在asp.net编码中需要帮助 [英] need help in asp.net coding

问题描述

我在gridview中有两个下拉列表和标签。如何将数据绑定到gridview内的第一个下拉列表，并获取dropdown1_selectedindexchanged中的secound下拉列表中的值，该值也在gridview中？

如何在gridview内的标签中获取自动生成值？



i收到以下代码的错误

I have two dropdownlists and lable in my gridview. how bind data to first dropdownlist inside the gridview and get the value in secound dropdownlist on dropdown1_selectedindexchanged which is also in gridview?
how to get autogenerate value in lable inside the gridview?

i was getting an error for below code

public void autogenerateidingrid()
    {

        lbl = (Label)GridView1.FooterRow.FindControl("lblsrno");
        str7 = this.GridView1.Rows.Count.ToString();
        i = Convert.ToInt32(str7) + 1;
            lbl.Text = i.ToString();

    }

nullreferenceexception未被用户代码处理

对象引用未设置为一个对象的实例。

请对此有任何帮助........

nullreferenceexception unhandled by user code
Object reference not set to an instance of an object.
please any one help on this........

推荐答案

你有一个一堆问题。



两个最明显的，是 FindControl 可以返回 null （VB.NET中 Nothing ）如果找不到您的控件。这将导致您的 NullReferenceException 。



其次，您从GridView获取行数，转换它到一个字符串，然后立即将其转换回整数。为什么？您不需要将整数转换为字符串并返回整数！



此外，如果GridView中有空行怎么办？这些将被计算在内，除非那是你想要的。您计算绑定到网格的数据源中的行，而不是网格中的行。

You're got a bunch of problems with this.

The two most obvious, are that FindControl can return null (Nothing in VB.NET) if it can't find your control. This will cause your NullReferenceException.

Second, you're getting the count of rows from the GridView, converting it to a string and then immediately converting it back to an integer. WHY? You don't need to convert an integer to a string and back to an integer!

Also, what if there are blank rows in the GridView?? Those will be counted, unless that's what you want. You count the row in the data source you bound to the grid, not the rows in the grid.

您没有显示带有对象引用未设置为实例消息的异常的位置一个对象被抛出。解决方案1显示了它可能发生的位置。



不用担心：这是最容易检测和修复的情况之一。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

You did not show where the exception with the message "Object reference not set to an instance of an object" is thrown. Solution 1 shows you where it might happen.

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

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