如何在dataGridView1_SelectionChanged事件中处理null异常 [英] how to handle null exception in dataGridView1_SelectionChanged event

问题描述

嗨朋友我在dataGridView1_SelectionChanged事件中收到以下错误。

第一个选择它的工作原理如果我更改选择我得到错误



System.NullReferenceException未处理

Message =对象引用未设置为对象的实例。



我的代码是如下：请纠正我错误的地方



<前lang =c＃> 私人 < span class =code-keyword> void dataGridView1_SelectionChanged（ object sender，EventArgs e）
{
int rowindex;
// MessageBox.Show（dataGridView1.CurrentRow.Index.ToString（））;
rowindex = dataGridView1.CurrentRow.Index; // 此处出错
if （rowindex > = 0 ）
{
DataGridViewRow row = this .dataGridView1.Rows [rowindex];
txtpaX.Text = row.Cells [ X]。Value.ToString（） ;
txtpaY.Text = row.Cells [ Y]。Value.ToString（） ;
lblinfo.Text = row.Cells [ item]。Value.ToString（） ;

xposition = int .Parse（txtpaX.Text）;
yposition = int .Parse（txtpaY.Text）;
flag = 1 ;
}
}

解决方案

如果您遇到System.NullReferenceException未处理消息的异常，这并不意味着您需要处理此异常。在这种情况下，很难发明更糟糕的想法。你不应该允许这个例外。



很难理解当你有针对性的异常来源时你怎么能忽略解决方案。 dataGridView1.CurrentRow 是否为空？因此，只需检查一下：

  var  currentRow = dataGridView1.CurrentRow; 
  if （currentRow ==  null ）
 {
  //  使用currentRow做一些事情 
} 
  else  
 {
  //  做其他事情 
} 

这是您需要使用调试器自行解决，检测和修复的最简单问题之一。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 [ ^ ]。





此外，永远不要使用自动生成的名称，如 dataGridView1 。它们违反（良好）Microsoft命名约定，并且不会缩进以供永久使用。总是将它们重命名（你给了一个重构引擎）给一些语义敏感的名字。

-SA

Hi friends i am getting the following error in dataGridView1_SelectionChanged event.
for the first selection its works and if i change the selection i get the error

System.NullReferenceException was unhandled
Message=Object reference not set to an instance of an object.

My code is as follows.please correct me where i made mistake

private void dataGridView1_SelectionChanged(object sender, EventArgs e)
        {
            int rowindex;
           // MessageBox.Show(dataGridView1.CurrentRow.Index.ToString());
            rowindex = dataGridView1.CurrentRow.Index;//error here           
            if (rowindex >= 0)
            {
                DataGridViewRow row = this.dataGridView1.Rows[rowindex];
                txtpaX.Text = row.Cells["X"].Value.ToString();
                txtpaY.Text = row.Cells["Y"].Value.ToString();
                lblinfo.Text = row.Cells["item"].Value.ToString();

                xposition = int.Parse(txtpaX.Text);
                yposition = int.Parse(txtpaY.Text);
                flag = 1;
            }
        }

解决方案

If you have an exception with the message "System.NullReferenceException was unhandled", it does not mean that you need to handle this exception. It's hard to invent worse idea, in this case. You should not allow this exception.

It's hard to understand how could you overlook the solution when you have the source of exception pin-pointed. Isn't it obvious that dataGridView1.CurrentRow was null? Therefore, just check it up:

var currentRow = dataGridView1.CurrentRow;
if (currentRow == null)
{
   // do something using currentRow
}
else
{
  // do something else
}

This is one of the simplest problems you need to be able to solve by yourself using the debugger, 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[^].


Also, never use the auto-generated names like dataGridView1. They violate (good) Microsoft naming conventions and are not indented for permanent use. Always rename them (you have given a refactoring engine) to some semantically sensible names.

—SA

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