library TestDLL;

{ Important note about DLL memory management: ShareMem must be the
  first unit in your library's USES clause AND your project's (select
  Project-View Source) USES clause if your DLL exports any procedures or
  functions that pass strings as parameters or function results. This
  applies to all strings passed to and from your DLL--even those that
  are nested in records and classes. ShareMem is the interface unit to
  the BORLNDMM.DLL shared memory manager, which must be deployed along
  with your DLL. To avoid using BORLNDMM.DLL, pass string information
  using PChar or ShortString parameters. }

uses
  SysUtils,
  Classes;
function ADD(X,Y:Integer):Integer;stdcall;
begin
  Result :=X+Y;
end;
function Txx(x,y:Double):Double ;stdcall ;
begin
  Result :=x+y+y+x;
end;
function EncryVncPassStrHex(Str: PChar): PChar;stdcall;

var
  TempResult: String;
begin
  TempResult := WideCharToString(str);
  Result := PChar(TempResult);
end;
{$R *.res}
exports
   ADD,
   EncryVncPassStrHex,
   Txx;
begin
end.

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace UseDLL
{
    /// <summary>
    /// Description of MainForm.
    /// </summary>
    public class UseDLL
    {
        [DllImport("TestDLL.DLL",EntryPoint="ADD",CharSet= CharSet.Auto,CallingConvention=CallingConvention.StdCall)]
        public static extern int add(int x,int y);
        [DllImport("TestDLL.DLL",EntryPoint="Txx",CharSet= CharSet.Auto,CallingConvention=CallingConvention.StdCall)]
        public static extern double PP(double x,double y);
        [DllImport("TestDLL.DLL", EntryPoint="EncryVncPassStrHex",CharSet=CharSet.Auto, CallingConvention = CallingConvention.StdCall)]
         public static extern string EncryVncPassStrHex( string Str);
    }

    public partial class MainForm : Form
    {
        public MainForm()
        {
            //
            // The InitializeComponent() call is required for Windows Forms designer support.
            //
            InitializeComponent();
            
            //
            // TODO: Add constructor code after the InitializeComponent() call.
            //
        }
        
        void Button1Click(object sender, EventArgs e)
        {
            int x=10;
            int y=100;
            int z=UseDLL.add(x,y);
            this.textBox1.Text=z.ToString();
            double oo=0.123;
            double pp=0.254;            
            this.button1.Text=UseDLL.PP(oo,pp).ToString();
            this.Text =UseDLL.EncryVncPassStrHex("我是XXX");
        }
    }
}