Erro在tostring覆盖中使用tostring [英] Erro with useing tostring in tostring override

问题描述

您好，我是编码的新手，我在使用ToString覆盖时遇到问题。当我尝试在其他ToString覆盖中使用已经重载的类的ToString时，我遇到了这个错误：非静态需要对象引用字段，方法或属性Freight.ToString（）。请帮助!!!



我尝试过：

Hello, i am new in coding and i have a problem with using ToString override.When i try to use already overloaded class`s ToString in other ToString override I am hitting this error : "An object reference is required to non static field,method,or property Freight.ToString()".Please help!!!

What I have tried:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace TruckCmopany
{
    class Truck
    {
        private string name;
        private double weightCapacity;
        private List<Freight> freights;

        public Truck(string name,double weightCapacity)
        {
            this.Name = name;
            this.WeightCapacity = weightCapacity;
            List<Freight> freights = new List<Freight>();
        }
        public string Name
        {
            get { return name; }
            set { name = value; }
        }
        public double WeightCapacity
        {
            get { return weightCapacity; }
            set { weightCapacity = value; }
        }
        public override string ToString()
        {
            
            StringBuilder sb = new StringBuilder();
            sb.Append(this.Name).Append(" - ");
            if (freights.Count==0)
            {
                sb.Append("Nothing loaded");
            }
            else
            {
                sb.Append(string.Join(", ", freights)).Append(Freight.ToString());


            }
            return sb.ToString();
        }
        public IReadOnlyCollection<Freight> Freights
        {
            get => freights.AsReadOnly();
        }

        public void AddFreight(Freight freight)
        {

        }

    }
}

推荐答案

在 ToString（）代码名称或货运可以 null 所以检查一下这个案例并处理它。

In your ToString() code Name or freights can be null so check that case and handle it.

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!

private List<Freight> freights;

public Truck(string name,double weightCapacity)
{
    this.Name = name;
    this.WeightCapacity = weightCapacity;
    List<Freight> freights = new List<Freight>();
}

在此代码中，您定义了一个名为货运的私有类级变量，但是在构造函数中，您正在创建一个名为freights的第二个变量并为其分配一个List，但是该变量仅存在于构造函数中，类级freights仍为null。

In this code you are defining a private class-level variable called "freights", however in the constructor you are creating a second variable called freights and assigning a List to that, however that variable only exists within the constructor, the class-level "freights" is still null.

sb.Append(string.Join(", ", freights)).Append(Freight.ToString());

你只能在一个类型的实例上调用ToString;

You can only call "ToString" on an instance of a type;

Freight f = new Freight();
f.ToString(); // f is an instance of Freight so this is valid
Freight.ToString(); // Freight is a type, not an instance of a type so this is not valid

我怀疑你可以删除这个ToString和只有这个

I suspect you can just remove this ToString and have only this

sb.Append(string.Join(", ", freights));

这段代码要做的是遍历货运中的每个项目，并在其上调用ToString（）将该项目转换为字符串，然后用逗号分隔列表。只要你在Freight类上有一个ToString覆盖，那么该代码可能会得到你想要的东西。

What this code is going to do is go through each item in freights and call ToString() on it to convert that item to a string, and then comma separate the list. As long as you have a ToString override on the Freight class as well then that code will probably get you what you want.

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