ComboBox控件未显示任何项目

Question

我试图学习C＃的基础知识，并决定制作一个简单的Windows窗体来演示Dictionary类，但是当我启动程序时，尽管我向其中加载了一些数据，但Combo- / ListBox控件仍然为空。 希望你能帮我这个忙。

using System;
using System.Collections.Generic;
using System.Windows.Forms;

namespace ClassDictionaryExample
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        Dictionary<string, string[]> CountryList = new Dictionary<string, string[]>();

        private void Form1_Load(object sender, EventArgs e)
        {
            CountryList["Bulgaria"] = new string[] { "Sofia University St Kliment Ohridski", "Technical University of Sofia", 
                "Plovdiv University Paisii Hilendarski" };
            CountryList["Romania"] = new string[] { "Alexandru Ioan Cuza University", "Babes-Bolyai University", 
                "University of Bucharest" };
            CountryList["Serbia"] = new string[] { "University of Belgrade", "University of Novi Sad", "University of Niš" };

            foreach (var CountryKey in CountryList.Keys)
            {
                comboBoxCountry.Items.Add(CountryKey);
            }

            comboBoxCountry.SelectedIndex = 0;
        }

        private void comboBoxCountry_SelectedIndexChanged(object sender, EventArgs e)
        {
            string selectedCountry = comboBoxCountry.SelectedItem.ToString();

            if (comboBoxCountry.SelectedIndex == 0)
                listBoxUniversities.Items.Clear();
            else 
            {
                listBoxUniversities.Items.Clear();
                listBoxUniversities.Items.AddRange(CountryList[selectedCountry]);
            }
        }
    }
}

Answer 1

我试图重现您的问题，但没有。 您需要记住，必须为代码分配2个事件。 一个在FormLoaded上，另一个在ComboboxSelectionChanged上。

当所选索引为零时，我已经减少了您的代码以显示大学。

public Form1()
{
    InitializeComponent();

    this.Load += Form1_Load;
    comboBoxCountry.SelectedIndexChanged += comboBoxCountry_SelectedIndexChanged;
}

private Dictionary<string, string[]> _countryList;
public Dictionary<string, string[]> CountryList
{
    get
    {
        if (_countryList == null)
        {
            _countryList = new Dictionary<string, string[]>();
            _countryList["Bulgaria"] = new string[] { "Sofia University St Kliment Ohridski", "Technical University of Sofia", "Plovdiv University Paisii Hilendarski" };
            _countryList["Romania"] = new string[] { "Alexandru Ioan Cuza University", "Babes-Bolyai University", "University of Bucharest" };
            _countryList["Serbia"] = new string[] { "University of Belgrade", "University of Novi Sad", "University of Niš" };
        }

        return _countryList;
    }
}

private void Form1_Load(object sender, EventArgs e)
{
    foreach (var CountryKey in CountryList.Keys)
        comboBoxCountry.Items.Add(CountryKey);

    comboBoxCountry.SelectedIndex = 0;
}

private void comboBoxCountry_SelectedIndexChanged(object sender, EventArgs e)
{
    string selectedCountry = comboBoxCountry.SelectedItem.ToString();

    listBoxUniversities.Items.Clear();
    listBoxUniversities.Items.AddRange(CountryList[selectedCountry]);
}