Error: Calling C++ dll function in C#

taibc

Hi, I am trying to use functions in C++ dll from C#, but I got an error: "attempt to read or write protected memory. This is often indication that other memory is corrupt" Anyone know how to fix ? Here is C++ functions:

typedef void *DGNHandle;

__declspec(dllexport) DGNHandle CPL_DLL DGNOpen( const char *, int );
__declspec(dllexport) DGNElemCore CPL_DLL *DGNReadElement( DGNHandle )

Here is structure in C++:

typedef struct {
int offset;
int size;

int         element\_id;     /\*!&lt; Element number (zero based) \*/
int         stype;          /\*!&lt; Structure type: (DGNST\_\*) \*/
int         level;          /\*!&lt; Element Level: 0-63 \*/
int         type;           /\*!&lt; Element type (DGNT\_) \*/
int         complex;        /\*!&lt; Is element complex? \*/
int         deleted;        /\*!&lt; Is element deleted? \*/

int         graphic\_group;  /\*!&lt; Graphic group number \*/
int         properties;     /\*!&lt; Properties: ORing of DGNPF\_ flags \*/
int         color;          /\*!&lt; Color index (0-255) \*/
int         weight;         /\*!&lt; Line Weight (0-31) \*/
int         style;          /\*!&lt; Line Style: One of DGNS\_\* values \*/

int         attr\_bytes;     /\*!&lt; Bytes of attribute data, usually zero. \*/
unsigned char \*attr\_data;   /\*!&lt; Raw attribute data \*/

int         raw\_bytes;      /\*!&lt; Bytes of raw data, usually zero. \*/
unsigned char \*raw\_data;    /\*!&lt; All raw element data including header. \*/

} DGNElemCore;

And below converted codes are in C#:

[StructLayout(LayoutKind.Sequential )]
public class DGNElemCore
{
public int attr_bytes;
public byte[] attr_data;
public int color;
public int complex;
public int deleted;
public int element_id;
public int graphic_group;
public int level;
public int offset;
public int properties;
public int raw_bytes;
public byte[] raw_data;
public int size;
public int style;
public int stype;
public int type;
public int weight;

[DllImport("DgnLib.dll", EntryPoint = "DGNOpen")]
public static extern IntPtr DGNOpen(string fileName, int bUpdate);
[DllImport("DgnLib.dll", EntryPoint = "DGNReadElement")]
p

Lost User

The error message is quite clear about what went wrong, but you will need to use your debugger to track down exactly where it occurs.

One of these days I'm going to think of a really clever signature.

taibc

Hi, It throwed error at the last line: element = DgnFile.DGNReadElement(dgnFile.oDgnFile)

Lost User

As I said before, you need to use your debugger to find out what is happening. There is no way we can guess what is going on inside the program.

One of these days I'm going to think of a really clever signature.

taibc

Thank you. But, the exception window only show that messeage. And, the below is output from debug:

Quote:

A first chance exception of type 'System.AccessViolationException' occurred in mscorlib.dll The thread 'vshost.RunParkingWindow' (0x518) has exited with code 0 (0x0). The thread '' (0x1fdc) has exited with code 0 (0x0). The program '[7952] CLCLis.vshost.exe: Managed (v2.0.50727)' has exited with code 0 (0x0). The program '[7952] CLCLis.vshost.exe: Program Trace' has exited with code 0 (0x0).

I still haven't found the reason for this error

Lost User

Well it looks as if some code is passing a bad pointer to a function in mscorlib.dll. Where does this DLL come from, is it your code or from a third party? If the latter then contact the people who wrote it and ask for assistance.

One of these days I'm going to think of a really clever signature.

taibc

Thank you. I used Visual Studio (project type: win 32 console application) to create this dll file from a C++ files collection (.h and .cpp) of a third-party (Here is the link of source code: http://dgnlib.maptools.org/dl/dgnlib-1.11.zip )

Lost User

Since you have the source code you can step through it in your debugger to trace the error.

One of these days I'm going to think of a really clever signature.

taibc

How I can trace the error through source codes while I am using dll file ? and I think the source codes don't have error, because I still can use it in Visual C++, but I got the error while trying in C#

Lost User

taibc wrote:

How I can trace the error through source codes while I am using dll file ?

Just the same, build a debug version of the DLL and the debugger should be able to step into it.

taibc wrote:

I think the source codes don't have error

That may well be true, but only by extensive testing can you be sure.

One of these days I'm going to think of a really clever signature.

Lost User

I have just looked again at the layout of your C++ and C# structures; why are they different?

One of these days I'm going to think of a really clever signature.

