There appears to be multiple ways to do this, but the examples I've tried haven't worked for me.
I read somewhere that using unsafe pointers would be the way to go for more complex structures that require pointers. (sorry but I haven't been able to find the source again)
My latest attempt looks something like this:
unsafe public struct ComplexStruct {
public Int32 NumSubStruct;
public SubStruct*[] arr;
}
unsafe public static extern UInt32 DLL_Call(ref ComplexStruct p);
function {
unsafe {
ComplexStruct p = new ComplexStruct();
p.NumSubStruct = 2;
p.arr= new SubStruct*[p.NumSubStruct];
SubStruct p1 = new SubStruct();
SubStruct p2 = new SubStruct();
p.arr[0] = &p1;
p.arr[1] = &p2;
testLogHandlerHandle = DLL_Call(ref p);
}
}
I get an error which says that SubStruct cannot be marshaled (Signature is not interop compatible).
Is it possible to pass the object without marshaling? (the DLL should be loaded in the same process space as the C# application).
If not, what would be the simplest way to pass a copy of the object?
Note: changing the DLL is a non-issue. I am vaguely aware of some other solutions, such as C++/CLI, but I only have a relatively small number of structure types that need to be passed in such a way.
EDIT:
A few notes:
The array needs to by dynamic
The actual structs a bit more complicated (there's 4 layers of nested structures like this) so I think adding them would detract from the question, but for completeness, here is how I would declare these in C++:
struct ComplexStruct {
unsigned int NumSubStruct;
SubStruct** arr;
};
struct SubStruct {
unsigned int NumSubStruct;
//some more datamembers here as well
SubSubStruct** arr;
};
(the extra indirection isn't necessary, but I figured it might be useful to be able to declare the array of SubStruct pointers as IntPtrs)
Like this maybe?
unsafe public struct ComplexStruct
{
public Int32 NumSubStruct;
public SubStruct** arr;
}
unsafe static void Main(string[] args)
{
ComplexStruct p = new ComplexStruct();
p.NumSubStruct = 2;
SubStruct[] s = new SubStruct[p.NumSubStruct];
// no need to new here, valuetype array
s[0].value = 1;
s[1].value = 2;
fixed (SubStruct* sp = s)
fixed (SubStruct** spp = new SubStruct*[] { sp })
{
p.arr = spp;
testLogHandlerHandle = DLL_Call(ref p);
}
}
You should use something like this and this.
[StructLayout(LayoutKind.Sequential)]
public struct ComplexStruct
{
public Int32 NumSubStruct;
[MarshalAs(UnmanagedType.ByValArray, SizeConst = MAX_NO_SUB_STRUCTS)]
public SubStruct[] arr;
}
public static extern UInt32 DLL_Call(IntPtr p);
Then marshal it with something like this:
IntPtr ptr = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(ComplexStruct)));
try
{
// Copy the struct to unmanaged memory.
Marshal.StructureToPtr(p, ptr, false);
testLogHandlerHandle = DLL_Call(ptr);
}
finally
{
// Free the unmanaged memory.
Marshal.FreeHGlobal(ptr);
}
Try to get rid of all the unsafe stuff, I doubt its necessary!
Related
Unmanaged and Managed Memory Regions
I am attempting to execute unmanaged code from a C-library. One of the methods takes a void* as a parameter but under the hood it's cast to a struct of type nc_vlen_t
C struct for nc_vlen_t
/** This is the type of arrays of vlens. */
typedef struct {
size_t len; /**< Length of VL data (in base type units) */
void *p; /**< Pointer to VL data */
} nc_vlen_t;
Executing the method is correct and it works, I am concerned more about the pinning and safe handling of managed and unmanaged memory regions. I want to be as certain as possible that I am not going to cause memory leaks or a SEGFAULT. I wrote a struct that will be marshalled to and from the nc_vlen_t when I execute the C-library method calls.
C# struct
[StructLayout(LayoutKind.Sequential)]
public struct VlenStruct {
public Int32 len;
public IntPtr p; // Data
}
The struct consists of a size_t that indicates the array length and a void * to the data. Inside the library it has attributes that allow it to cast the (void*) to the appropriate numeric types and I've had great success with that so far.
