Send data from form to Sub form

That's means i just create all my variables in public properties? Is that pubilc react as global variable in C# project.

If you create public property you get access to data from subform. This data belongs to instance of the subform (not static/global variable), you can get access to this data if you have access to instance of subform.
You can also create internal properties if you will be using that subform inside project (dll).

Try this code I have modified your original code and replace it with this one.Please check this :  

namespace Main_Forms
{
    public partial class Form1 : Form
    {
        Form2 frm2;
        Sub_From.SubForm2 Sfrom;
        public string answer;
        public Form1()
        {
            InitializeComponent();
            
            button2.Enabled = false;
            textBox1.Enabled = false;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            button1.Enabled = false;
            button2.Enabled = true;
            textBox1.Enabled = true;
            frm2 = new Form2();
            if(frm2.ShowDialog()==DialogResult.Yes)
            {
                Sfrom = new Sub_From.SubForm2(this); //**Changes**
                Sfrom.FormClosing += new FormClosingEventHandler(SubForm_FormClosing);
                Sfrom.Show();
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            if (Sfrom != null)
            {
                Sfrom.Close();
                //Sfrom.Dispose();
                Sfrom = null;
            }         
            
        }
        private void SubForm_FormClosing(object sender, FormClosingEventArgs e)
        {
            SubForm2 sub2 = (SubForm2)sender;//**Changes**
            sub2.Hide();//**Changes**
            this.Show();//**Changes**
            this.textBox1.Text = sub2.valueans;   //**Changes**
        }
    }
}
============================================================================
namespace Main_Forms
{
    public partial class Form2 : Form
    {
        //Sub_From.SubForm2 Sform;
        
        public Form2()
        {
            InitializeComponent();
            
        }

        private void button1_Click(object sender, EventArgs e)
        {
            this.DialogResult = DialogResult.Yes;
           
        }

        private void button2_Click(object sender, EventArgs e)
        {
            this.Close();
        }
   }
}

===========================================================================
namespace Sub_From
{
    public partial class SubForm2 : Form
    {
       
        public string valueans
        {
            get { return textBox1.Text; }
        }

        public SubForm2()
        {
            InitializeComponent();
        }
        Form parentFrm;//**Changes**

	//**Changes**
        public SubForm2(Form parent)
        {
            InitializeComponent();
            parentFrm = parent;
        }
    }
}

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