Calling MFC DLL from C#

[alexgm23]

Hi,

I'm trying to call a MFC DLL function from C# code, but I just don't know how to do it.

I've got the DLL's function names using "Dependency Walker" but they look like this:

?AddApl@CDocsHeadings@@QAEHPAVCDocsApl@@@Z
?AddItem@CDocsHeadings@@QAEHPAVCDocsItems@@H@Z
?AddNew@CDocsHeadings@@UAEXXZ
?AddRef@CDocsHeadings@@QAEXDVCString@@@Z
.....

sgsql.dll.jpg

I've got the C++ header files used to make the DLL, so I know how many parameters and what parameter types the DLL's functions expect.

Any idea on how can I accomplish this?


Alejandro Gutierrez

[craig1231]

Those look like debug strings. Build -> Clean Solution. Ensure the Configuration is set to Release. Then Build -> Build Solution

Craig

[alexgm23]

Thanks Craig.

I already found out what those strings mean and how to deal with them, but I still need (too much) help on import a function from the DLL that has a return value of type 'CString &'.

Can you help me on this?

Thanks in advance.

[craig1231]

Well this can be attacked in many different ways, but heres how I would do it;

Have the function return 'const char*' instead of 'CString'

CString theString( "This is a test" );
char* mychar = new char[theString.GetLength()+1];
_tcscpy(mychar, theString);
return mychar;

then in C# use DLLImport to import the function which it will now return a normal C# string that is recognised and terminated by a null value.

If you still have any problems, let me know

Craig

[alexgm23]

Thanks again!

After doing a research I got the same answer, and I think that's the best. But I haven't been explicit enough. The fact is that I can't add such wrapper function returning something more suitable like char* because I don't have the DLL code.

There's a way to instantiate a C++ class from a DLL. You can call a function that returns a pointer such as a class constructor (returns this). Then in C# you have to declare a struct that properly holds the this pointer:

[StructLayout(LayoutKind.Sequential, Pack = 4)]
public unsafe struct __CString
{
    public IntPtr* _vtable;
    public int m_Value;
}
 

Then you can make a C# wrapper class for the C++ class CString:

public unsafe class CString : IDisposable
{
    private __CString* _cpp;
 
    [DllImport("mfc42.dll", EntryPoint = "#535", CallingConvention = CallingConvention.ThisCall)]
    private static extern int _CString_Constructor(__CString* ths, CString str);
 
    [DllImport("mfc42.dll", EntryPoint = "#537", CallingConvention = CallingConvention.ThisCall)]
    private static extern int _CString_Constructor(__CString* ths, [MarshalAs(UnmanagedType.LPStr)] String str);
 
    [DllImport("mfc42.dll", EntryPoint = "#540", CallingConvention = CallingConvention.ThisCall)]
    private static extern int _CString_Constructor(__CString* ths);
 
    [DllImport("mfc42.dll", EntryPoint = "#800", CallingConvention = CallingConvention.ThisCall)]
    private static extern void _CString_Destructor(__CString* ths);
 
    [DllImport("mfc42.dll", EntryPoint = "#2915", CallingConvention = CallingConvention.ThisCall)]
    [return: MarshalAs(UnmanagedType.LPStr)]
    private static extern String _CString_GetBuffer(__CString* ths, int Length);
 
    public CString(CString str)
    {
        _cpp = (__CString*)Memory.Alloc(sizeof(__CString));
        _CString_Constructor(_cpp, str);
    }
 
    public CString(String str)
    {
        _cpp = (__CString*)Memory.Alloc(sizeof(__CString));
        _CString_Constructor(_cpp, str);
    }
 
    public CString()
    {
        _cpp = (__CString*)Memory.Alloc(sizeof(__CString));
        _CString_Constructor(_cpp);
    }
 
    public void Dispose()
    {
        _CString_Destructor(_cpp);
        Memory.Free(_cpp);
    }
 
    public String GetBuffer()
    {
        return _CString_GetBuffer(_cpp, 255);
    }
}
 

Actually, I can instatiate a CString in this way without any problems:

class Program
{
    static void Main(string[] args)
    {
        CString s = new CString("Hello, World!");
        Console.WriteLine(s.GetBuffer());
    }
}
 

But when I try to do this:

class Program
{
    static void Main(string[] args)
    {
        Sample s = new Sample(1000);
        Console.WriteLine(s.AsString().GetBuffer());
    }
}
 
/*
  CString code...
*/
 
/*
  Sample Class Wrapper
*/
 
[StructLayout(LayoutKind.Sequential, Pack = 4)]
public unsafe struct __Sample
{
    public IntPtr* _vtable;
    public int m_Value;
}
 
public unsafe class Sample : IDisposable
{
    private __Sample* _cpp;
 
    [DllImport("OldMFC.dll", EntryPoint = "??0Sample@@QAE@H@Z", CallingConvention = CallingConvention.ThisCall)]
    private static extern int _Sample_Constructor(__Sample* ths, int Value);
 
    [DllImport("OldMFC.dll", EntryPoint = "??1Sample@@UAE@XZ", CallingConvention = CallingConvention.ThisCall)]
    private static extern void _Sample_Destructor(__Sample* ths);
 
    [DllImport("OldMFC.dll", EntryPoint = "?AsString@Sample@@QAEAAVCString@@XZ", CallingConvention = CallingConvention.ThisCall)]
    private static extern CString _Sample_AsString(__Sample* ths);
 
    [DllImport("OldMFC.dll", EntryPoint = "?GetValue@Sample@@QAEHXZ", CallingConvention = CallingConvention.ThisCall)]
    private static extern int _Sample_GetValue(__Sample* ths);
 
    public Sample(int Value)
    {
        _cpp = (__Sample*)Memory.Alloc(sizeof(__Sample));
        _Sample_Constructor(_cpp, Value);
    }
 
    public void Dispose()
    {
        _Sample_Destructor(_cpp);
        Memory.Free(_cpp);
    }
 
    public CString AsString()
    {
        return _Sample_AsString(_cpp);
    }
}
 
/*
  Memory Class code...
*/
 

I get this error:

Unspecied error
Exception from HRESULT: 0x80004005 (E_FAIL)

The DLL function declaration is:

CString & Sample::AsString();

Any idea about it?

Alex G.