What I want to understand is the best way to handle the memory regions. After reading some articles and other SO questions this is my best guess for how to handle it. I have a class that acts as an arbiter for creating and managing the structs and their memory. I rely on a destructor to free the handle which will unpin the array so that the GC can do it's job.
C# Vlen Helper
public class Vlen {
private GCHandle handle;
private VlenStruct vlen_t;
public Vlen() {
isNull = true;
}
public Vlen(Array t) {
isNull = false;
handle = GCHandle.Alloc(t, GCHandleType.Pinned); // Pin the array
vlen_t.len = t.Length;
vlen_t.p = Marshal.UnsafeAddrOfPinnedArrayElement(t, 0); // Get the pointer for &t[0]
}
~Vlen() {
if(!isNull) {
handle.Free(); // Unpin the array
}
}
public VlenStruct ToStruct() {
VlenStruct retval = new VlenStruct();
retval.len = vlen_t.len;
retval.p = vlen_t.p;
return retval;
}
private bool isNull;
}
C Method Declaration
//int cmethod(const int* typep, void *data)
// cmethod copies the array contents of the vlen struct to a file
// returns 0 after successful write
// returns -1 on fail
[DllImport("somelib.dll", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true, CallingConvention=CallingConvention.Cdecl)]
public static extern Int32 cmethod(ref Int32 typep, ref VlenStruct data);
If I use this class to create the struct is there a possibility that the GC will clean the array before the C-library is called in this situation:
C# Use-case
{
double[] buffer vlenBuffer = new double[] { 0, 12, 4};
Vlen data = new Vlen(vlenBuffer); // The instance now pins buffer
VlenStruct s = data.ToStruct()
Int32 type = VLEN_TYPE;
cmethod(ref type, ref s);
}
Is it possible for the data instance to be cleaned and thereby unpin buffer which could cause unpredictable behavior when executing the external library method?
Yes, you certainly have a problem here. As far as the jitter is concerned, the lifetime of your "data" object ends just before the ToStruct() method returns. Check this answer for the reason why. Which permits the finalizer to run while your unmanaged code is running. Which unpins your array. It would actually take another garbage collection to corrupt the data that the unmanaged code uses. Very rare indeed but not impossible. You are not likely to get an exception either, just random data corruption.
One workaround is to extend the lifetime of the Vlen object beyond the call, like this:
Vlen data = ...
...
cmethod(ref type, ref s);
GC.KeepAlive(data);
Which works but doesn't win any prizes, easy to forget. I would do this differently:
public static void CallHelper<T>(int type, T[] data) {
var hdl = GCHandle.Alloc(data, GCHandleType.Pinned);
try {
var vlen = new nc_vlen();
vlen.len = data.Length;
vlen.data = hdl.AddrOfPinnedObject();
cmethod(ref type, ref vlen);
}
finally {
hdl.Free();
}
}
Usage:
var arr = new int[] { 1, 2, 3 };
CallHelper(42, arr);
Which, beyond avoiding the early collection problem, also keeps the array pinned as short as possible. Do note that ref on the first argument of this function is pretty strange, you would not expect this function to alter the data type.
this is my first stackoverflow post. I have been stucking in this issue for days. I try to import the usbi2cio.dll which is a C Dll to a C# based project. I went through most of the similar posts within the site, while still I couldn't fix my issue, since my case might be little different.
So here is the original definition of the API and related struct as a parameter:
LONG _stdcall DAPI_ReadI2c(HANDLE hDevInstance, I2C_TRANS * TransI2C);
typedef struct _I2C_TRANS {
BYTE byTransType;
BYTE bySlvDevAddr;
WORD wMemoryAddr;
WORD wCount;
BYTE Data[256];
}I2C_TRANS, *PI2C_TRANS;
//In my C# code, I did the translation like this:
[StructLayoutAttribute(LayoutKind.Sequential), Serializable]
public struct I2C_TRANS
{
public byte byTransType;
public byte bySlvDevAddr;
public ushort wMemoryAddr;
public ushort wCount;
[MarshalAsAttribute(UnmanagedType.ByValArray, SizeConst = 256, ArraySubType = UnmanagedType.I1)]
public byte[] Data;
public I2C_TRANS(int size)
{
Data = new byte[size];
this.byTransType = 0x00;
this.bySlvDevAddr = 0x00;
this.wMemoryAddr = 0;
this.wCount = 0;
}
};
public I2C_TRANS TransI2C = new I2C_TRANS(256);
public IntPtr[] hDevice = new IntPtr[DAPI_MAX_DEVICES];
...
