Dynamic P/Invoke for custom native exported methods

By Philippe Leefsma

 

When P/Invoking an exported ObjectARX or Win32 method, you usually do not need to specify the full path of the dll/executable that contains that method, as it will be already loaded either by AutoCAD or the operating system in case of a Win32 API.

Here is a couple of examples:

[DllImport("user32.dll",

    CharSet = CharSet.Auto,

    CallingConvention = CallingConvention.StdCall)]

public static extern int SetWindowsHookEx(

    int idHook, HookFunc lpfn, IntPtr hInstance, int threadId);

 

[DllImport("acad.exe",

    CharSet = CharSet.Unicode,

    CallingConvention = CallingConvention.Cdecl,

    EntryPoint = "acedCmd")]

public static extern int acedCmd(System.IntPtr vlist);

 

However it might happen that you have exported a C++ method from one of your custom dll or arx and do not have the dll loaded inside AutoCAD process. In that case you will have to specify the full path of that dll inside the DllImport attribute, as follow:

[DllImport(@"C:\Temp\myDll.dll")]

public static extern int MyExportedMethod();

 

We can see that the approach is not convenient, because of the hardcoded path inside the DllImport attribute.

I will expose two ways to workaround that limitation. In order to test them, let’s first create an exported method from a custom arx. It requires 3 steps:

1/ Declare the exported method in some header file:

extern "C" HWND _declspec(dllexport) getViewHandle();

2/ Implement the method:

/////////////////////////////////////////////////////////////

//Use: Exported View Handle

//

/////////////////////////////////////////////////////////////

HWND getViewHandle()

{

       CView *pView = acedGetAcadDwgView();

 

       if (pView != NULL)

       {

              return pView->m_hWnd;

       }

 

       return NULL;

}

3/ Declare the exported method in a .def file:

LIBRARY       "myArx"

 

       EXPORTS getViewHandle

That’s it concerning our custom exported method, we are now able to P/invoke “getViewHandle” from .Net as follow and the arx does not have to be loaded in AutoCAD for this to work:

[DllImport(@"C:\Temp\myDll\myArx.arx",

    CharSet = CharSet.Unicode,

    CallingConvention = CallingConvention.Cdecl,

    EntryPoint = "getViewHandle")]

private static extern IntPtr getViewHandle();

So how to avoid the hardcode path in DllImport?

1/ First approach:  Rely on the SetDllDirectory Win32 API and dynamically specify the path of the dll on your system.

 

/////////////////////////////////////////////////////////////

// Use: Dynamic P/Invoke using Win API SetDllDirectory

// Author: Philippe Leefsma

/////////////////////////////////////////////////////////////

[DllImport("Kernel32.dll")]

public static extern bool SetDllDirectory([In]string lpPathName);

 

[DllImport("myArx.arx",

    CharSet = CharSet.Unicode,

    CallingConvention = CallingConvention.Cdecl,

    EntryPoint = "getViewHandle")]

private static extern IntPtr getViewHandle();

 

[CommandMethod("DllDirPInvoke")]

public void DllDirPInvoke()

{

    Document doc = Application.DocumentManager.MdiActiveDocument;

    Database db = doc.Database;

    Editor ed = doc.Editor;

 

    bool b = SetDllDirectory(@"C:\Temp\");

 

    IntPtr res = getViewHandle();

 

    ed.WriteMessage("\nViewHandle: " + res.ToString());

}

2/ Handle loading and unloading of the dll yourself using LoadLibraryEx / FreeLibrary, then access the method with GetProcAddress:

 

/////////////////////////////////////////////////////////////

// Use: Dynamic P/Invoke

// Author: Philippe Leefsma

/////////////////////////////////////////////////////////////

private const uint LOAD_LIBRARY_AS_DATAFILE = 0x00000002;

private const uint DONT_RESOLVE_DLL_REFERENCES = 0x00000001;

private const uint LOAD_WITH_ALTERED_SEARCH_PATH = 0x00000008;

private const uint LOAD_IGNORE_CODE_AUTHZ_LEVEL = 0x00000010;

private const uint LOAD_LIBRARY_AS_DATAFILE_EXCLUSIVE = 0x00000040;

 

[DllImport("kernel32.dll")]

private static extern IntPtr LoadLibraryEx(

    string lpFileName, IntPtr hFile, uint dwFlags);

 

[DllImport("kernel32.dll")]

private static extern bool FreeLibrary(IntPtr dllPointer);

 

[DllImport("kernel32.dll", CharSet = CharSet.Ansi)]

private static extern IntPtr GetProcAddress(

    IntPtr dllPointer, string functionName);

 

 

//Default calling convention used by interop is __stdcall,

//so need to specify

[UnmanagedFunctionPointerAttribute(CallingConvention.Cdecl)]

private delegate IntPtr DllFuncDelegate();

 

[CommandMethod("DynamicPInvoke")]

public void DynamicPInvoke()

{

    Document doc = Application.DocumentManager.MdiActiveDocument;

    Database db = doc.Database;

    Editor ed = doc.Editor;

 

    string lpFileName = @"C:\Temp\myArx.arx";

 

    string FunctionName = "getViewHandle";

 

    IntPtr moduleHandle = LoadLibraryEx(

        lpFileName,

        IntPtr.Zero,

        LOAD_WITH_ALTERED_SEARCH_PATH);

 

    if (moduleHandle != IntPtr.Zero)

    {

        IntPtr pProc = GetProcAddress(moduleHandle, FunctionName);

 

        if (pProc != IntPtr.Zero)

        {

            DllFuncDelegate delegateFunc = (DllFuncDelegate)

                Marshal.GetDelegateForFunctionPointer(

                    pProc,

                    typeof(DllFuncDelegate));

 

      &
#160;     IntPtr res = delegateFunc();

 

            ed.WriteMessage("\nView Handle: " + res.ToString());

        }

 

        bool free = FreeLibrary(moduleHandle);

    }

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