将headrow动态插入GridView时出错 [英] Error in dynamically inserting headrow into GridView

问题描述

我试图将headrow动态插入GridView（名为gv）。相关代码如下：

I tried to dynamically insert the headrow into a GridView (named gv). The related code is below:

<pre lang="c#">
   GridViewRow headerRow = new GridViewRow(1, 0, DataControlRowType.Header, DataControlRowState.Insert);
   Add_Cells();   // return a GridViewRow w/ cell values
   headerRow.Attributes.Add("class", "header");
   gv.Controls[0].Controls.AddAt(0, headerRow);          // #1, Error 1
   // gv.HeaderRow.Parent.Controls.AddAt(0, headerRow);  // #2, Error 2
</pre>

如果我使用＃1代码，错误1是

If I use #1 code, Error 1 is

[ArgumentOutOfRangeException: Specified argument was out of the range of valid values.  Parameter name: index]

如果我使用＃2代码，错误2是

If I use #2 code, Error 2 is

[NullReferenceException: Object reference not set to an instance of an object.]

我的代码有什么问题？谢谢，如果你能指出的话。

What is wrong in my code? Thanks if you can point out.

推荐答案

＃1： gv.Controls 不为空，但元素数为零，因此 gv.Controls [0] 将索引不存在的元素。您需要在访问任何元素之前检查计数。

＃2：从上一行开始，我们知道 gv 不为空。但是 gv.HeaderRow 可以为null。如果没有，链中的下一个属性可以为null（我没有检查它是否可能），依此类推。您需要在解除引用之前检查它，使用调试器。

#1: gv.Controls is not null, but the element count is zero, so gv.Controls[0] would index non-existing element. You need to check the count before accessing any element.
#2: From the previous line, we know that gv is not null. But gv.HeaderRow can be null. If not, next property in chain can be null (I did not check it it's possible), and so on. You need to check it before dereferencing, use the debugger.

您遇到了两个最容易检测和修复的异常。每次遇到这些问题时都不能问这些问题，了解如何自己处理这些问题非常重要。

You faced two different exceptions which are the easiest to detect and fix. You cannot ask such questions every time you face such problems, it's very important to learn how to deal with them all by yourself.

首先，让我们看一下null异常。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。在弄清楚之后，修复代码：要么确保成员/变量被正确初始化为非空引用，要么检查它是否为null，如果为null，则执行其他操作。

First, let's look at the null exception. 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.

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没有设置为对象的实例则会出错。

有时，你不能在调试器，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 。

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.

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 。

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.

现在，超出范围异常。所有支持该属性的集合 Count 提供从 0 到的索引对元素的访问权限-1 。在通过索引访问任何元素之前，您可能不知道此索引处的元素是否存在，因此应在运行时检查它。

Now, out of range exception. All collections which support the property Count provide access to the elements by index from 0 to Count−1. Before accessing any element by index, you may not know that the element at this index exists, so should check it up during runtime.

祝你好运，

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