缺少方法异常-查找所需的方法

Question

我有一个绑定到用户类列表的数据网格视图。

当我创建新行时，我得到了一个缺少的方法异常“找不到User.cs上的构造函数”。 问题是我已经有一个默认的构造函数，所以我想知道是否有一种方法可以找出参数是什么，以便我可以实现类构造函数。

这是类和构造函数

    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>();
    }

错误：

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

您没有默认的构造函数。 默认构造函数是不带任何参数的构造函数：

public User()
{
    ...
}

您所拥有的是一个构造函数，其中所有参数都具有默认值。 这是一个很大的差异，因此原因是.NET中参数默认值的工作方式：
在.NET中，编译器将默认值复制到调用该构造函数（或任何其他方法）的每个位置。 因此，基本上，参数的默认值只是语法糖。

例：
假设您有一个这样的方法：

public void Method(int para = 12)
{
}

现在，在代码中的某处，您可以这样称呼它：

Method();

编译器将对此进行更改，而实际被编译的代码将如下所示：

public void Method(int para)
{
}

Method(12);