How to call this Delphi function from a DLL in C#?

Question

I am given a Delphi DLL that contains functions that I need to call in C#. One of the functions takes two char arrays, where one is an encrypted password and the other is the key.

TCString = array[0..254] of Char;
...
function Decrypt(const S, Key: TCString): TCString; stdcall;

I tried to figure out how to call this function on my own but I keep getting "Cannot marshal 'return value': Invalid managed/unmanaged type combination." I am using byte since the Char type in Delphi is AnsiChar which is 8 bits.

[DllImport("path", CallingConvention = CallingConvention.StdCall, CharSet = CharSet.Ansi)]
        public static extern byte[] Decrypt(byte[] S, byte[] Key);

What is the correct way to call this in C#?

Answer 1

I think I would be inclined to wrap the fixed length array in a C# struct.

public struct CString
{
    [UnmanagedType.ByValArray(SizeConst=255)]
    byte[] value;
}

This allows the size to be specified in one place only.

The next hurdle is the return value. The Delphi ABI treats a return value that cannot fit into a register as an additional hidden var parameter. I'll translate that as a C# out parameter.

Finally the two input parameters are declared as const . That means that they are passed by reference.

So the function would be:

[DllImport(dllname, CallingConvention = CallingConvention.StdCall)]
public static extern void Decrypt(
    [In] ref CString S, 
    [In] ref CString Key, 
    out CString Result

);

I've intentionally avoided any use of text in this because this would appear to be a function that operates on binary data. Many Delphi programmers treat AnsiChar arrays interchangeably with byte arrays in such situations which is often confusing.