C# 初始化嵌套 classes 和外部 class 产生 Null 引用

Question

我有以下代码：

using System;
using System.Collections.Generic;

public class XTRAsystem
{
    public string version
    { get; set; }

    public XTRAapi api
    { get; set; }

    public XTRAsystem(string system_version)
    {
        this.version = system_version;
        XTRAapi api = new XTRAapi(Guid.NewGuid());
        Console.WriteLine("New XTRA system (" + this.version + ") initialized.");
    }

    public class XTRAapi
    {
        public Guid id
        { get; set; }

        public string name
        { get; set; }

        public XTRAapi(Guid Id)
        {
            this.id = Id;
            Console.WriteLine("New T24 API created.");
        }
    }
}

public class testCase
{
    public int caseNumber
    { get; set; }

    public List<string> data;

    public XTRAsystem refSystem
    { get; set; }

    public testCase()
    {
        data = new List<string>();
        Console.WriteLine("New Test Case created.");
    }
}

上面两个类嵌套是有原因的。现在，在我的 Main 程序中，下面的代码会产生 Null 引用异常。有人可以帮助我吗？

using System;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Start..");

            XTRAsystem mySystem = new XTRAsystem("Mickey");
            testCase[] myTest = new testCase[100];

            Console.WriteLine("Capacity is {0}", myTest.Length);

            //  'Object reference not set to an instance of an object' for all the below
            myTest[0].caseNumber = 1;

            myTest[0].data.Add("first test");
            myTest[0].data.Add("second test");
            myTest[0].data.Add("third test");

            myTest[0].refSystem = mySystem;
        }
    }
}

根据您的专业知识和经验，是否有任何其他方法可以生成现在会产生错误的功能（行 myTest[0]... 等）非常感谢

Answer 1

您必须先初始化每个数组项，然后才能使用属性

testCase[] myTest = new testCase[100];

Console.WriteLine("Capacity is {0}", myTest.Length);
myTest[0] = new testCase();
           
myTest[0].caseNumber = 1;

// also, initialize your list
myTest[0].data = new List<string>();

C# 初始化嵌套 classes 和外部 class 产生 Null 引用

C# Initializing nested classes and external class produces Null reference

c#

nested

compiler-errors

class