Error: Calling C++ dll function in C#

[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

[taibc]

Hi. I got the answer from msdn forum: http://social.msdn.microsoft.com/Forums/en-US/csharpgeneral/thread/2b6c3bc0-1c78-4d50-97d6-3100326aea09/#caa9aa68-8e57-4235-be16-29603e41f3f0[^] But, I am getting another error.

[taibc]

Hi BobJanova, I resolved that error as your sugesstion. However, I am getting another error while trying to get the text value of a char array in C. For example: I have a struct in C that inclue a char array like below:

struct DGNElemText
{
.....

char text[1]; /*!< Actual text (length varies, \0 terminated*/

....
}

And the converted codes in C#:

[StructLayout(LayoutKind.Sequential)]
public class DGNElemText
{
.....

IntPtr text;
}

And I got the text value by using Marshal.PtrToStringAnsi(...). But I always received an empty text value. It is not expected value such as: "Hello", "abc123",... I still got the same value when trying to use the another way:

[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 1)]
public string text;

Do you know why ?