer = Encoding.Default.GetBytes("one two three");
length = (ushort)buffer.Length;
Marshal.Copy(buffer, 0, name, buffer.Length);
}
Although this interface design is just asking for a buffer overrun. How are you supposed to know how big the unmanaged buffer is? It would make more sense for the length parameter to be passed by ref. On input it would tell you how big the buffer is. On output you would have recorded how many bytes you copied into the buffer.
Cast the char* as a char[]. That should do the trick.
Casting the char will not do. The char * data is 'unmanaged', native data. And C# uses 'managed', .NET data.
You need to make a wrapper for your call and use marschall to convert the data from 'unmanaged' to 'managed'.
I need to pass a pointer to a structure to my DLL, any ideas how would I go about doing that?
In my C DLL:
typedef struct
{
int length;
unsigned char *value;
} Sample;
__declspec(dllexport) void __stdcall helloWorld( Sample *sample );
In my C# code:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.InteropServices;
namespace CSharpConsole
{
class Program
{
[StructLayout(LayoutKind.Sequential, Pack = 1)]
private struct Sample
{
public Int32 length;
// What Should I Declare Here?
}
[DllImport("C:\\CTestDLL.dll")]
private static extern void helloWorld( Sample sample ); // How would I make this a pointer?
void HelloWorld()
{
Sample sample = new Sample();
sample .length = 20;
// How can I fill up the values of value?
helloWorld( sample ); // How should I pass it inside here
return;
}
static void Main(string[] args)
{
Program program = new Program();
program.HelloWorld();
}
}
}
To pass a pointer to a value type into a P/Invoke function just declare the parameter as a ref or out. This implicitly takes a reference to the parameter and passes that:
[DllImport("C:\\CTestDLL.dll")]
private static extern void helloWorld(ref Sample sample);
Since your structure has an array in it, you'll have to take care to declare it properly for this to work. I strongly recommend, if possible, that you turn it into a fixed-length array, as it will make the marshaler much, much happier:
typedef struct
{
int length;
unsigned char value[MAX_LENGTH];
} Sample;
This becomes:
public struct Sample
{
int length;
[MarshalAs(UnmanagedType.LPArray, SizeConst = MAX_LENGTH)] byte[] value;
}
If that is not possible, then the runtime has no way of knowing how much data to marshal back; in that case, you probably will have to resort to manual marshaling of your data:
public struct Sample
{
int length;
IntPtr value;
}
var sample = new Sample();
helloWorld(ref sample);
byte[] value = new byte[sample.length];
Marshal.Copy(sample.value, value, 0, sample.Length);
However, based on your comment to another answer, it looks like you just need to get a block of bytes out of the C DLL into C#. For that you don't really need a structure at all, and eliminating it would simplify things a lot. If you just want to pass in an array and have it filled in and returned to you, try something like this:
(This assumes you have control over both C and C# code bases; if not then the ref suggestion is the way to accomplish what you need.)
// In C# code:
[DllImport(#"C:\CTestDll.dll")]
private static extern void helloWorld(
int length,
[MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 0)] byte[] buffer);
byte[] buffer = new byte[1024 * 8];
helloWorld(1024 * 8, buffer);
// In C:
__declspec(dllexport) void __stdcall helloWorld(int, unsigned char *);
void helloWorld(int cb, unsigned char *buf)
{
memcpy(buf, DATASRC, cb);
}
In C, an unsigned char is, by definition, the same size as a C# byte - 8 bits, no sign. In C#, a char is actually two bytes. The runtime will automatically "convert" an unsigned char * to a byte[] for you. (There's not really a conversion at all; they are just different names for the same type.)
Default Marshaling for Value Types - gives some information on marshalling structs.
Calling Win32 DLLs in C# with P/Invoke - a little over half way down the page there's a table showing the type conversions between the standard unmanaged and managed types.
There are a few ways to pass around and convert the types.
As Michael Edenfield pointed out you can generally just use the ref keyword to indicate that a parameter should by passed by reference which is basically a pointer.
However sometimes things don't cooperate, particularly when it comes to strings or complex data types so this is where the IntPtr type comes in.
You have a couple of options for using IntPtrs.
