Combobox.SelectedIndex错误：对象引用未设置为对象的实例。 [英] Combobox.SelectedIndex Error: Object reference not set to an instance of an object.?

问题描述

****** My .xaml ***********

****** My .xaml ***********

<ComboBox Name="cbb_plus" Grid.Column="0" Margin="4px" Style="{StaticResource ResourceKey=ComboBoxFlatStyle}" SelectionChanged="cbb_plus_SelectionChanged_1" >
                                <ComboBox.Items>
                                <ComboBoxItem Content="+0" Height="24px" IsSelected="True"/>
                                <ComboBoxItem Content="+1" Height="24px"/>
                                <ComboBoxItem Content="+2" Height="24px"/>
                                </ComboBox.Items>
                            </ComboBox>
                            <ComboBox Name="cbb_level" Grid.Column="1" Margin="4px" Style="{StaticResource ResourceKey=ComboBoxFlatStyle}"  SelectionChanged="cbb_level_SelectionChanged_1" >
                                <ComboBox.Items>
                                <ComboBoxItem Content="Lv. 1" Height="24px" IsSelected="True"/>
                                <ComboBoxItem Content="Lv. 2" Height="24px"/>
                                <ComboBoxItem Content="Lv. 3" Height="24px"/>
                                </ComboBox.Items>
                            </ComboBox>

*********我的代码***********

********* My code ***********

private void cbb_plus_SelectionChanged_1(object sender, SelectionChangedEventArgs e)
        {
            int lv = cbb_level.SelectedIndex; // Error: Object reference not set to an instance of an object.
        }

请告诉我为什么？

Please tell me why?

推荐答案

您没有显示带有对象引用未设置为对象实例消息的异常位置。



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.

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

你的 cbb_level 当你的代码后面有一个名字 cbb_plus_SelectionChanged_1 。如果在创建cbb_level组合之前调用它，则可能导致您提到的错误。



如果cbb_plus_SelectionChanged_1是拼写错误，只需更改名称即可。如果没有，请将代码更改为：

Your cbb_level combobox is calling an event handler cbb_level_SelectionChanged_1 while your code behind has the name cbb_plus_SelectionChanged_1. If this is called before the cbb_level combo is created it could result in the error you mention.

If the cbb_plus_SelectionChanged_1 is a typo, just change the name. If not, change code behind to :

if(cbb_level != null) { int lv = cbb_level.SelectedIndex; }

希望这会有所帮助。



现在我看到了上面的组合是cbb_plus，所以只要确保cbb_level！= null，那应该没问题。[/ EDIT]

hope this helps.

Now I saw the combo above was cbb_plus, so just make sure the cbb_level != null and that should be fine.[/EDIT]

这篇关于Combobox.SelectedIndex错误：对象引用未设置为对象的实例。的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！