J4amieC

The OP has laid them out alphabetically in c#, im pretty sure this will be the issue, non?

taibc

I only convert the field type: unsigned char * (in C) into byte[] (in C#)

Lost User

Yes but you have moved the elements into different positions within the structure. How can that work?

One of these days I'm going to think of a really clever signature.

Lost User

True; I did not check that originally, it never occurred to me that anyone would think of doing such a thing. :doh:

One of these days I'm going to think of a really clever signature.

BobJanova

This isn't my area but:

Defining the fields in the struct in a different order definitely won't work. You need to declare in the same order (hopefully the reason for this is obvious).
You've defined it as a class in C#, not a struct. I think you need it to be a struct, even though it's a pointer type declaration in C++. You might even need to declare it IntPtr and use Marshal methods to make a struct out of it.
What is DGNHandle? That C++ function definition isn't valid unless it's a macro.

taibc

Thanks BobJanova and Richard MacCutchan very much. I changed the order of fields and using "struct" instead of "class" in C#. DGNHandle is "Opaque handle representing DGN file, used with DGN API" Now, I got another error: Exception of type 'System.ExecutionEngineException' was thrown. Please see my detail codes:

[DllImport("DgnLib.dll", EntryPoint = "DGNReadElement")]
public static extern IntPtr DGNReadElement(IntPtr DGNHandle);

    public static DGNElemCore ReadElement(IntPtr DGNHandle)
    {

        DGNElemCore element = new DGNElemCore ();
        IntPtr itr = DGNReadElement(DGNHandle);

        element = (DGNElemCore)Marshal.PtrToStructure(itr, typeof(DGNElemCore)); **// throw the exception**

        return element;

    }

I am wondering: Can I convert a structure pointer (DGNElemCore *) in C to IntPtr (in C#), then use Marshal to get DGNElemCore. The function in C is:

__declspec(dllexport) DGNElemCore CPL_DLL *DGNReadElement( DGNHandle );

Thank you very much !

taibc

Hi, I found out the reason of the error. After changing the order of fields in Class and using "IntPrt" instead "byte[]" in C# (for char * in C). I can run without error. Thank you very much !

taibc

I am getting another error when casting between two structs in C#: I have the function in C:

__declspec(dllexport) DGNElemCore CPL_DLL *DGNReadElement( DGNHandle );

This function will read a file and return a pointer to either DGNElemCore or DGNElemText, and I can use casting between them. But, when I convert codes into C# as below, I can't cast types of these structs:

[DllImport("DgnLib.dll", EntryPoint = "DGNReadElement")]
public static extern IntPtr DGNReadElement(IntPtr DGNHandle);
public static DGNElemCore ReadElement(IntPtr DGNHandle)
{
DGNElemCore element = new DGNElemCore ();
IntPtr itr = DGNReadElement(DGNHandle);
MessageBox.Show(itr.GetType ().ToString ());

        element = (DGNElemCore)Marshal.PtrToStructure(itr, typeof(DGNElemCore));                          
        return element;            
    }

And the codes for testing:

DGNElemCore element = new DGNElemCore();
element = DgnFile.ReadElement(DGNHandle);

while (element != null)
{
if (element.type == 17 ) // Element is a DGNElemText
{
DGNElemText txtElement = new DGNElemText();
txtElement = (DGNElemText)element; **// throw error: Unable to cast object of type DgnLib.DGNElemCore to type DgnLib.DGNElemText**

     ...... // More codes

}

}

Structs in C#:

[StructLayout(LayoutKind.Sequential )]
public class DGNElemCore
{
public int offset;
public int size;
public int element_id;
public int stype;
public int level;
public int type;
public int complex;
public int deleted;
public int graphic_group;
public int properties;
public int color;
public int weight;
public int style;
public int attr_bytes;
public IntPtr attr_data;
public int raw_bytes;
public IntPtr raw_data;

[StructLayout (LayoutKind.Sequential)]
public class DGNElemText : DGNElemCore
{
// Fields
DGNElemCore core;

    public int font\_id;
    public int justification;
    public double length\_mult;
    public double height\_mult;
    public double rotation;
    public DGNPoint origin;
    public string text;
}

mphill4744

I am not sure if you are a member of the Bentley communities but there is a specific programming forum that can address these questions with the developers and other users. I am just starting to learn C# and use both CP and Bentley sites for help. http://communities.bentley.com/products/microstation/microstation_programming/w/microstation_v8i_programming_wiki/default.aspx[^]

I'll tell you what, then. Why don't you call me some time when you have no class? - Thornton Melon