Using Delphi DLL in C#

Question

I have a third party "mystery dll" written with Delphi(unknown version), working example in delphi (past 2009), dire need to use said dll in my C# code, and almost no relevant knowledge on how to do it.

Here is Delpi example in using this dll:

type
TD_Query = function(host: WideString; port : Word;pud,query : WideString):WideString; stdcall;
procedure TForm11.Button6Click(Sender: TObject);
var
   Handle         : LongWord;
   D_Query        : TD_Query;
   sss            : WideString;
begin

 Handle := LoadLibrary('kobrasdk.dll');
 sss:='';
 if Handle <> 0 then
 begin
  @D_Query := GetProcAddress(Handle, 'D_Query');
  sss:=D_Query('host',8201,'pud','query');
  FreeLibrary(Handle);
 end;
end;

And here is my attempts to interpret it in C#:

class Program
{
    [DllImport("C:\\Games\\kobrasdk.dll", CallingConvention = CallingConvention.StdCall,
        CharSet = CharSet.Ansi)]
    [return: MarshalAs(UnmanagedType.LPStr)]
    public static extern string D_Query(string host, ushort port, string pud, string query);


    static void Main(string[] args)
    {
        D_Query("test", 8201, "test", "test");
    }
}

Unfortunately, what I have is an error: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.

From what I read during the day, I probably fudged up with return type, or parameter types. Help?

Answer 1

The Delphi ABI differs from the Microsoft ABI for certain types. A Delphi WideString is a managed type (in Delphi terminology) and as return types use an ABI that is incompatible with Microsoft tools.

The Delphi ABI translates a managed return type into a hidden var parameter. So the compiler transforms:

function(host: WideString; port: Word; pud, query: WideString): WideString; stdcall;

into

procedure(var result: WideString; host: WideString; port: Word; pud, query: WideString); 
  stdcall;

You can therefore access your original Delphi function from C# by importing the function in its transformed guise.

[DllImport(@"...", CallingConvention = CallingConvention.StdCall)]
public static extern void My_D_Query(
    [MarshalAs(UnmanagedType.BStr)]
    out string result,
    [MarshalAs(UnmanagedType.BStr)]
    string host,
    ushort port,
    [MarshalAs(UnmanagedType.BStr)]
    string pud,
    [MarshalAs(UnmanagedType.BStr)]
    string query
);

Answer 2

I mostly figured it out. For some reason unclear to me, C# cant handle WideString return values. If you have access to delphi source code, it might be appropriate to exchange function with procedure, and pass return value as "out" parameter. In my case, I did not have access to source, so I was forced to write a proxy DLL to do so. For example above, "proxy" dll code:

  type
  TD_Query = function(host : WideString;port : Word;pud,query : WideString):WideString; stdcall;

procedure My_D_Query(host: WideString; port: Word; pud, query: WideString; out return : WideString); stdcall;
var
   Handle: LongWord;
   D_Query : TD_Query;
   sss : WideString;
begin
 Handle := LoadLibrary('kobrasdk.dll');
 sss:='';
 if Handle <> 0 then
 begin
  @D_Query:=GetProcAddress(Handle, 'D_Query');
  sss:=D_Query(host,port,pud,query);
  FreeLibrary(Handle);
 end;
return := sss;
end;

Then C# code to access it:

[DllImport("C:\\MyDll.dll", CallingConvention = CallingConvention.StdCall,
    CharSet = CharSet.Ansi)]
public static extern void My_D_Query(
[MarshalAs(UnmanagedType.BStr)]
        string host,
        int port,
[MarshalAs(UnmanagedType.BStr)]
        string pud,
[MarshalAs(UnmanagedType.BStr)]
        string query,
[MarshalAs(UnmanagedType.BStr)]
        out string result
);

Its not pretty, but for me, it was the answer.