如何使用后台工作程序将数据从数据库加载到datagridview [英] how to use background worker to load data from database to datagridview

问题描述

我尝试从数据库导入数据，在加载时我需要在进度条中显示它，所以我添加了进度条和backgroundWorker但是我得到一个错误对象引用没有设置为对象的实例我不知道为什么。



我在这里添加了我的代码：

I have tried to Import Data From Database and while loading i need to show it in a progress bar so i added progress bar and backgroundWorker but I get an Error "Object reference no set to an instance of an object" I dont kno why.

I have added My code here:

Private Sub BackgroundWorker1_DoWork(sender As Object, e As DoWorkEventArgs) Handles BackgroundWorker1.DoWork

       Dim Obj As RetriveTableData = DirectCast(e.Argument, RetriveTableData)
       Dim SqlcmdString As String = "" 

       Dim DT As DataTable
       Dim i As Integer = 1
      Dim ds As DataSet
       Dim dr As DataRow
       Try
           Using conn As New SqlConnection(ConnString)
               Using Sqlcmd As New SqlCommand("Query", conn)
                   Sqlcmd.CommandType = CommandType.Text
                   Using adapter As New SqlDataAdapter(Sqlcmd)
                       Using ds As New DataSet
                           DT = ds.Tables(0)
                           If DT.Rows.Count > 0 Then - (Error After This Line)
                               For Each dr As DataRow In DT.Rows
                                    convert.toint32(adapter[""].ToString());
                                   Obj.sceneID = dr("ID").ToString()
                                   Obj.Quality = dr("Quality").ToString()
                                    adapter.Fill(DT)
                                   DataGridView1.DataSource = DT
                                    Thread.Sleep(100)
                                    To Report progress.
                                   BackgroundWorker1.ReportProgress(i, Obj)
                                   If BackgroundWorker1.CancellationPending Then
                                         Set the e.Cancel flag so that the WorkerCompleted event
                                         knows that the process was cancelled.
                                       e.Cancel = True
                                       BackgroundWorker1.ReportProgress(0)
                                       Return
                                   End If
                                   i += 1

                               Next
                           End If
                       End Using
                   End Using
               End Using
           End Using
       Catch ex As Exception
           MessageBox.Show(ex.Message)
       End Try
end sub

推荐答案

这是我们遇到的最常见问题之一，也是我们最不能回答的问题，但你最有能力回答自己。



让我解释一下错误的含义：你试图使用变量，属性或方法返回值，但它包含null - 这意味着那里变量中没有类的实例。

它有点像一个口袋：你的衬衫里有一个口袋，用来握笔。如果你进入口袋并发现那里没有笔，你就不能在一张纸上签名 - 如果你尝试的话，你会得到非常有趣的外观！空口袋给你一个空值（这里没有笔！）所以你不能做任何你检索笔通常做的事情。它为什么空？这就是问题 - 可能是你今天早上离开家时忘了拿起你的笔，或者你昨晚把它拿到昨天的衬衫口袋里时可能会把笔留下来。



我们无法分辨，因为我们不在那里，更重要的是，我们甚至看不到你的衬衫，更不用说口袋里的东西了！



回到计算机，你做了同样的事情，不知何故 - 我们看不到你的代码，更不用说运行它了，找不到包含null的东西。

但是你可以 - 而Visual Studio将在这里帮助你。在调试器中运行您的程序，当它失败时，VS将向您显示它发现问题的行。然后，您可以开始查看它的各个部分，以查看哪个值为null，并开始回顾代码以找出原因。因此，在包含错误行的方法的开头放置一个断点，然后从头再次运行程序。这一次，VS会在错误发生前停止，让你通过查看代码来查看你的价值来检查发生了什么。



但我们做不到那 - 我们没有您的代码，如果我们拥有它，我们不知道如何使用它，我们没有您的数据。所以试试吧 - 看看你能找到多少信息！

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterdays shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!

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