Returning string from delphi dll to c#

Question

I am trying to separate an encryption function from our legacy code to a dll which I can call from C#, but I am having issues getting it to work and I keep getting access violations when calling the dll.

I am not sure where the AV happens because delphi has a hard time hitting my breakpoints when the dll is attached to another process.

I got it to work yesterday using David Heffernan's answer here: Returning a string from delphi dll to C# caller in 64 bit But my success was short-lived as I changed the string parameters to regular string 's (delphi) saw it didn't work and changed them back to to AnsiString (our encryption routine expects Ansi). Since I changed these param types. I have not been able to get it to work again.

Here is my Delphi Code:

procedure Encrypt(const Source: AnsiString; const Key: AnsiString; var OutPut:PAnsiChar; const OutputLength: Integer);
var
  EncryptedString, EncodedString: AnsiString;
begin
  EncryptedString := Crypt(Source, Key);
  EncodedString := Encode(EncryptedString);
  if Length(EncodedString) <= OutputLength then
    System.AnsiStrings.StrPCopy(Output, EncodedString);
end;

exports
  Encrypt;

My C# caller:

[DllImport("AsmEncrypt.dll", CharSet = CharSet.Ansi)]
public static extern void Encrypt(string password, string key, StringBuilder output, int outputlength);
// using like this:
Encrypt(credentials.Password, myKey, str, str.Capacity);

My best bet right now is that I've goofed some of the arguments to the dll since it seems to crash before it reaches an OutputDebugStr() I had put on first line of Encrypt()

All help will be greatly appreciated

Answer 1

From this docs page :

The AnsiString structure contains a 32-bit length indicator, a 32-bit reference count, a 16-bit data length indicating the number of bytes per character, and a 16-bit code page.

So an AnsiString isn't simply a pointer to an array of characters -- it's a pointer to a special structure which encodes a bunch of information.

However, .NET's P/Invoke machinery is going to pass a pointer to an array of characters. Delphi is going to try and interpret that as a pointer to its special AnsiString structure, and things aren't going to go well.

I think you're going to have a hard time using AnsiString in interop. You're better off choosing a string type which both .NET and Delphi know about. If you then need to convert that to AnsiString , do that in Delphi.

Answer 2

Change the Delphi function to

procedure Encrypt(Source, Key, OutPut: PAnsiChar; OutputLength: Integer); stdcall;

in order to make this code work.

You should probably also make the length argument IN/OUT so that the caller can resize the string builder object once the call returns. That would also allow the callee to signal any errors to the caller, another flaw in your current design.

I must also say that using AnsiString as a byte array is a recipe for failure. It's high time you started doing encryption right. If you have text, then encode it as a byte array with a specific encoding, usually this means UTF-8. Then encrypt that byte array to another byte array.