我如何在c＃中操作listBox操作 [英] How do I operate listBox operations in c#

问题描述

大家好，



我对listBoxes有疑问。在我的项目中有两个listBox。我在这些ListBox上更新，添加，删除，清理操作。其中一个没有问题，但另一个给出，

PresentationFramework.dll中出现未处理的类型'System.NullReferenceException'异常

附加信息：对象引用不是设置为一个对象的实例



错误，使用相同的代码。



例如当我想要的时候清除listBox中的所有项目，我这样做;



lboxCity.Items.Clear（）;并且它给出了上面提到的错误。

当我调试代码时，我认识到代码lboxCity.Items.Clear（）;

编译器去

private void lboxCity_SelectionChanged（对象发送者，SelectionChangedEventArgs e）{}

做一个里面的操作。



我也在运行listBox时尝试了相同的调试。

代码之后

lboxRegion.Items.Clear （）;编译器不运行函数

private void lboxRegion_SelectionChanged（object sender，SelectionChangedEventArgs e）



这些只是我的观察，我不确定问题在哪里。



如果不清楚请注意



摘要，同时操作列表框操作，它给出了nullReferencesException。

Hi everyone,

I have a problem about listBoxes. In my project there are two listBoxes. I do update, add,delete,clean operations on these ListBoxes. There is no problem with one of them but the other gives,
An unhandled exception of type 'System.NullReferenceException' occurred in PresentationFramework.dll
Additional information: Object reference not set to an instance of an object

error, with same codes.

For example when i want to clear all items in listBox, i do ;

lboxCity.Items.Clear(); and it gives the error mentioned above.
When i debug the code i recognize that after the code lboxCity.Items.Clear();
compiler goes
private void lboxCity_SelectionChanged(object sender, SelectionChangedEventArgs e){}
an do the operations inside.

Also I tried the same debug in running listBox.
After the code
lboxRegion.Items.Clear(); compiler does not run the function
private void lboxRegion_SelectionChanged(object sender, SelectionChangedEventArgs e)

These are just my observation, i am not sure where is the problem.

If it is not clear please note

Summary, while operating listbox operations, it gives nullReferencesException.

推荐答案

对于lboxCity.Items.Clear（）;使用空引用错误失败的方法意味着没有创建lboxCity，或者您已经删除了控件，或者您已经为名称'lboxCity指定了其他内容，例如'null。



在执行lboxCity.Items.Clear（）的行上的代码中放置一个断点。当应用程序停在那里时，通过将鼠标悬停在它上来检查lboxCity的值。



第二个错误：一旦清除了ListBox的Items集合，为什么如果SelectionChanged事件发生了：没有什么可以选择的。

For the lboxCity.Items.Clear(); method to fail with a null reference error means lboxCity was not created, or you have done something to delete the Control, or you have assigned something else, like 'null, to the name 'lboxCity.

Put a break-point in your code on the line where you execute lboxCity.Items.Clear(); and when the application stops there, examine the value of lboxCity by hovering the mouse over it.

The second error: once you have cleared the Items collection of a ListBox why should the SelectionChanged Event ever fire: there's nothing there to select.

你好，



没有人可以克服问题。我很迷茫。而代码

lboxCity.Items.Clear（）;执行系统给出nullreferencesexception。

请注意，在执行代码之前，lBoxCity已经填满。



谢谢

Hi there,

Is not there anyone that can overcome the problem. I am very confused. While the code
lboxCity.Items.Clear(); is executed system give nullreferencesexception.
and please note that, just before executing the the code lBoxCity is filled.

Thanks

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