TransI2C.byTransType = byTransType;
TransI2C.bySlvDevAddr = bySlvDevAddr;
TransI2C.wMemoryAddr = wMemoryAddr;
TransI2C.wCount = wCount;// no larger than 64
...
if((hDevice[0] = DAPI_OpenDeviceInstance(devName, 0)) != INVALID_HANDLE_VALUE)
//the returned lReadCnt should be equal to wCount.
Public int lReadCnt = DAPI_ReadI2c(hDevice[0], ref TransI2C);
For some reason, the struct in the read I2C transaction can't be well passed through, As a result, the function returns 0 value without errors(what I expect is the same value with wCount). For some other similar API and struct, it works well. So what might be the cause for this issue?
//Here is the P/Invoke declaration:
[DllImportAttribute("UsbI2cIo.dll", EntryPoint = "DAPI_ReadI2c", CallingConvention = CallingConvention.StdCall)]
public static extern int DAPI_ReadI2c(IntPtr hDevInstance, ref I2C_TRANS TransI2C);
I had a similar problem, and I fixed it by writing my own C library called Bridge, that would deal with the complex C API but expose simple methods that could be interfaced with C# easily.
For example in the method below I could pass a byte array to my C code.
From a C# point of view I would only deal with byte, int16 or int32 or byte array.
[DllImport(DLL)]
private static extern System.Int32 __SPI_Helper_Write(IntPtr lpBuffer, System.Int32 len);
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've been fighting this problem for some time now and am hoping someone can help. I'm converting a VB6 application to C#. Everything I'm about to show works perfect in VB6, but fails in C#.
I'm making an API call into a C dll, passing a structure. The structure gets values modified in the C side and I should see the modified values in my return structure.
Note, Sig is an int, Status is an int, and CLIENTID is a UINT_PTR.
Here are the C structures:
typedef struct
{
short vers;
short flags;
short cmd;
short objtype;
DWORD id;
DWORD client;
char buf[MAX_TOOLBUF];
DWORD toolID;
NMSG msg;
DWORD caller;
CLIENTID clientID;
DWORD ticket;
ToolResult PTR result;
long spare[4];
} Request;
typedef struct
{
DWORD hwnd;
DWORD message;
DWORD wParam;
DWORD lParam;
} NMSG;
typedef struct
{
Sig sig;
short cnt;
Status error;
DWORD ticket;
DWORD toolID;
long spare[4];
} ToolResult;
In my C# code, I have the following structs defined to map to the C structs above:
[StructLayout(LayoutKind.Sequential)]
public struct NMSG
{
public int hWnd;
public int msg;
public int wParam;
public int lParam;
}
[StructLayout(LayoutKind.Sequential)]
public struct Request
{
public Int16 vers;
public Int16 flags;
public Int16 cmd;
public int16 objType;
public int id;
public int Client;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 260)]
public string buf;
public int toolID;
public NMSG msg;
public int caller;
public IntPtr clientID;
public int ticket;
public ToolResult result;
[MarshalAs(UnmanagedType.ByValArray, SizeConst= 4) ]
public int64[] spare;
}
This is the method decoration in the C code:
SendRequest(Request PTR req)
{
....
}
Here is my PInvoke declaration in C# for the API in C that uses these structures:
[DllImport("TheCDLL.dll", EntryPoint = "_Request#4", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
public static extern void Request(ref Request req);
And here is my C# code that fills the structure and makes the API call:
Request req = new Request();
req.vers = 1;
req.toolID = 4000;
req.Client = 0;
req.cmd = 11;
req.objType = 1;;
req.id = 1;
req.msg.hWnd = Hwnd; // verified this is a valid window handle
req.msg.msg = 1327;
req.msg.wParam = 101;
req.msg.lParam = 0;
req.spare = new int[4];
req.buf = new string(' ', 260);
req.flags = 11;
Request(ref req);
The problem is, in the VB6 code the structure item 'result' gets filled with a value after I make the API call. In C#, 'result' is always just 0. I'm guessing I must have something wrong with the way I'm marshaling? I've read dozens of articles and have tried lots of different ways to get this working with no success. Any help is appreciated!
