c# - 继承和构造函数中的错误

Question

我目前正在研究一个涉及继承和构造函数的程序。我实际上是 C# 的新手，所以我只是想知道为什么下面的代码在第 57 行有错误，它说贷款不包含采用 0 个参数的构造函数。请帮我分析一下哪里出了问题。非常感谢。使用系统；

class DemoCarLoan3
{
    static void Main()
    {
        Loan aLoan = new Loan(333, "Hanson", 7000.00);
CarLoan aCarLoan = new CarLoan(444, "Carlisle", 30000.00, 2011, "BMW");
        Console.WriteLine("Loan #{0} for {1} is for Php{2}",
        aLoan.LoanNumber, aLoan.LastName,
        aLoan.LoanAmount.ToString("0,000.00"));
        Console.WriteLine("Loan #{0} for {1} is for Php{2}",
aCarLoan.LoanNumber, aCarLoan.LastName,
aCarLoan.LoanAmount.ToString("0.00"));
        Console.WriteLine(" Loan #{0} is for a {1} {2}",
        aCarLoan.LoanNumber, aCarLoan.Year,
        aCarLoan.Make);
        Console.ReadLine();
    }
}

class Loan
{
    public const double MINIMUM_LOAN = 5000;
    protected double loanAmount;
    public int LoanNumber { get; set; }
    public string LastName { get; set; }
    public double LoanAmount
    {
        set
        {
            if (value < MINIMUM_LOAN)
                loanAmount = MINIMUM_LOAN;
            else
                loanAmount = value;
        }
        get
        {
            return loanAmount;
        }
    }

    public Loan(int num, string name, double amount)
    {
        LoanNumber = num;
        LastName = name;
        LoanAmount = amount;
    }
}

class CarLoan : Loan
{
    private const int EARLIEST_YEAR = 2006;
    private const int LOWEST_INVALID_NUM = 1000;
    private int year;

    public CarLoan(int num, string name, double amount, int year, string make)
    {
        Year = year;
        Make = make;
    }

    public int Year
    {
        set
        {
            if (value < EARLIEST_YEAR)
            {
                year = value;
                loanAmount = 0;
            }
            else
                year = value;
        }
        get
        {
            return year;
        }
    }

    public string Make { get; set; }

    public new int LoanNumber
    {
        get
        {
            return base.LoanNumber;
        }
        set
        {
            if (value < LOWEST_INVALID_NUM)
                base.LoanNumber = value;
            else
                base.LoanNumber = value % LOWEST_INVALID_NUM;
        }
    }
}

Answer 1

只需添加对基本构造函数的调用

public CarLoan(int num, string name, double amount, int year, string make) 
    : base(num, name, amount)
{
    Year = year;
    Make = make;
}

当您在不调用基类构造函数的派生类中声明构造函数时，它会自动调用那里不存在的具有 0 个参数的构造函数。如果您按原样使用代码，它将编译为

public CarLoan(int num, string name, double amount, int year, string make) 
    : base()
{
    Year = year;
    Make = make;
}

但这是无效的，因为基类中没有参数为 0 的构造函数。因此，要么创建一个，要么调用现有的一个。