为什么我在这里得到空引用？ [英] Why am i getting a nullreference here?

问题描述

我在这部分代码中得到一个空引用...

I am getting a null reference in this part of the code...

dr["Column" + counter.ToString()] = cell.Value.ToString();

metrogrid就像datagridview一样



我尝试过：

the metrogrid is just like datagridview

What I have tried:

DataTable dt = new DataTable();
            for (int i = 0; i <= metroGrid1.Columns.Count; i++)
            {
                dt.Columns.Add("Column" + i.ToString() );
            }
foreach (DataGridViewRow row in metroGrid1.Rows)
            {
                DataRow dr = dt.NewRow();
                int counter = 0;
                foreach (DataGridViewCell cell in row.Cells)
                {
                        dr["Column" + counter.ToString()] = cell.Value.ToString();
                        counter++;
                        
                }
                dt.Rows.Add(dr);
            }

现在我注意到datagridview从原始数据中添加了1个数据因此，如果您的原始数据是50，当您将其传输到datagridview时变为51.如何防止它读取添加的数据因为它会抛出空错误？谢谢！

now i notice that the datagridview add a 1 data from your original data so if your original data is 50, when you transfer it to datagridview is became 51. how can i prevent it from reading the added data cos it throws a null error? thanks!

推荐答案

好吧..所以额外的行来自新行，DataGridView显示供用户添加新行。



只需设置 DataGridView.AllowUserToAddRows属性 [ ^ ]到 False 并且该行消失了..

如果您需要，用户可以添加行..

您可以使用 DataGridViewRow.IsNewRow属性（System.Windows.Forms） [ ^ ]



作为补充..看看这个c# - 如何在没有任何数据源的情况下从DataGridView创建DataTable？ - 堆栈溢出 [ ^ ]通过索引访问列肯定更快，因为使用您使用的串联字符串ColumnName进行搜索。



和秒。就像谢尔盖已经提到的那样，你需要检查 cell.value 是否为空！我很确定它可以，你只会在那里得到你的下一个Null Exception !!!

okay.. so that extra row is from the new line, the DataGridView shows for users to add a new line.

just set the DataGridView.AllowUserToAddRows Property[^] to False and that line is gone..
if you need that users can add rows..
you can check if the row you are reading is just an empty new line with the DataGridViewRow.IsNewRow Property (System.Windows.Forms)[^]

as addition .. have a look at this c# - How to make a DataTable from DataGridView without any Datasource? - Stack Overflow[^] accessing the columns by index is sure faster as searching it with that concatenated string ColumnName you use.

and second. Like Sergey already mentioned, you need to check if cell.value is null! I'm pretty sure it can be and you just will get your next Null Exception there!!!

请看我对这个问题的评论。列出可能为null的内容真是太无聊了。它可能是 dr [Column+ counter.ToString（）] ，几乎 cell ，但是 cell.Value 可能再次为空。你必须简单地检查一下，找到问题的根源。每次这样的事情发生时你都不能问这样的问题，但是真的必须学会自己处理这些非常基本的情况。



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/变量通过使用及其实例（非静态）成员进行解引用，这需要此成员/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。但是如果下一个记录功能的条件对象引用未设置为对象的实例则会出错。



有时候，你不能这样做在调试器下，由一个或另一个原因。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 。



祝你好运，

-SA

Please see my comments to the question. It's just boring to list what potentially could be null. It could be dr["Column" + counter.ToString()], hardly cell, but cell.Value could again be null. You had to simply check it all, to find the root of the problem. You cannot ask such question every time such things happen, but really have to learn to deal with such very basic situations all by 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 dereferences 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

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