C#Visual Basic继承

Question

我刚刚开始使用C#,我已经坚持了两个星期这个问题.我有一个主窗体,从类和子类获取值.我的问题是当我尝试创建一个CorporateClass的对象时,VB告诉我我的两个变量(CarSizeInteger和DiscountInteger)是未分配的.我的问题是为什么.我在程序的早期实现了它们.救命!我绝望地被卡住了!

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;


namespace EX02_CarRentals
{
    public partial class RentalForm : Form
    {
        public RentalForm()
        {
            InitializeComponent();
        }

        private void CloseButton_Click(object sender, EventArgs e)
        {
            Close();
        }

        private void CalculateButton_Click(object sender, EventArgs e)
        {
            int DaysInteger, BeginningOdometerInteger, EndingOdometerInteger, CarSizeInteger, DiscountInteger;

            if (LicenseTextBox.Text != "")

                if (CompactRadioButton.Checked || MidSizeRadioButton.Checked || LuxuryRadioButton.Checked)
                {
                    int.TryParse(DaysRentedTextBox.Text, out DaysInteger);

                    int.TryParse(BeginningOdometerTextBox.Text, out BeginningOdometerInteger);

                    if (BeginningOdometerInteger > 0)
                    {
                        int.TryParse(EndingOdometerTextBox.Text, out EndingOdometerInteger);
                        if (EndingOdometerInteger > 0)
                        {
                            if (CompactRadioButton.Checked)
                                CarSizeInteger = (int)CarSize.Compact;

                            else if (MidSizeRadioButton.Checked)
                                CarSizeInteger = (int)CarSize.MidSize;

                            else CarSizeInteger = (int)CarSize.Luxury;
                        }
                        {
                            if (CorporateRadioButton.Checked || InsuranceRadioButton.Checked)
                            {
                                if (CorporateRadioButton.Checked)
                                    DiscountInteger = (int)Discount.Corporate;

                                else if (InsuranceRadioButton.Checked)
                                    DiscountInteger = (int)Discount.Insurance;

                                //create an instance of the Corporate Class
                                CorporateClass aCorpRental = new CorporateClass(BeginningOdometerInteger, EndingOdometerInteger, CarSizeInteger, DaysInteger, DiscountInteger);
                                AmountDueTextBox.Text = (aCorpRental.getAmountDue()).ToString("C");
                            }
                            else
                            {
                                //create an instance of the Rental Class

                                RentalRate ARental = new RentalRate(BeginningOdometerInteger, EndingOdometerInteger, CarSizeInteger, DaysInteger);
                                AmountDueTextBox.Text = (ARental.getAmountDue()).ToString("C");
                            }
                        }
                    }
                }
            }
        }

        private void DaysRentedTextBox_TextChanged(object sender, EventArgs e)
        {

        }

    }
}

Answer 1

在C#中,您不能在给定值之前使用变量的值,否则您将获得未分配的变量错误.让我们看看为什么你的代码会给你这个.

我会专注于CarSizeInteger.您尝试初始化的位置有三个CarSizeInteger:

if (EndingOdometerInteger > 0)
{
    if (CompactRadioButton.Checked)
        CarSizeInteger = (int)CarSize.Compact;
    else if (MidSizeRadioButton.Checked)
        CarSizeInteger = (int)CarSize.MidSize;
    else CarSizeInteger = (int)CarSize.Luxury;
}

请注意,这三个任务都包含在if (EndingOdometerInteger > 0)块中.如果EndingOdometerInteger > 0是假的,则不会发生任何分配.因此,当您尝试稍后使用时CarSizeInteger,编译器会检测CarSizeInteger到可能未初始化,这是一个错误.

要避免此错误,您可以提供CarSizeInteger默认值:

int CarSizeInteger = 0;

或者在if语句中添加一个else块来初始化CarSizeInteger:

if (EndingOdometerInteger > 0)
{
    // ...
}
else
{
    CarSizeInteger = 0;
}

在任何情况下,只需确保在尝试读取其值之前为每个变量赋值.