缺少方法異常-查找所需的方法

Question

我有一個綁定到用戶類列表的數據網格視圖。

當我創建新行時，我得到了一個缺少的方法異常“找不到User.cs上的構造函數”。 問題是我已經有一個默認的構造函數，所以我想知道是否有一種方法可以找出參數是什么，以便我可以實現類構造函數。

這是類和構造函數

    public string Username { get; set; }
    public byte[] HashedPassword;
    public byte[] Salt ;
    public string sSalt { get { return Encoding.ASCII.GetString(Salt); } set; }
    public string sPass { get { return Encoding.ASCII.GetString(HashedPassword); } set; }
    public bool Admin { get; set; }
    public List<AnswerClass> answers { get; set; }
    public Tuple<int, int> sessionScore;

    public User(string UsernameArg = "", byte[] PasswordArg = null, byte[] SaltArg = null, bool AdminArg = false)
    {
        sessionScore = new Tuple<int, int>(0, 0);
        Username = UsernameArg;
        HashedPassword = PasswordArg;
        Salt = SaltArg;
        Admin = AdminArg;
        answers = new List<AnswerClass>();
    }

錯誤：

System.MissingMethodException was unhandled
HResult=-2146233069
Message=Constructor on type 'QuizProject_SourceControl_.User' not found.
Source=mscorlib
StackTrace:
   at System.RuntimeType.CreateInstanceImpl(BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes, StackCrawlMark& stackMark)
   at System.Activator.CreateInstance(Type type, BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes)
   at System.Activator.CreateInstance(Type type, BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture)
   at System.SecurityUtils.SecureCreateInstance(Type type, Object[] args, Boolean allowNonPublic)
   at System.ComponentModel.BindingList`1.AddNewCore()
   at System.ComponentModel.BindingList`1.System.ComponentModel.IBindingList.AddNew()
   at System.Windows.Forms.CurrencyManager.AddNew()
   at System.Windows.Forms.DataGridView.DataGridViewDataConnection.AddNew()
   at System.Windows.Forms.DataGridView.DataGridViewDataConnection.OnNewRowNeeded()
   at System.Windows.Forms.DataGridView.OnRowEnter(DataGridViewCell& dataGridViewCell, Int32 columnIndex, Int32 rowIndex, Boolean canCreateNewRow, Boolean validationFailureOccurred)
   at System.Windows.Forms.DataGridView.SetCurrentCellAddressCore(Int32 columnIndex, Int32 rowIndex, Boolean setAnchorCellAddress, Boolean validateCurrentCell, Boolean throughMouseClick)
   at System.Windows.Forms.DataGridView.OnCellMouseDown(HitTestInfo hti, Boolean isShiftDown, Boolean isControlDown)
   at System.Windows.Forms.DataGridView.OnCellMouseDown(DataGridViewCellMouseEventArgs e)
   at System.Windows.Forms.DataGridView.OnMouseDown(MouseEventArgs e)
   at System.Windows.Forms.Control.WmMouseDown(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.DataGridView.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.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 QuizProject_SourceControl_.Program.Main() in d:\Programming\Repos\QuizProject(SourceControl)\QuizProject(SourceControl)\Program.cs:line 19
   at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
   at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
   at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
   at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()
InnerException: 

Answer 1

您沒有默認的構造函數。 默認構造函數是不帶任何參數的構造函數：

public User()
{
    ...
}

您所擁有的是一個構造函數，其中所有參數都具有默認值。 這是一個很大的差異，因此原因是.NET中參數默認值的工作方式：
在.NET中，編譯器將默認值復制到調用該構造函數（或任何其他方法）的每個位置。 因此，基本上，參數的默認值只是語法糖。

例：
假設您有一個這樣的方法：

public void Method(int para = 12)
{
}

現在，在代碼中的某處，您可以這樣稱呼它：

Method();

編譯器將對此進行更改，而實際被編譯的代碼將如下所示：

public void Method(int para)
{
}

Method(12);