“对象引用未设置为对象的实例” [英] "Object reference not set to an instance of an object"

问题描述

我在设置全局对象的值时遇到问题。我收到对象引用未设置为对象的实例的错误。一旦frmMain（）运行，它就会创建组件，创建对象，然后从UI更新对象。当代码进入blowerObjectUpdate（）并点击

I am having a problem with setting a value of a global object. I am getting the error of "Object reference not set to an instance of an object". Once the frmMain() runs it creates the components, creates the object, and then updates the objects from the UI. I am getting the error when the code goes into the blowerObjectUpdate() and hits the

Global._Blower[i].Model 

异常时，我收到错误抛出这一点。



我把手表放在我的全局对象上，我可以看到对象正在被创建，但我可以设置它的值。请提供任何建议。

The exception is be thrown at this point.

I have put watches on my global object and I can see that the object is being created, but I can set its value. Please offer any suggestions.

using Globals;

namespace WetVacClient
{
   public partial class frmMain : Form
  {
public frmMain()
        {
            InitializeComponent();
            Global.InstantiateBlowerObj();
            blowerObjectUpdate();
        }
 public class Blower
       {
        public bool Valid;
        public string Model;
        public string Series;
        public string Brand;
        public double CFR;
        public double Slip_RPM;
        public double FHP;
        public int Max_RPM;
        public double Pmax;
        public int RPM;
        public double Inlet_Temp;
        public double Inlet_Pressure;
        public double Discharge_Temp;
        public double Discharge_Pressure;
        public double HP;
        public double Torque;
         }
public void blowerObjectUpdate ()     
        {
          for (int i = 1; i < 4; i++)
           int i = 1;
            { 
                switch (i)
                {
                    case 1:
                    case 2:
             Global._Blower[i].Model = comboboxBlower1Model.SelectedItem.ToString();
                    break;
                }
            }
        }
   }
}

namespace Globals
{
  
  
    public static class Global
    {
        public static Blower[] _Blower = new Blower[4];

        public static void InstantiateBlowerObj()
        {
            for (int i = 1; i < 4; i++)
                _Blower[i] = new Blower();
        }
    }
}

仅供参考我希望将对象数组初始化为1而不是0以对齐我的用户界面和制作统一。这不是问题，因为我在for循环中尝试使用i = 0.

推荐答案

如果抛出异常的行的位置是准确的，显然在 Global._Blower [i] .Model ， Global._Blower [i] 返回null一些我。你进一步尝试通过获取 .Model 来取消引用这个空对象，这会引发异常。





是的，我没注意 SelectedItem 也可以为null，效果相同。理解我在下面解释的原则更为重要。

[结束编辑]



这就是全部。那么该怎么办？修复它。

---

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

If your location of the line throwing the exception is accurate, it's apparent that in Global._Blower[i].Model, Global._Blower[i] returns null for some i. You further try to dereference this null object by getting .Model, which throws the exception.


And yes, I did not pay attention that SelectedItem could also be null, with the same effect. It's more important to understand the principles I explained below.
[END EDIT]

That's all. So, what to do? Fix it.

---

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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