You can create an IntPtr to a block of unmanaged memory of a specified size using:
IntPtr pointer = Marshal.AllocHGlobal(sizeOfBufferInBytes);
//Do stuff with the pointer
Marshal.FreeHGlobal(pointer); //Don't forget to release the memory
This is obviously a bit dangerous because you're manually allocating unmanaged memory.
You'll need to Marshal the data from the buffer back into a managed type, using something like Marshal.Copy(), Marshal.PtrToStructure(), Buffer.BlockCopy(), etc.
Alternatively you can create a managed object and pin it in memory while you need to, get a pointer to it and pass that to your method.
MyObject instance = new MyObject();
GCHandle gch = GCHandle.Alloc(instance, GCHandleType.Pinned);
importedMethod(gch.AddrOfPinnedObject()); //AddrOfPinnedObject() gives you an IntPtr
gch.Free(); //Release the pinned memory so the garbage collector can deal with it
This avoids the necessity for manually marshalling back to the correct data type but this is not always an option depending on the type of MyObject and whether it's blittable.
This works for me:
DLL:
typedef struct
{
int length;
unsigned char *value;
} Sample;
extern "C" __declspec(dllexport) void __stdcall helloWorld( Sample *sample )
{
MessageBoxA(NULL, (LPCSTR) sample->value, (LPCSTR) sample->value, 0);
}
C#:
class Program
{
[StructLayout(LayoutKind.Sequential, Pack = 1)]
private class Sample
{
public Int32 length;
public String value;
}
[DllImport("C:\\Users\\Kep\\Documents\\Visual Studio 2010\\Projects\\SODLL\\Debug\\DLL.dll")]
private static extern void helloWorld(Sample sample);
static void Main(string[] args)
{
Sample s = new Sample();
s.length = 10;
s.value = "Huhu";
helloWorld(s);
}
}
Important thing is to mark it as a class, not a struct in C#.
With this, you could also use constructors etc. in C#:
class Program
{
[StructLayout(LayoutKind.Sequential, Pack = 1)]
private class Sample
{
public Int32 length;
public String value;
public Sample(String s)
{
length = s.Length;
value = s;
}
}
[DllImport("C:\\Users\\Kep\\Documents\\Visual Studio 2010\\Projects\\SODLL\\Debug\\DLL.dll")]
private static extern void helloWorld(Sample sample);
static void Main(string[] args)
{
Sample s = new Sample("Huhu");
helloWorld(s);
}
}
I have a C function with prototype,
void* VoidPointer(void*);
Now I need to marshal it in C#(using DllImport). But I do not know how to mention the parameters in C# code.
static public extern WHAT_RETURN_TYPE VoidPointer( WHAT_PARAMETER_TYPE );
How to make a call with the proper parameters in the C# code (SAMPLE USE)
I am new to C# and need to solve this asap ( in several attempts have got errors like this cannot convert from 'int' to 'System.IntPtr')
Thanks.
c# supports void pointers. Just declare the function as
[DllImport("test.dll")]
public static extern unsafe void* VoidPointer(void* AValue);
public unsafe void Test()
{
int* a;
int b = 0;
a = (int*)VoidPointer(&b);
}
(this only works if the void pointers are referencing integers ofcourse)
I have a native regular C++ Dll which I want to call from C# code, so i created C++/CLI class (as described here and here) which will include managed C++ code and which can be called by any C# code directly and which can make calls inturn to native unmanaged C++.
One of function in native C++ dll has parameter of type int *. How do I declare in wrapper function and how can i convert it into int *?
It is the C/C++ way of passing a value by reference. You should use the ref or out keyword:
[DllImport("something.dll")]
private static extern void Foo(ref int arg);
In C++/CLI that would look roughly like this:
public ref class Wrapper {
private:
Unmanaged* impl;
public:
void Foo(int% arg) { impl->Foo(&arg); }
// etc..
};
[DllImport("some.dll")]
static extern void SomeCPlusPlusFunction(IntPtr arg);
IntPtr is a type that is roughly equivalent to void *.
From your comment, you'd be best off doing something like this (C#):
int size = 3;
fixed (int *p = &size) {
IntPtr data = Marshal.AllocHGlobal(new IntPtr(p));
// do some work with data
Marshal.FreeHGlobal(data); // have to free it
}
but since AllocHGlobal can take an int, I don't know why you wouldn't do this:
IntPtr data = Marshal.AllocHGlobal(size);