Using Delphi's stuct arrays and strings in C#

Question

I've been trying to invoke a method that have been created in Delphi in the following way:

 function _Func1(arrParams: array of TParams): Integer;stdcall;    

 type 
   TParams = record
   Type: int;
   Name: string;
   Amount : Real;
 end;

My code is:

[DllImport("some.dll", EntryPoint = "_Func1", CallingConvention = CallingConvention.StdCall)]
public static extern int Func(
  [MarshalAs(UnmanagedType.LPArray, ArraySubType=UnmanagedType.Struct)] TParams[] arrParams)

And the struct is:

[StructLayoutAttribute(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct TParams
{
  public int Type;
  [MarshalAs(UnmanagedType.AnsiBStr)]
  public string Name;
  public double Amount;
}

When I am calling this method I'm getting the error: Cannot marshal field 'Name' of type 'TParams': Invalid managed/unmanaged type combination (String fields must be paired with LPStr, LPWStr, BStr or ByValTStr).

However none of those combinations works, as Delphi's strings are prefixed with its length and it is Ansi for sure (I've tried it with other string parameters). Does anyone have a clue how to solve this?

Answer 1

There are two main problems with this, use of open arrays and use of Delphi string.

Open arrays

Delphi open arrays are implemented by passing a pointer to the first element of the array and an extra parameter specifying the index of the last item, high in Delphi terminology. For more information see this answer.

Delphi strings

The C# marshaller cannot interop with a Delphi string. Delphi strings are private types, only to be used internally to a Delphi module. Instead you should use a null-terminated string, PAnsiChar.

---

Putting it all together you can write it like this:

Delphi

type 
  TParams = record
    _Type: Integer;//Type is a reserved word in Delphi
    Name: PAnsiChar;
    Amount: Double;
  end;

function Func(const arrParams: array of TParams): Integer; stdcall;

C#

[StructLayoutAttribute(LayoutKind.Sequential)]
public struct TParams
{
  public int Type;
  public string Name;
  public double Amount;
}

[DllImport("some.dll")]
public static extern int Func(TParams[] arrParams, int high);

TParams[] params = new TParams[len];
...populate params
int retval = Func(params, params.Length-1);

Answer 2

To compliment David's answer, you can marshal to a Delphi string, but it's ugly. In C#, you have to replace all of your strings in the struct with IntPtr.

private static IntPtr AllocDelphiString(string str)
{
    byte[] unicodeData = Encoding.Unicode.GetBytes(str);
    int bufferSize = unicodeData.Length + 6;

    IntPtr hMem = Marshal.AllocHGlobal(bufferSize);

    Marshal.WriteInt32(hMem, 0, unicodeData.Length); // prepended length value

    for (int i = 0; i < unicodeData.Length; i++)
        Marshal.WriteByte(hMem, i + 4, unicodeData[i]);

    Marshal.WriteInt16(hMem, bufferSize - 2, 0); // null-terminate

    return new IntPtr(hMem.ToInt64() + 4);
}

This can directly be sent to Delphi, where it'll properly be read as a string.

Remember that you must free this string when you're done with it. However, GlobalFree() can't be called directly on the pointer to the string, because it doesn't point to the start of the allocation. You'll have to cast that pointer to a long, then subtract 4, then cast it back to a pointer. This compensates for the length prefix.