c# - passing values between forms (winforms)

Question

Wierd behaviour when passing values to and from second form.

ParameterForm pf = new ParameterForm(testString);

works

ParameterForm pf = new ParameterForm();
pf.testString="test";

doesn't (testString defined as public string)

maybe i'm missing something? Anyway I'd like to make 2nd variant work properly, as for now - it returns null object reference error.

Thanks for help.

Posting more code here:

calling

Button ParametersButton = new Button();
ParametersButton.Click += delegate
                {
                    ParameterForm pf = new ParameterForm(doc.GetElementById(ParametersButton.Tag.ToString()));
                    pf.ShowDialog(this);
                    pf.test = "test";
                    pf.Submit += new ParameterForm.ParameterSubmitResult(pf_Submit);
                };

definition and use

   public partial class ParameterForm : Form
    {
        public string test;
        public XmlElement node;
        public delegate void ParameterSubmitResult(object sender, XmlElement e);
        public event ParameterSubmitResult Submit;

        public void SubmitButton_Click(object sender, EventArgs e)
        {
            Submit(this,this.node);
            Debug.WriteLine(test);
        }
     }

result: Submit - null object reference test - null object reference

Answer 1

• pf.ShowDialog(this); is a blocking call, so pf.Submit += new ParameterForm.ParameterSubmitResult(pf_Submit); is never reached: switch the order.

• Submit(this,this.node); throws a null object reference because no event is assigned to it (see above). Generally, you should always check first: if (Submit != null) Submit(this,this.node);

• You should change ``pf.ShowDialog(this);topf.Show(this);` so that your main form isn't disabled while your dialog box is open, if that's what you want, or use the model below (typical for dialog boxes.)

---

I'm not sure what pf_Submit is supposed to do, so this might not be the best way to go about it in your application, but it's how general "Proceed? Yes/No" questions work.

Button ParametersButton = new Button();
ParametersButton.Click += delegate
    {
        ParameterForm pf = new ParameterForm(testString);
        pf.ShowDialog(this); // Blocks until user submits
        // Do whatever pf_Submit did here.
    };

public partial class ParameterForm : Form
{
    public string test;     // Generally, encapsulate these
    public XmlElement node; // in properties

    public void SubmitButton_Click(object sender, EventArgs e)
    {
        Debug.WriteLine(test);
        this.Close(); // Returns from ShowDialog()
    }
 }