UPDATE: Now that I've updated the code based on the suggestions below (fixing types and cleaning up the code as originally written), I'm getting an exception:
System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
I've found that this happens as soon as I changed to 'public Int16 vers' from 'public int vers' in my Request structure. Thoughts?
UnmanagedType.Struct actually isn't for substructures; it makes the Marshaler pass the related object as a VARIANT. That isn't what you want here. Try taking off all the MarshalAs statements except for strings (for the length), arrays (also for the length), and enumerated types (I don't see any here, but in my DLLs they've been UnmanagedType.I4) and see if that works.
There's also some extra things in your C# struct that aren't in your C version, a couple variables don't match between the two, and your given C function doesn't return anything but your C# extern is expecting an IntPtr. These do need to match for everything to go smoothly. (I'm guessing it's just simplification for the site, but it is something to keep in mind.)
I am porting a c++ code into c#.There are 2 structures, as explained
below.
// Must be a total of 180 bytes
typedef struct MAINTENANCE_PARAM`
{
BYTE bCommand;
BYTE bSubCommand;
USHORT usDataLength;
union // Must be no larger than 176 bytes
{
MyStruct1 myStruct1;
MyStruct2 myStruct2;
BYTE bDummy[176];
} data;
} MAINTENANCE_PARAM;
typedef struct _OCI_PARAM {
SHORT sType; // Size: 2
LONG lValue; // 4
SHORT sScale; // 2
LONG lValueTabIdx; // 4
} OCI_PARAM[OCI_MAXPARAM]; //Size: OCI_MAXPARAM = 15 // 15 * 12 = 180
My code uses the memcpy in the following fashion.
MAINTENANCE_PARAM maintainanceParam;
OCI_PARAM ociParam
// Copy recieved structure to correct format
memcpy((void*)& maintainanceParam, (void*) ociParam, sizeof(OCI_PARAM));
As I know there is no code for memcpy in the C#. So how can I port the
above code into the C#. I am new to C#. I don't know much regarding
the C#. So can anybody tell me how exactly I can implement the above
line of code in C#. I need to copy 180 bytes from one structure to another structure object with different datatype
Thanks in Advance for any help.
Regards,
Ashish
Use the StructLayoutAttribute and FieldOffsetAttribute for your structs so that you can layout your fields explicitly, then import RtlMoveMemory (P/Invoke). You can modify the signature to take any pointer type (in unsafe context) or use the standard signature on PInvoke.net.
unsafe class Program {
[System.Runtime.InteropServices.DllImport("Kernel32.dll", EntryPoint = "RtlMoveMemory", SetLastError = false)]
private static extern void MoveMemory(void* dest, void* src, int size);
static void Main(string[] args) {
Maintenance_Param p1 = new Maintenance_Param();
p1.bCommand = 2;
p1.bSubCommand = 3;
p1.usDataLength = 3;
p1.myStruct1 = new MyStruct1();
Maintenance_Param p2 = new Maintenance_Param();
MoveMemory(&p2, &p1, sizeof(Maintenance_Param));
}
}
[System.Runtime.InteropServices.StructLayout(System.Runtime.InteropServices.LayoutKind.Explicit)]
public struct Maintenance_Param {
// fields should be private and be accessed over properties ...
[System.Runtime.InteropServices.FieldOffset(0)]
public byte bCommand;
[System.Runtime.InteropServices.FieldOffset(1)]
public byte bSubCommand;
[System.Runtime.InteropServices.FieldOffset(2)]
public ushort usDataLength;
[System.Runtime.InteropServices.FieldOffset(4)]
public MyStruct1 myStruct1;
[System.Runtime.InteropServices.FieldOffset(4)]
public MyStruct2 myStruct2;
}
public struct MyStruct1 {
int value;
}
public struct MyStruct2 {
int value;
}
MemCpy DOES exist in .NET! Try googling for OpCodes.Cpblk. Here is an example: http://www.abstractpath.com/weblog/2009/04/memcpy-in-c.html