Question

I’m building a Notepad. I have a Find and Replace form. When I click a button that form opens, user gives two input in two textboxes and press a button. Then the RichTextBoxes from the main form is supposed to get modified.

Here’s the form of the FindAndReplace form :

private void btnReplaceAll_Click(object sender, EventArgs e)
        {
            string findMe = txtFind.Text;
            string replaceMe = txtReplace.Text;
            Form1 f1 = new Form1();
            f1.MainText.Replace(findMe, replaceMe);
            //this.Hide();
        }

The problem is its not working.. I’m getting a NullReferenceException in the line f1.MainText.Replace(findMe, replaceMe);
Any idea?

Answer 1

Here you create a new instance of the form:

Form1 f1 = new Form1();

All properties are initialized to their default values (i.e. strings to null). Next you try to call the Replace method on the MainText property which is null and you get the exception:

f1.MainText.Replace(findMe, replaceMe);

You need to first initialize this property:

f1.MainText = "blablabla";
f1.MainText = f1.MainText.Replace(findMe, replaceMe);

---

UPDATE:

When you create the FindAndReplace form you could pass to its constructor the current value of the text:

public class Form1 : Form
{
    protected void FindAndReplace_Click(object sender, EventArgs e) 
    {
        var findAndReplaceForm = new FindAndReplaceForm(MainText.Text);
        findAndReplaceForm.ShowDialog();
        MainText.Text = findAndReplaceForm.NewText;
    }
}

public class FindAndReplaceForm : Form
{
    private readonly string _originalText;

    public FindAndReplaceForm(string originalText)
    {
        _originalText = originalText;
    }

    public string NewText 
    { 
        get 
        {
            return (_originalText ?? string.Empty).Replace(findMe, replaceMe);
        }
    }
}