accessing Delphi DLL import with C#

Greetings

I've asked before on other forums but so far I'm stumped.

I have a Delphi DLL library that communicates with an MCU and it works fine as it is. However I'm not into pascal and would rather continue learning C#. I'm trying to use the Delphi DLL from within the C# structure as it already handles the communication with the mcu.

List of functions and procedures provided by the DLL

Code:

---

Type TData = array [0..255] of byte;

function Receive(Adr, Len: Integer): TData; stdcall;

---

This dll reads data from the mcu.
Adr= Starting address
Len= Total length of the data being read
The function returns a TData array where the item with index 0 contains the starting address and the item with index 1 contains the length of the data received.
Following items (index 2 and higher) contain the data received. If the Receive operation fails, the length value returned in the array is set to zero.

I have been trying the following without success.

Code:

---

[DllImport("mrds.dll", CallingConvention = CallingConvention.StdCall, CharSet = CharSet.Ansi)]
public static unsafe extern byte[] Receive(int Adr, int Len);

           
private void btnReceive_Click(object sender, EventArgs e)
{
    txtBox1.Text = "";
    StatusLbl1.Text = "";
    byte[] Data = new byte[8];
    int i, Adr =0x02, Len = 8;
    string s = "";
            for (i = 0; i <= 7; i ++ ) Data[i] = 0;
            Data[0] = 0x02;
            Data= Receive(Adr, Len);
            for (i = 1; i <= 8; i++)
            s = s + Data[i + 1].ToString();
            txtBox1.Text = s;
}

---

Everything works up to

Code:

---

Data= Receive(Adr, Len);

---

where it returns an error "MarshalDirectiveException was unhandled"

I'm lost, should I be using PInvoke or do I use a Marshal?

Do you have any access to the Delphi source code?

Yes, I have the source code, same function
Partial code shown as the owner asked it not be published.

Delphi6

Code:

---

function Receive(Adr, Len: Integer): TData;
function TDM1.Receive(Adr, Len: Integer): TData;
var Output: TData;
VaComm1.ReadChar(Ch)
Output[Len+1]:=ReadByte(true);
Result:=Output;

end;

---

Ok so, the first thing you might want to do is to change that function calling convention to stdcall. Delphi by default uses fastcall, but .net doesn't support it. This should limit any issues between your delphi dll and .net to datatype differences -- possibly eliminating the current error altogether. But unlikely. Still, it will guarantee you won't have other type of issues, like getting unexpected results between similar function calls.

The error itself is well within PInvoke usage. So don't think it means you are using the wrong methodology. It means that it is encountering an attribute it doesn't support when marshaling data between the unmanaged and managed code. I suspect a data type conversion that isn't being supported. But I'm not actually sure. I'm suspecting the return type which isn't being properly mapped between the delphi Type TData and C# byte array. I'd probably try a unmanaged array here, before looking anywhere else.

What if a new Function was created in the original mrds.dll that could store and export the

Code:

---

Result:=Output;

---

? Or am I looking at this the wrong way?

The source mrds.dpr file identifies the Receive function as this

Code:

---

function Receive(Adr, Len: Integer): Unit1.TData; stdcall;

begin

Result:=DM1.Receive(Adr, Len);

end;

---

I wouldn't know. Don't ask me about Delphi. I have very little idea :)

However I just now noticed from your initial post that the function is being declared as stdcall. So that's covered.
If you don't want to use an unmanaged array, you could try to find some way to return from the delphi dll a true byte array, and not one wrapped inside a type. Do note however that I'm fishing here. I just know this is an attribute problem. You only work with two datatypes here. And Int can't be the problem because both languages share the same type properties.

Thanks Mario

I'll stick with a C# solution, the Send Function has no issues passing int and byte[] arrays through the delphi DLL so it must be a unmanaged Marshalling issue I am just not seeing for the Receive function.