如何在我的代码中处理空引用异常？ [英] How to handle null reference exception in my code?

问题描述

您好，



我想通过textbox textchange事件使用c＃.net wpf应用程序计算税或服务税。但我的代码在运行时遇到错误申请表或此表格; -

我的代码： -

Hello,

I want to calculate tax or service tax using c# .net wpf application through textbox textchange event.but my code is getting error when in run the application or this form;-
my code:-

private void txtServiceTax_TextChanged(object sender, TextChangedEventArgs e)
       {
           decimal st, total, nt;
           total = Convert.ToDecimal(txtTotal.Text); \\Error is getting this line
           st = Convert.ToDecimal(txtServiceTax.Text);
           nt = (total * st) / 100;
           txtNetPayBill.Text =Convert.ToString(nt);
       }

NullRefException ..在total = Convert.ToDecimal（txtTotal.Text）;中获取错误。 />


请帮帮我。



先谢谢。



Ankit Agarwal

软件工程师

NullRefException.. getting error in "total=Convert.ToDecimal(txtTotal.Text);".

Please help me.

Thanks in Advance.

Ankit Agarwal
Software Engineer

推荐答案

你指出的行中的null exception只有在 txtTotal 为空，这是可能的，但不容易相信。我建议你在调试器下仔细检查它。但是不要使用转换，使用 decimal.TryParse ，处理由于输入错误而导致解析失败的情况文本框。



每次遇到此异常时都不能提出这样的问题。自己处理这些基本问题很重要。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。但是如果下一个记录功能的条件对象引用未设置为对象的实例则会出错。



有时候，你不能这样做在调试器下，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 。



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 。



祝你好运，

-SA

The "null exception" in the line you pointed out could be thrown only if txtTotal is null, which is possible, but not so easy to believe. I would advise you to double check it under the debugger. But don't use Convert, use decimal.TryParse, handle the situations when the parsing fails because of wrong input in the text box.

You cannot ask such questions every time you have this exception. It's important to handle such basic problems by yourself. 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

可能输错了小数分隔符。检查您的文化信息中的数字格式。



或者使用TryParse：

Probably entering wrong decimal separator. Check number format in your culture info.

Or use TryParse:

decimal st, total, nt;
boolean converted;
converted = Decimal.TryParse(txtTotal.Text, total);
if (converted)
// do stuff)
else {
// some message box?
txtTotal.Focus();
return;
}

使用 Decimal.TryParse [ ^ ]



代码更改：

use Decimal.TryParse[^]

Code change:

decimal total,st;
 decimal.TryParse(txtTotal.Text, out total);
 decimal.TryParse( txtServiceTax.Text, out st);

为空异常，可能的情况是文本框被清除了某处

尝试使用下面的代码处理null，但是你的一些问题仅限代码，其中文本框已从DOM中删除

for null exception, possible cases are the textbox is cleared somewhere
try to handle null using below code, but some issue in your code only, where the textbox is removed from the DOM

if( txtTotal != null) {
           decimal total,st;
           decimal.TryParse(txtTotal.Text, out total);
           }

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