窗体加载时隐藏标签？ [英] Hide Tabs when windows form load?

问题描述

namespace WindowsFormsApplication2
{
    
    public partial class Form1 : Form
    {
        public Form1()
        {

            TabPage removedTab = tabControl1.TabPages[1];

            tabControl1.TabPages.Remove(removedTab);
            InitializeComponent();
        }

        private void button2_Click(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {
            SqlConnection con= new SqlConnection();
            
            con.ConnectionString = "Data Source=PROMOD-PC;Initial Catalog=Unicella_Data;Integrated Security=True";
            con.Open();
            String QueryStr = "INSERT INTO Login_Table(Password) VALUES ('" + User_Password.Text + "')";

            SqlCommand cmd = new SqlCommand(QueryStr, con);
            cmd.ExecuteNonQuery();

            MessageBox.Show("Saving is done!");

            if (UserType_CMBbox.SelectedItem.ToString() == "UserType_CMBbox")
            {
                using (SqlConnection sqlConnection = new SqlConnection("Data Source=PROMOD-PC;Initial Catalog=Unicella_Data;Integrated Security=True"))
                {
                    String cmbItem = UserType_CMBbox.SelectedText;
                    SqlCommand sqlCmd = new SqlCommand("SELECT * FROM Login_Table WHERE User_Type='" + cmbItem + "'", sqlConnection);
                    sqlConnection.Open();
                    SqlDataReader sqlReader = sqlCmd.ExecuteReader();

                    while (sqlReader.Read())
                    {
                        UserName_CMBbox.Items.Add(sqlReader["User_Name"].ToString());
                    }

                    sqlReader.Close();
                }

            }

            //con.Close
            

        }

i在我的应用程序中有四个标签
第一个选项卡显示用户身份验证。

我需要隐藏第二，第三，第四个选项卡，直到我输入正确的密码

当我输入正确的密码时它将显示那些标签。



所以我是如何做到的（我已经通过多种方式完成了它。但它没有用）

i have four tabs in my application
1st tab shows user authentication.
I need to hide 2nd, 3rd, 4th tabs until i input my correct password
when i input correct password it will show those tabs.

so how i do that (i have done it in several ways. but it doesn't worked)

public Form1()
        {
 
            TabPage removedTab = tabControl1.TabPages[1];
 
            tabControl1.TabPages.Remove(removedTab);
            InitializeComponent();
        }

此代码显示错误。它说对象引用未设置为对象的实例。

this code show me an error. it says "Object reference not set to an instance of an object."

推荐答案

几乎可以肯定，它发生的原因是最终只有一个或更少的选项卡，然后 tabControl1.TabPages [1] 返回null。您尝试通过下一行中的调用取消引用此null，并抛出此异常。



您应该使用调试器。每次发生时你都不能问这样的问题。你真的需要自己学习如何处理这些问题。不用担心。这是检测和修复的最简单的案例之一。它只是意味着某些引用类型的某个成员/变量通过使用和它的实例（非静态）成员解除引用，这要求此成员/变量为非null，但实际上它似乎为null。只需在调试器下执行它，它将停止抛出异常的执行。在该行上设置一个断点，重新启动应用程序并再次到达这一点。评估下一行中涉及的所有引用，并查看哪一个为null，而不需要为null。解决这个问题之后，修复代码：要么确保将成员/变量正确初始化为非空引用，要么将其检查为null，如果为null，则执行其他操作。



另请参阅：想要在按钮点击时显示下一条记录。但是如果下一条记录功能的条件对象引用没有设置为对象的实例 [ ^ ]。



有时候，你不能在调试器下，由于一个或另一个原因做到这一点。一个非常讨厌的情况是，只有在调试信息不​​可用时构建软件时才会出现问题。在这种情况下，你必须使用更难的方式。首先，你需要确保你永远不会通过静默处理异常来阻止异常的传播（这是开发人员对自己的犯罪，但很常见）。您需要在每个线程的最顶层堆栈帧上捕获绝对所有异常。如果处理类型 System.Exception 的异常，则可以执行此操作。在处理程序中，您需要记录所有异常信息，尤其是 System.Exception.StackTrace ：

http://msdn.microsoft.com/en-us/library/system.exception.aspx [ ^ ]，

http://msdn.microsoft.com/en-us/library/system.exception。 stacktrace.aspx [ ^ ]。



堆栈跟踪只是一个字符串，显示从throw语句到处理程序的异常传播的完整路径。通过阅读，您总能找到目的。对于日志记录，使用类 System.Diagnostics.EventLog ：

http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog.aspx [ ^ ]。

-SA

Almost certainly, it happens because eventually you have only one or less tabs, and then tabControl1.TabPages[1] returns null. You try to dereference this null by the call in next line, and it throws this exception.

You should use the debugger. You cannot ask such question every time it happens. You really need to learn how to deal with such problems yourself. Not to worry. This is one of the very easiest cases to detect and fix. It simply means that some member/variable of some reference type is dereferenced by using and of its instance (non-static) members, which requires this member/variable to be non-null, but in fact it appears to be null. Simply execute it under debugger, it will stop the execution where the exception is thrown. Put a break point on that line, restart the application and come to this point again. Evaluate all references involved in next line and see which one is null while it needs to be not null. After you figure this out, fix the code: either make sure the member/variable is properly initialized to a non-null reference, or check it for null and, in case of null, do something else.

Please see also: want to display next record on button click. but got an error in if condition of next record function "object reference not set to an instance of an object"[^].

Sometimes, you cannot do it under debugger, by one or another reason. One really nasty case is when the problem is only manifested if software is built when debug information is not available. In this case, you have to use the harder way. First, you need to make sure that you never block propagation of exceptions by handling them silently (this is a crime of developers against themselves, yet very usual). The you need to catch absolutely all exceptions on the very top stack frame of each thread. You can do it if you handle the exceptions of the type System.Exception. In the handler, you need to log all the exception information, especially the System.Exception.StackTrace:
http://msdn.microsoft.com/en-us/library/system.exception.aspx[^],
http://msdn.microsoft.com/en-us/library/system.exception.stacktrace.aspx[^].

The stack trace is just a string showing the full path of exception propagation from the throw statement to the handler. By reading it, you can always find ends. For logging, it's the best (in most cases) to use the class System.Diagnostics.EventLog:
http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog.aspx[^].

Good luck,

—SA

扩展谢尔盖所说的话...



问题是您要删除标签的位置：

Expanding on what Sergey said...

The problem is WHERE you are removing the tab:

public Form1()
        {
 
            TabPage removedTab = tabControl1.TabPages[1];
 
            tabControl1.TabPages.Remove(removedTab);
            InitializeComponent();
        }

InitializeComponent调用很重要，因为它设置了表单上的所有类型。在调用之前，您的表单上没有定义任何控件（嗯，它们已定义，但未实例化）。



为了解决删除第一个标签页的问题，你可以将你的代码移到InitializeComponent调用之下：

The InitializeComponent call is important because it sets up all the types on the form. Until this is called, your form does not have any controls defined on it (well, they are defined, but not instantiated).

In order to fix your problem of removing the first tab page, you can just move your code below the InitializeComponent call:

public Form1()
        {
            InitializeComponent(); 
            TabPage removedTab = tabControl1.TabPages[1];
 
            tabControl1.TabPages.Remove(removedTab);

        }

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