pass listbox item to a textbox on another form c#

问题:

I am making this simple Windows forms app in Visual studio in c#. I have two forms. On form1 I have a textbox,listbox and two buttons (one to insert into listbox from textbox and another to open form2). On form2 I only have a textbox. I just simply want, when click on a button (for opening form2) on form1, form2 to open and textbox to contain (on formLoad) selected item from listbox from form1. But when I click on button it says "Object reference not set to an instance of an object". What am I doing wrong? I am pretty sure it's something simple but I just can't get it.

Thanks in advance!

Here is my code:

on form1:

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

    private void btnOpenForm2_Click(object sender, EventArgs e)
    {
        Form2 f2 = new Form2();
        f2.ShowDialog();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        listBox1.Items.Add(textBox1.Text);
    }
    public string Transfer
    {
        get { return listBox1.SelectedItem.ToString(); }
    }

and on form2:

 public partial class Form2 : Form
  {
    public Form2()
    {
        InitializeComponent();
    }

    private void Form2_Load(object sender, EventArgs e)
    {
        Form1 f1 = new Form1();
        textBox1.Text = f1.Transfer;
    }

回答1:

Because in the Form2_Load event you always create a new instance of Form1 and then access the Transfer property which accesses listBox1.SelectedItem which is not set for the newly created form.

You should rather pass a referece to form 1 in the button event:

on form1:

private void btnOpenForm2_Click(object sender, EventArgs e)
{
    Form2 f2 = new Form2(this);
    f2.ShowDialog();
}

and on form2:

public partial class Form2 : Form
{
   Form1 f1;
   public Form2(Form1 f1)
   {
       this.f1 = f1;
       InitializeComponent();
   }

   private void Form2_Load(object sender, EventArgs e)
   {
       textBox1.Text = this.f1.Transfer;
   }
}

回答2:

In your Form2_Load method, you're creating a new instance of the object Form1, separate from your existing item.

Instead, you need to either: a) Pass a reference to your current Form1 object to Form2, so that Form2 can access the Transfer property. or b) Add a new property to Form2 (called Transfer, say), and then when you create Form2, assign the current textbox value to this property, like so:

Form2 f2 = new Form2();
f2.Transfer = listBox1.SelectedItem.ToString();
f2.ShowDialog();

You could also do this by adding a parameter to the constructor of Form2, although that's really a design decision.

回答3:

because you haven't selected your listbox item,value listBox1.SelectedItem is null.Practice doing try catch block

回答4:

You are creating new Form1 instance here, which is not related to Form1 instance which you used to open Form2:

private void Form2_Load(object sender, EventArgs e)
{
    Form1 f1 = new Form1(); // here is new instance of Form1 created
    textBox1.Text = f1.Transfer;
}

So, this new Form1 instance does not have selected item and you have error. I suggest you to pass selected item value to Form2 when you opening Form2:

private void btnOpenForm2_Click(object sender, EventArgs e)
{
    Form2 f2 = new Form2(Transfer); // pass selected item value to constructor
    f2.ShowDialog();
}

All you need is changing Form2 constructor to accept this string:

public Form2(string transfer)
{
    InitializeComponent();
    textBox1.Text = transfer;
}