Another C# marshaling question

  • Thread starter Thread starter Laurent
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Laurent

Hi again,



I created a thread some days ago, while I was trying to access a C++ DLL
using my C# program. First of all, I want to thanks all the guys who helped
me. But I still have a problem... My C++ DLL has a function which has the
following prototype:

long FF_Function(void* inInstance, char* inDate, FF_Struct**
outStructure, long* outNumber);

The structure has the following prototype:

struct FF_Struct
{
long m_Index;
char m_ID[FF_ID_LEN];
char m_Date[FF_DATETIME_LEN];
};


I know that the function should return me an array of FF_Struct, and
that the parameter outNumber has the number of elements stored in the
FF_Struct array. Now, I'm tring to translate it in C#, and here is what I
did (which does not work, of course):


public class FF_AO_OrderList
{
public int index;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = FF_ID_LEN)]
public string numeroCommande;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = FF_DATETIME_LEN)]
public string dateCommande;
}


[DllImport("FF_myDLL.dll")]
public static extern int FF_Function
(
IntPtr inInstance,
[MarshalAs(UnmanagedType.LPTStr)]
string date,
out FF_Struct[] outStructure,
out int outNumber
);


I can access the function, but it returns me an error message ("The
parameters specified for the interface are incorrect."). I don't know how to
deal with this structure and its double pointer. Even in unsafe mode (then I
use a FF_Struct** parameter), I have the same problem.


Does anyone knows what I'm missing ? I'd greatly appreciate some help,
as I'm completely out of ideas... Thanks !!!!



Laurent
 
Laurent,

You are going to have to manually marshal the structure. Your structure
declaration needs to be changed:

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi)]
public class FF_Struct
{
public int index;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = FF_ID_LEN)]
public string numeroCommande;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = FF_DATETIME_LEN)]
public string dateCommande;
}

Declaring as a class will tell the CLR that it can re-arrange the order
of the fields as it wishes, which is not what you want in this case.

You will also have to change your function declaration to:

[DllImport("FF_myDLL.dll")]
public static extern int FF_Function(
IntPtr inInstance,
[MarshalAs(UnmanagedType.LPTStr)]
string date,
ref IntPtr outStructure,
ref int outNumber
);

Then, what you have to do on return is marshal the array, like so:

// Allocate the array.
FF_Struct[] outStructs = new FF_Struct[outNumber];

// Cycle through unmanaged memory, and marshal.
for (int index = 0; index < outNumber; index++)
{
// Get the pointer of the location in memory.
IntPtr elementPointer = new IntPtr(outStructure.ToInt64() + (index *
Marshal.SizeOf(typeof(FF_Struct))));

// Now marshal that value.
outStructs[index] = Marshal.PtrToStructure(elementPointer,
typeof(FF_Struct));
}

The only problem here is that you have to figure out how to release the
memory that was allocated for the structure. The function is allocating it,
but you aren't handling that in this situation.

Hope this helps.


--
- Nicholas Paldino [.NET/C# MVP]
- (e-mail address removed)


Laurent said:
Hi again,



I created a thread some days ago, while I was trying to access a C++
DLL using my C# program. First of all, I want to thanks all the guys who
helped me. But I still have a problem... My C++ DLL has a function which
has the following prototype:

long FF_Function(void* inInstance, char* inDate, FF_Struct**
outStructure, long* outNumber);

The structure has the following prototype:

struct FF_Struct
{
long m_Index;
char m_ID[FF_ID_LEN];
char m_Date[FF_DATETIME_LEN];
};


I know that the function should return me an array of FF_Struct, and
that the parameter outNumber has the number of elements stored in the
FF_Struct array. Now, I'm tring to translate it in C#, and here is what I
did (which does not work, of course):


public class FF_AO_OrderList
{
public int index;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = FF_ID_LEN)]
public string numeroCommande;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = FF_DATETIME_LEN)]
public string dateCommande;
}


[DllImport("FF_myDLL.dll")]
public static extern int FF_Function
(
IntPtr inInstance,
[MarshalAs(UnmanagedType.LPTStr)]
string date,
out FF_Struct[] outStructure,
out int outNumber
);


I can access the function, but it returns me an error message ("The
parameters specified for the interface are incorrect."). I don't know how
to deal with this structure and its double pointer. Even in unsafe mode
(then I use a FF_Struct** parameter), I have the same problem.


