System.EntryPointNotFoundException:找不到入口点

我在执行VB6时将其转换为c#，生成此错误

来自VB6

Declare Function OpenCommPort Lib "C:'Program Files'MR705API.dll" Alias "?OpenCommPort@@YGHPADPAPAX@Z" (ByVal PortName As String, ByRef hCom As Long) As Long
Declare Function CloseCommPort Lib "C:'Program Files'MR705API.dll" Alias "?CloseCommPort@@YGHPAX@Z" (ByVal hCom As Long) As Long
Declare Function SetLED Lib "C:'Program Files'MR705API.dll" Alias "?SetLED@@YGHPAXEE@Z" (ByVal hCom As Long, ByVal Led As Byte, ByVal Addr As Byte) As Long
Declare Function ActiveBuzzer Lib "C:'Program Files'MR705API.dll" Alias "?ActiveBuzzer@@YGHPAXEE@Z" (ByVal hCom As Long, ByVal DelayTime As Byte, ByVal Addr As Byte) As Long
Declare Function Iso14443Reqa Lib "C:'Program Files'MR705API.dll" Alias "?Iso14443Reqa@@YGHPAXEPAEE@Z" (ByVal hCom As Long, ByVal ReqAMode As Byte, ByVal ATQ As String, ByVal Addr As Byte) As Long
Declare Function Iso14443Anticoll Lib "C:'Program Files'MR705API.dll" Alias "?Iso14443Anticoll@@YGHPAXEPAE1E@Z" (ByVal hCom As Long, ByVal AnticollMode As Byte, ByVal Uid As String, ByVal MultiTag As String, ByVal Addr As Byte) As Long

IN C#

        [DllImport ("MR705API.dll")]
        public static extern long OpenCommPort(String portName, ref long hCom ); 
        [DllImport ("MR705API.dll")]
        public static extern long CloseCommPort(long hCom);
        [DllImport ("MR705API.dll")]
        public static extern long SetLED(long hCom, byte Led , byte Addr);
        [DllImport ("MR705API.dll")]
        public static extern long ActiveBuzzer (long hcom, byte DelayTime, byte Addr);
        [DllImport ("MR705API.dll")]
        public static extern long Iso14443Reqa (long hcom, byte ReqAMode, string ATQ, byte Addr);

以及我如何使用它。

public void doReader() {
            Result = OpenCommPort("COM9", ref HANDLE);
            ....
            ....
}

异常

System.EntryPointNotFoundException: Unable to find an entry point named 'OpenCommPort' in DLL 'MR705API.dll'.
   at TrueReader.MainForm.OpenCommPort(String portName, Int64& hCom)
   at TrueReader.MainForm.doReader() in c:'Users'sattha'Documents'SharpDevelop Projects'TrueReader'TrueReader'MainForm.cs:line 59
   at TrueReader.MainForm.Timer1Tick(Object sender, EventArgs e) in c:'Users'sattha'Documents'SharpDevelop Projects'TrueReader'TrueReader'MainForm.cs:line 54
   at System.Windows.Forms.Timer.OnTick(EventArgs e)
   at System.Windows.Forms.Timer.TimerNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
   at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
   at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.Run(Form mainForm)
   at TrueReader.Program.Main(String[] args) in c:'Users'sattha'Documents'SharpDevelop Projects'TrueReader'TrueReader'Program.cs:line 27

有人能指引我吗？我做错了什么？？或者我错过了什么。

我不知道这个.dll是怎么写的，用哪种语言写的。

但它以前曾在VB6中使用过。