從列表框 c# 中選擇參數時組合框不顯示項目

Question

我希望每次我從 ListBox 中選擇某些內容時，我的 ComboBox 都顯示一組參數，但它沒有在 ComboBox 中顯示任何內容。

這是我到目前為止...

    private void Form1_Load(object sender, EventArgs e)
    {
        listBox4.Items.Add("BE");
        listBox4.Items.Add("MBA");
        listBox4.Items.Add("Pharmacy");
    }

    private void listBox4_SelectedIndexChanged(object sender, EventArgs e)
    {

    }

    private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
    {
        if ((string)listBox4.SelectedItem == "BE")
        {

            comboBox1.Items.Add("CSE");
            comboBox1.Items.Add("IT");
            comboBox1.Items.Add("ME");
            comboBox1.Items.Add("EX");
            comboBox1.Items.Add("CE");
        }

        if ((string)listBox4.SelectedItem == "Pharmacy")
        {
            comboBox1.Items.Add("Pharmaceutical Chemistry");
            comboBox1.Items.Add("Pharmacology");
        }

        if ((string)listBox4.SelectedItem == "MBA")
        {
            comboBox1.Items.Add("Retail Management");
            comboBox1.Items.Add("HR");
        }
    }

這是輸出

Answer 1

您將代碼放在錯誤的事件中。

        // This is where your code belongs.
        private void listBox4_SelectedIndexChanged(object sender, EventArgs e)
        {
            if ((string)listBox4.SelectedItem == "BE")
            {

                comboBox1.Items.Add("CSE");
                comboBox1.Items.Add("IT");
                comboBox1.Items.Add("ME");
                comboBox1.Items.Add("EX");
                comboBox1.Items.Add("CE");
            }
            if ((string)listBox4.SelectedItem == "Pharmacy")
            {
                comboBox1.Items.Add("Pharmaceutical Chemistry");
                comboBox1.Items.Add("Pharmacology");
            }
            if ((string)listBox4.SelectedItem == "MBA")
            {
                comboBox1.Items.Add("Retail Management");
                comboBox1.Items.Add("HR");
            }
        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            // THIS WAS THE WRONG PLACE
        }   

Answer 2

1. 您應該為控件指定有意義的名稱，以便更容易確定來自何處的內容。

2. 每當填充 ComboBox 時，您應該先清除它。

3. 最重要的是，您正在檢查 ComboBox 而不是 ListBox 上的SelectedIndexChanged 。 如果向上移動會發生什么？

private void Form1_Load(object sender, EventArgs e)
{
    listBox4.Items.Add("BE");
    listBox4.Items.Add("MBA");
    listBox4.Items.Add("Pharmacy");
}

private void listBox4_SelectedIndexChanged(object sender, EventArgs e)
{
    if ((string)listBox4.SelectedItem == "BE")
    {

        comboBox1.Items.Add("CSE");
        comboBox1.Items.Add("IT");
        comboBox1.Items.Add("ME");
        comboBox1.Items.Add("EX");
        comboBox1.Items.Add("CE");
    }

    if ((string)listBox4.SelectedItem == "Pharmacy")
    {
        comboBox1.Items.Add("Pharmaceutical Chemistry");
        comboBox1.Items.Add("Pharmacology");
    }

    if ((string)listBox4.SelectedItem == "MBA")
    {
        comboBox1.Items.Add("Retail Management");
        comboBox1.Items.Add("HR");
    }
}

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
}

Answer 3

 private void Form1_Load(object sender, EventArgs e)
    {
        listBox4.Items.Add("BE");
        listBox4.Items.Add("MBA");
        listBox4.Items.Add("Pharmacy");
    }

    private void listBox4_SelectedIndexChanged(object sender, EventArgs e)
    {
comboBox1.Items.Clear();
if ((string)listBox4.SelectedItem == "BE")
        {

            comboBox1.Items.Add("CSE");
            comboBox1.Items.Add("IT");
            comboBox1.Items.Add("ME");
            comboBox1.Items.Add("EX");
            comboBox1.Items.Add("CE");
        }

        if ((string)listBox4.SelectedItem == "Pharmacy")
        {
            comboBox1.Items.Add("Pharmaceutical Chemistry");
            comboBox1.Items.Add("Pharmacology");
        }

        if ((string)listBox4.SelectedItem == "MBA")
        {
            comboBox1.Items.Add("Retail Management");
            comboBox1.Items.Add("HR");
        }
    }

    private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
    {

    }

Answer 4

好吧，您應該更新（刪除舊的並添加新的） comboBox1.Items上的listBox4更改：

   // Please, notice "listBox4"
   private void listBox4_SelectedIndexChanged(object sender, EventArgs e) {
     String selected = listBox4.SelectedItem as String;

     // we don't want blinking - too many re-draws
     combobox1.BeginUpdate();

     try { 
       //DONE: do not forget to remove old items
       combobox1.Items.Clear();

       if (selected == "BE") {
         combobox1.Items.AddRange("CSE", "IT", "ME", "EX", "CE");
       else if (selected == "Pharmacy") {
         combobox1.Items.AddRange("Pharmaceutical Chemistry", "Pharmacology");
       else if (selected == "MBA") 
         combobox1.Items.AddRange("Retail Management", "HR");  
     finally {
       combobox1.EndUpdate();
     }  
   }

而且似乎comboBox1沒有用，至少現在是：

 private void comboBox1_SelectedIndexChanged(object sender, EventArgs e) {
   //TODO: put here logic on comboBox1 change, e.g. on "Retail Management" selection
 }