如何修复未设置为对象实例的对象引用。 [英] How do I fix object reference not set to an instance of an object.

问题描述

我尝试从xml文件中读取数据，格式如下：

Im try to read data from xml file with format like this:

<npc_templates>	
	<npc_template npc_id="280001">
		<equipment>
			<item>100000005
			<item>100000006

我的尝试：



我尝试使用此代码读取设备/项目元素：

What I have tried:

And im trying with this code to read equipment/item elements:

public class NpcTemplate
    {
        [XmlAttribute("npc_id")]
        public int npc_id;
        
        [XmlElement("equipment")]
        public NpcEquippedGear equipment;
    }

和NpcEquippedGear类：

and NpcEquippedGear class:

public class NpcEquippedGear : IEnumerable<KeyValuePair<EquipmentSlot, ItemTemplate>>
    {
        [XmlElement("item")]
        public ItemTemplate[] items2;

        public Dictionary<EquipmentSlot, ItemTemplate> items;
        public int mask;
        public NpcEquippedGear()
        {
        }

        public void Add(object o)
        {

        }

        public int getItemsMask()
        {
            if (items == null)
                init();
            return mask;
        }

        public void init()
        {
            lock(this)
            {
                if (items == null)
                {
                    items = new Dictionary<EquipmentSlot, ItemTemplate>();
                    foreach (ItemTemplate item in items2)// Error Object reference not set to an instance of an object.
                    {
                        if (items[item.EquipmentSlot] == null)
                        {
                            items.Add(item.EquipmentSlot, item);
                            mask |= (int)item.EquipmentSlot;
                        }
                    }
                }
            }
        }

        public ItemTemplate GetItem(EquipmentSlot slot)
        {
            return items != null ? items[slot] : null;
        }

        public IEnumerator GetEnumerator()
        {
            return GetEnumerator();
        }

        IEnumerator<KeyValuePair<EquipmentSlot, ItemTemplate>> IEnumerable<KeyValuePair<EquipmentSlot, ItemTemplate>>.GetEnumerator()
        {
            return items.GetEnumerator();
        }
    }

我试试但是在运行程序时告诉mw错误

That all i try but while run program its tell mw error

Object reference not set to an instance of an object.

在NpcEquippedGear类foreach循环

at NpcEquippedGear class foreach loop

推荐答案

这是最常见的一个我们被问到的问题，也是我们最不能回答的问题，但你最有能力回答自己。



我来解释一下错误是什么意思：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。

它有点像一个口袋：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!

感谢你们的帮助。

我通过重新编码解决了我的问题NpcEquippedGear类

Thanks about your help guys.
Im resolved my issue by recode NpcEquippedGear class

public class NpcEquippedGear
    {
        [XmlElement("item")]
        public int[] items;
        private Dictionary<EquipmentSlot, ItemTemplate> dicItems;
        private int mask;
        public int getItemsMask()
        {
            if (dicItems == null)
                init();
            return mask;
        }
        private void init()
        {
            lock (this)
            {
                if (dicItems == null)
                {
                    dicItems = new Dictionary<EquipmentSlot, ItemTemplate>();
                    foreach (int itemId in items)
                    {
                        ItemTemplate item = Datastore.ItemsById[itemId];
                        if (!dicItems.ContainsKey(item.EquipmentSlot))
                        {
                            dicItems.Add(item.EquipmentSlot, item);
                            mask |= (int)item.EquipmentSlot;
                        }
                    }
                }
            }
        }
        public ItemTemplate GetItem(EquipmentSlot slot)
        {
            return dicItems != null ? dicItems[slot] : null;
        }
    }

目前你已经

Currently u've

foreach (ItemTemplate item in items2)// Error Object reference not set to an instance of an object.

只需这一行



if（items2！= null）



喜欢这个

just put this line

if(items2 !=null)

like this

if(items2 !=null)
foreach (ItemTemplate item in items2)// Error Object reference not set to an instance of an object.

它将解决你的null异常，但总是好的在其中初始化数组或列表构造函数或使用Initialize方法约定



Plz标记问题已解决!!!!

It will solve your null exception but it is always good to initialize Arrays or List within contructor or use Initialize method convention

Plz mark the issue resolved!!!!

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