Pass list to another form without reloading it

Question

Hi,
I have two Forms, Form A and Form B C# Winform application .

When the user click on the button in Form A , will show Form B , then Form B will generate a list of integer, this list should be sent to Form A (which is already opened) , and Form A has a button to print the list

FormA >> public List<int> MyArrayList = new List<int>();
Form A code to open Form B

var exm = new FormB();  
            exm.Show();  

Form B code to generate the List

 MyList.Add[1];  
 MyList.Add[4];  
 MyList.Add[5];  
 MyList.Add[6];  

Form B code to open Form A and send the Array

 var fm= new FormA();  
           fm.MyArrayList = this.MyArrayList ;  
            this.Close();  

But I received error

FormA.Mylist.**get** returned null  

will work only if I FormA.show() is used from FormB

so please how may I send the ArrayList from FormB to FormA without closing FormA and open it again.

Answer 1

This is easy using an event. In the following example, MainForm is the startup form and ChildForm will pass an array to MainForm.

Full source

MainForm

namespace SendToMainForm  
{  
    public partial class MainForm : Form  
    {  
        public MainForm()  
        {  
            InitializeComponent();  
        }  
  
        private void OpenChildFormButton_Click(object sender, EventArgs e)  
        {  
            ChildForm childForm = new() { Owner = this };  
  
            childForm.PassMonths += ChildFormOnPassMonths;  
            childForm.Show(this);  
            childForm.Top = Top;  
            childForm.Left = Left +  Width + 10;  
        }  
  
        private void ChildFormOnPassMonths(string[] months)  
        {  
            listBox1.Items.AddRange(months);  
        }  
    }  
}  

ChildForm

namespace SendToMainForm  
{  
    public partial class ChildForm : Form  
    {  
        public delegate void OnPassMonths(string[] monthNames);  
        public event OnPassMonths PassMonths;  
  
        public ChildForm()  
        {  
            InitializeComponent();  
        }  
  
        private void SendDataToMainFormButton_Click(object sender, EventArgs e)  
        {  
            var months = System.Globalization  
                .DateTimeFormatInfo.CurrentInfo  
                .MonthNames.Take(12).ToArray();  
  
            PassMonths?.Invoke(months);  
        }  
    }  
}  
  

For other types such as int

namespace SendToMainForm  
{  
    public partial class ChildForm : Form  
    {  
        public delegate void OnPassMonths(int[] monthNames);  
        public event OnPassMonths PassMonths;  
  
        public ChildForm()  
        {  
            InitializeComponent();  
        }  
  
        private void SendDataToMainFormButton_Click(object sender, EventArgs e)  
        {  
            PassMonths?.Invoke(new[] { 1, 2, 3 });  
        }  
    }  
}  

And

namespace SendToMainForm  
{  
    public partial class MainForm : Form  
    {  
        public MainForm()  
        {  
            InitializeComponent();  
        }  
  
        private void OpenChildFormButton_Click(object sender, EventArgs e)  
        {  
            ChildForm childForm = new() { Owner = this };  
  
            childForm.PassMonths += ChildFormOnPassMonths;  
            childForm.Show(this);  
            childForm.Top = Top;  
            childForm.Left = Left +  Width + 10;  
        }  
  
        private void ChildFormOnPassMonths(int[] data)  
        {  
            listBox1.Items.AddRange(Array.ConvertAll(data, x => x.ToString()));  
        }  
    }  
}  

Answer 2

Do you close FormA when you open FormB? If not, rather than creating a new FormA from inside FormB why not pass in a reference to FormA into FormBs constructor and set it as a member variable? Then you'll be able to access the original form for the lifetime of the new form.