我不知道为什么我得到这个错误系统.nullreferenceexception：对象引用未设置为对象的实例。 [英] I don't know why I am getting this error system.nullreferenceexception: object reference not set to an instance of an object.?

问题描述

您好，



我正在尝试对我的代码进行测试，我得到一个像这样的System.NullReferenceException异常：对象引用未设置为实例一个对象。



这是我的代码：

Hello,

I am trying to rule a test on my code and I get an exception like this System.NullReferenceException: Object reference not set to an instance of an object.

Here is my code :

public class Class1
    {
        XLDriver CANDemo = new XLDriver();
        XLClass.xl_driver_config driverConfig = new XLClass.xl_driver_config();

        public int GetDescription()
        {
            XLDefine.XL_Status status;
            status = CANDemo.XL_OpenDriver();
            Console.WriteLine("Open Driver       : " + status);
            //c.GetDescription();
            status = CANDemo.XL_GetDriverConfig(ref driverConfig);
            Console.WriteLine("Get Driver Config : " + status);
            return 0;
        }
    }

我的测试代码：

And my test code :

[TestClass]
public class UnitTest1
{
    static CANInterface.Class1 can;
    [ClassInitialize]
    public static void InitClass(TestContext context)
    {
        can = new CANInterface.Class1();
    }

    [TestMethod]
    public void TestMethod1()
    {
        can.GetDescription();
    }
}

我的GetDescription方法中的方法是从dll导入的。当我运行测试代码时，我得到了该错误，但是来自XLDriver对象，我不知道为什么。 XLDriver类也来自dll。我该如何解决这个问题？



谢谢。



我尝试过：



我试图将XLDriver对象放在方法中，但它没有用。

The methods from my GetDescription method are imported from a dll. And when I run the test code I get that error but from the XLDriver object and I don't know why. The XLDriver class is also from the dll. How can I resolve this problem?

Thank You.

What I have tried:

I tried to put the XLDriver object also in the method but it didn't worked.

推荐答案

这是我们遇到的最常见问题之一，也是我们最不能回答的问题，但你最有能力回答自己。



让我解释一下错误的含义：您尝试使用变量，属性或方法返回值，但它包含null - 这意味着变量中没有类的实例。

这有点像一个口袋：你的衬衫里有一个口袋，用来握笔。如果你进入口袋并发现那里没有笔，你就不能在一张纸上签名 - 如果你尝试的话，你会得到非常有趣的外观！空口袋给你一个空值（这里没有笔！）所以你不能做任何你检索笔通常做的事情。它为什么空？这就是问题 - 可能是你今天早上离开家时忘了拿起你的笔，或者你昨晚把它拿到昨天的衬衫口袋里时可能会把笔留下来。



我们无法分辨，因为我们不在那里，更重要的是，我们甚至看不到你的衬衫，更不用说口袋里的东西了！



回到计算机，你做了同样的事情，不知何故 - 我们看不到你的代码，更不用说运行它了，找不到包含null的东西。

但是你可以 - 而Visual Studio将在这里帮助你。在调试器中运行您的程序，当它失败时，VS将向您显示它发现问题的行。然后，您可以开始查看它的各个部分，以查看哪个值为null，并开始回顾代码以找出原因。因此，在包含错误行的方法的开头放置一个断点，然后从头再次运行程序。这一次，VS会在错误发生前停止，让你通过查看代码来查看你的价值来检查发生了什么。



但我们做不到那 - 我们没有您的代码，如果我们拥有它，我们不知道如何使用它，我们没有您的数据。所以尝试一下 - 看看你能找到多少信息！

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterdays shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!

总的来说，这是一个非常糟糕的代码： PrintFunctionError 需要用户与控制台交互。只要在单元测试中调用该函数（或在Web服务器，Windows服务等上运行），它就会挂起。



返回值是并不意味着表明是否发生了异常。



变量/成员命名错误，起初我认为这个东西无法编译因为我没注意到 driverConfig 是一个成员变量并在实例化时初始化。



最后，这些函数不能进行单元测试：一个单元test 断言什么，可以在这里声明什么？那东西写在控制台上？

Overall, that's a terribly bad code: PrintFunctionError requires a user to interact with the console. Whenever that function will be called within a Unit Test (or run on a Web Server, a Windows Service etc.), it will just hang.

A return value is not meant to indicate if an exception happened or not.

Variables / members are badly named, at first I thought that thing can't compile because I failed to notice that driverConfig is a member variable and initialized at instantiation.

Finally, these functions cannot be unit tested: a unit test asserts something, and what can be asserted here? That something was written onto the console?

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