NullReferenceException 使用两个构造函数 class

Question

我是新来的。我无法在任何地方找到解决问题的方法。我正在给学校写一个项目。我有两种形式。在第一种形式中，当我将数据存储在 SQL 服务器数据库中时，我有一个 DataGridView 对象。我使用代码将用户名从 DataGridView 传递到第二种形式：

List<string> usersList = new List<string>();

for (int i = 0; i < dataGridViewUsers.RowCount; i++)
{
    usersList.Add(dataGridViewUsers.Rows[i].Cells[1].Value.ToString());
}

AddFileForm addFile = new AddFileForm(usersList);
addFile.Show();

在第二种形式中有以下代码：

    private List<string> userNames;
    private Files file;

    public AddFileForm()
    {
        file = new Files();
        userNames = new List<string>();
        InitializeComponent();
    }

    public AddFileForm(List<string> _userNames)
    {
        this.userNames = _userNames;
        InitializeComponent();
        listBoxUserList.DataSource = _userNames;
    }

    private AddFileForm(Files _file)
    {
        this.file = _file;
        InitializeComponent();
        listBoxUserList.SelectedValue = _file.userName;
        listBoxItems.SelectedValue = _file.directory;
    }

从第一个表单传递数据工作正常，但是当我尝试使用存储过程将数据从 listBoxUserList 和 listBoxItems 传递到数据库时，问题就出现了。我正在手动将数据添加到 listBoxItems。单击按钮时发生 NullReferenceException，此处：

private void buttonAdd_Click(object sender, EventArgs e)
{
    if (listBoxUserList.SelectedItems.Count != 0 && listBoxItems.SelectedItems.Count != 0)
    {
            file.userName = listBoxUserList.SelectedValue.ToString();
            file.directory = listBoxItems.SelectedValue.ToString();
                try
                {
                    DBConnection connection = new DBConnection();
                    connection.AddFile(file);

                    MessageBox.Show("File added to database.");
                }
                catch (Exception)
                {
                    MessageBox.Show("Failed database connection!");
                }
            }
            else
            {
                MessageBox.Show("First, select items to add to database");
            }
        }

两个列表框的值都不为空，我已经检查过了。主要问题可能出在 class 构造函数中。我的存储过程工作正常，当我是从第一种形式传递数据的注释构造函数时，它完美地将数据传递到数据库。那么，怎么了？我应该如何处理 class 的构造函数，它在两种情况下都可以属性工作？

我正在粘贴错误详细信息

System.NullReferenceException was unhandled
  HResult=-2147467261
  Message=Odwołanie do obiektu nie zostało ustawione na wystąpienie obiektu.
  Source=FilesEncoding
  StackTrace:
       w FilesEncoding.AddFileForm.buttonAdd_Click(Object sender, EventArgs e) w C:\Users\Grabarz\Documents\Visual Studio 2010\Projects\FilesEncoding\FilesEncoding\AddFileForm.cs:wiersz 110
       w System.Windows.Forms.Control.OnClick(EventArgs e)
       w System.Windows.Forms.Button.OnClick(EventArgs e)
       w System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
       w System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
       w System.Windows.Forms.Control.WndProc(Message& m)
       w System.Windows.Forms.ButtonBase.WndProc(Message& m)
       w System.Windows.Forms.Button.WndProc(Message& m)
       w System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
       w System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
       w System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
       w System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
       w System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
       w System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
       w System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
       w System.Windows.Forms.Application.Run(Form mainForm)
       w FilesEncoding.Program.Main() w c:\users\grabarz\documents\visual studio 2010\Projects\FilesEncoding\FilesEncoding\Program.cs:wiersz 18
       w System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       w System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       w Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       w System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       w System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       w System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       w System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       w System.Threading.ThreadHelper.ThreadStart()
  InnerException:

Answer 1

我不确定这段代码是否正确，但它终于开始工作了！我有类似的解决方案，here 但我不知道我的主题问题中的构造函数和这个有什么不同。在我看来，我的第一张唱片也应该完美无缺。这是我的代码：

private string userName;
private string fileDirectory;

public AddFileForm(Files _file) : this(_file.userName, _file.directory) {  }
public AddFileForm(string user, string directory)
{
    userName = user;
    fileDirectory = directory;
    InitializeComponent();
}

private void buttonAdd_Click(object sender, EventArgs e)
{
    userName = listBoxUserList.SelectedItem.ToString();
    fileDirectory = listBoxItems.SelectedItem.ToString();
        try
        {
            Files file = new Files();
            file.userName = userName;
            file.directory = fileDirectory;
            DBConnection connection = new DBConnection();
            connection.AddFile(file);
        }
        catch (Exception)
        {
            MessageBox.Show("Failed database connection!");
        }
    }

NullReferenceException 使用两个构造函数 class

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