将通用列表项添加到Winforms的列表框C＃

Question

Hi Im trying to add the items to a generic list. Then on a button click the generic list items should be transferred to my listbox1. This works however, it keeps adding them every time I click the button.

I tried using an if command, sometime like this

 if (listBox1.Items.Count == 0)
 {
     // this contained everything I am now adding under initialize component)
 }

But this generated the error. index value can not be 0. So in essence this works as is. But I do not want keep adding the items every time I click.

public Form1()
{
    InitializeComponent();
    list.Add("Januari");
    list.Add("Februari");
    list.Add("March");
    list.Add("April");
    list.Add("May");
    list.Add("June");
    list.Add("July");
    list.Add("August");
    list.Add("September");
    list.Add("Oktober");
    list.Add("November");
    list.Add("December");
}

private IList<string> list = new List<string>();

private void button1_Click(object sender, EventArgs e)
{
    for (int index = 0; index < list.Count; index++)
    {
        listBox1.Items.Add(list[index]);
    }

    int mnr = Convert.ToInt32(textBox1.Text);
    string mnm = Convert.ToString(listBox1.Items[mnr - 1]);
    textBox2.Text = mnm;      
}

Answer 1

This is just an alternative to @Kami's answer, if you don't use the IList<string> list anywhere else, then just add the items directly to the listbox:

public Form1()
{
    InitializeComponent();

    // This is only called once.
    InitializeListBox();
}

private void InitializeListBox()
{
    listBox1.Items.Add("Januari");
    listBox1.Items.Add("Februari");
    listBox1.Items.Add("March");
    listBox1.Items.Add("April");
    listBox1.Items.Add("May");
    listBox1.Items.Add("June");
    listBox1.Items.Add("July");
    listBox1.Items.Add("August");
    listBox1.Items.Add("September");
    listBox1.Items.Add("Oktober");
    listBox1.Items.Add("November");
    listBox1.Items.Add("December");
}

private void button1_Click(object sender, EventArgs e)
{
    int mnr = Convert.ToInt32(textBox1.Text);
    string mnm = Convert.ToString(listBox1.Items[mnr - 1]);
    textBox2.Text = mnm;
}

Answer 2

There are two things you can do here.

Firstly option is to add a call to clear() before you load the list. This will remove the existing items and reinsert them.

    private void button1_Click(object sender, EventArgs e)
    {
        listBox1.Items.Clear();

        for (int index = 0; index < list.Count; index++)
        {

            listBox1.Items.Add(list[index]);
        }

        int mnr = Convert.ToInt32(textBox1.Text);
        string mnm = Convert.ToString(listBox1.Items[mnr - 1]);
        textBox2.Text = mnm;
    }

Or alternatively, and probably the better solution is to move the loading into an init function. As the data only needs to be loaded once per form, this is the better solution.

    public Form1()
    {
        InitializeComponent();
        Init();
    }

    private void Init()
    {
        list.Add("Januari");
        list.Add("Februari");
        list.Add("March");
        list.Add("April");
        list.Add("May");
        list.Add("June");
        list.Add("July");
        list.Add("August");
        list.Add("September");
        list.Add("Oktober");
        list.Add("November");
        list.Add("December");

        listBox1.Items.Clear();

        for (int index = 0; index < list.Count; index++)
        {

            listBox1.Items.Add(list[index]);
        }
    }

    private IList<string> list = new List<string>();

    private void button1_Click(object sender, EventArgs e)
    {
        int mnr = Convert.ToInt32(textBox1.Text);
        string mnm = Convert.ToString(listBox1.Items[mnr - 1]);
        textBox2.Text = mnm;
    }