PrepareRequest(
    const Request:Pointer; 
    const RequestSize: Integer; 
    const Code:Pointer; 
    const CodeSize:Integer;
    var OutgoingTLD:Pointer; 
    var OutgoingTLDSize:Integer; 
    const Encrypt, Sign: WordBool): Integer; stdcall;

//вот так объявляю функцию
    public class Win32
    {
        [DllImport("MesProWrp.dll", CharSet = CharSet.Auto)]
        public static extern int PrepareRequest(object Request,
            int RequestSize,
            object Code,
            int CodeSize,
            [In, Out] ref object Result,
            [In, Out] ref int ResultSize,
            bool Encrypt,
            bool Sign);
    }

//вот так вызываю:
            object SomeObj = new Object();
            int size = 0;
            Win32.PrepareRequest(
                textBox1.Text,
                textBox1.Text.Length,
                "DYZL",
                "DYZL".Length,
                ref SomeObj,
                ref size,
                false,
                true);