Does anyone knows what I'm missing ? I'd greatly appreciate some help,
as I'm completely out of ideas... Thanks !!!!



Laurent
Click to expand...
 
Hi Nicholas,


Thank you for your answer, it works !!! I can access my function and the
returned value is as expected.

But I still have a little problem... this part of code doesn't work "as
it":

for (int index = 0; index < outNumber; index++)
{
// Get the pointer of the location in memory.
IntPtr elementPointer = new IntPtr(outStructure.ToInt64() + (index *
Marshal.SizeOf(typeof(FF_Struct))));

// Now marshal that value.
outStructs[index] = Marshal.PtrToStructure(elementPointer,
typeof(FF_Struct));
}


In fact, the first line is OK and the others have not been read
correctly. When I debug the code, Marshal.SizeOf(typeof(FF_Struct)) = 32. I
wrote the part of memory I have read into a binary file and examinated it
with Hex Editor. The size of my structure is 29. When I manually put this
value into your code, it works perfectly.

I think something is wrong with my FF_Struct class definition, but I
don't know why. I'm sure the size of the char arrays is OK. Prehaps the
problem comes from the '\0' terminated string ??? Do you have any clue ?

Thanks anyway for your help, you taught me a lot of stuffs today ! :)



Laurent



Nicholas Paldino said:
Laurent,

You are going to have to manually marshal the structure. Your
structure declaration needs to be changed:

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi)]
public class FF_Struct
{
public int index;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = FF_ID_LEN)]
public string numeroCommande;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = FF_DATETIME_LEN)]
public string dateCommande;
}

Declaring as a class will tell the CLR that it can re-arrange the order
of the fields as it wishes, which is not what you want in this case.

You will also have to change your function declaration to:

[DllImport("FF_myDLL.dll")]
public static extern int FF_Function(
IntPtr inInstance,
[MarshalAs(UnmanagedType.LPTStr)]
string date,
ref IntPtr outStructure,
ref int outNumber
);

Then, what you have to do on return is marshal the array, like so:

// Allocate the array.
FF_Struct[] outStructs = new FF_Struct[outNumber];

// Cycle through unmanaged memory, and marshal.
for (int index = 0; index < outNumber; index++)
{
// Get the pointer of the location in memory.
IntPtr elementPointer = new IntPtr(outStructure.ToInt64() + (index *
Marshal.SizeOf(typeof(FF_Struct))));

// Now marshal that value.
outStructs[index] = Marshal.PtrToStructure(elementPointer,
typeof(FF_Struct));
}

The only problem here is that you have to figure out how to release the
memory that was allocated for the structure. The function is allocating
it, but you aren't handling that in this situation.

Hope this helps.


--
- Nicholas Paldino [.NET/C# MVP]
- (e-mail address removed)


Laurent said:
Hi again,



I created a thread some days ago, while I was trying to access a C++
DLL using my C# program. First of all, I want to thanks all the guys who
helped me. But I still have a problem... My C++ DLL has a function which
has the following prototype:

long FF_Function(void* inInstance, char* inDate, FF_Struct**
outStructure, long* outNumber);

The structure has the following prototype:

struct FF_Struct
{
long m_Index;
char m_ID[FF_ID_LEN];
char m_Date[FF_DATETIME_LEN];
};


I know that the function should return me an array of FF_Struct, and
that the parameter outNumber has the number of elements stored in the
FF_Struct array. Now, I'm tring to translate it in C#, and here is what I
did (which does not work, of course):


public class FF_AO_OrderList
{
public int index;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = FF_ID_LEN)]
public string numeroCommande;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = FF_DATETIME_LEN)]
public string dateCommande;
}


[DllImport("FF_myDLL.dll")]
public static extern int FF_Function
(
IntPtr inInstance,
[MarshalAs(UnmanagedType.LPTStr)]
string date,
out FF_Struct[] outStructure,
out int outNumber
);


I can access the function, but it returns me an error message ("The
parameters specified for the interface are incorrect."). I don't know how
to deal with this structure and its double pointer. Even in unsafe mode
(then I use a FF_Struct** parameter), I have the same problem.


Does anyone knows what I'm missing ? I'd greatly appreciate some help,
as I'm completely out of ideas... Thanks !!!!



Laurent
Click to expand...
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