回答編集履歴

  コンボボックスの順番が分かりにくかったので変えています。

  comboBox1 comboBox5 comboBox6

  comboBox2

  comboBox3

  comboBox4

 

 

 

  using System;

  using System.Collections.Generic;

  using System.Data;

  using System.Linq;

  using System.Windows.Forms;

 

  namespace Questions230267

  {

  public partial class Form1 : Form

  {

  private readonly ComboBox[] itemComboBoxes;

  private readonly Dictionary<string, string[]> itemDict = new Dictionary<string, string[]>

  {

  { "項目１", new string[]{ "項目１の値", "1", "2", "3", "4" } },

  { "項目２", new string[]{ "項目２の値", "5", "6", "7", "8" } },

  { "項目３", new string[]{ "項目３の値", "9", "10", "11", "12" } },

  };

 

 

  public Form1()

  {

  InitializeComponent();

 

  // デザイナで指定してよい ここから

  comboBox1.Name = "コンボ１";

  comboBox2.Name = "コンボ２";

  comboBox3.Name = "コンボ３";

  comboBox4.Name = "コンボ４";

  comboBox1.TextChanged += ComboBox_TextChanged;

  comboBox2.TextChanged += ComboBox_TextChanged;

  comboBox3.TextChanged += ComboBox_TextChanged;

  comboBox4.TextChanged += ComboBox_TextChanged;

  comboBox5.TextChanged += ComboBox_TextChanged;

  comboBox6.DropDownStyle = ComboBoxStyle.DropDownList;

  // デザイナで指定してよい ここまで

 

 

  itemComboBoxes = new ComboBox[] { comboBox1, comboBox2, comboBox3, comboBox4 };

 

  var strsSource = itemDict.Keys.ToArray();

  foreach(var combo in itemComboBoxes)

  {

  combo.Items.Add(combo.Name);

  combo.Items.AddRange(strsSource);

  combo.SelectedIndex = 0;

  }

 

  comboBox5.Items.AddRange(itemComboBoxes.Select(x => x.Name).ToArray());

  }

 

  private void ComboBox_TextChanged(object sender, EventArgs e)

  {

  comboBox6.Items.Clear();

  var combo = itemComboBoxes.FirstOrDefault(x => x.Name == comboBox5.Text) as ComboBox;

  var item = combo?.Text;

  if(!string.IsNullOrEmpty(item) && itemDict.ContainsKey(item))

  {

  comboBox6.Items.AddRange(itemDict[item]);

  comboBox6.SelectedIndex = 0;

  }

  }

  }

  }

  ```

 

  文字を入力できるようにする（`DropDownStyle = ComboBoxStyle.DropDown`）のは必須条件でしょうか？（使い分けてるようなのでそうなんでしょうが）