Question

I have 2 forms in C# desktop application. Form1 and Form2.

Form1 contains a public method that adds item in ListBox control as follows:

public void AddToList(string item)
{
    listBox.Items.Add(item);
}

When I call this method directly on some button press then it works fine. But when I call this method from Form2, it doesn’t add anything in ListBox control on Form1. Code in Form2 is as below:

Form1 frm = new Form1();
frm.AddToList("something");

When I run this nothing happens. No error nothing. It just doesn’t add any item into ListBox.

What am I doing wrong?

Answer 1

You’re creating a new instance of Form1 and adding an item to it’s listbox, rather than getting an instance of the Form1 you no doubt already have and calling the method on that.

The naive approach is to have a parameter in the Form2 constructor that takes an instance of Form1 and saves it as an instance variable for use in this event handler.

I don’t much like that approach from a design perspective.

I would suggest creating a public event in Form2, having Form1 subscribe to that event and add a handler that add the item to the listbox. The Event in Form2 would look something like this:

public event EventHandler ButtonClick
{
    add
    {
        button1.Click += value;
    }
    remove
    {
        button1.Click += value;
    }
}

Then you’ll have a property that looks something like this:

public string SomeValueForm1NeedsOnButtonClick
{
    get
    {
        return texbox1.Text;
    }
}

Then in Form1 you’ll have something like:

Form2 otherForm = new Form2();
otherForm.ButtonClick += (sender, args) =>
{
    listbox1.Items.Add(otherForm.SomeValueForm1NeedsOnButtonClick);
};

This approach ensures that each form only knows as little as possible about each other form. It reduces Coupling between the two classes and makes it clearer to future users/readers of the forms exactly what communication takes place between them.