C#

public partial class ModalForm : Form
{
  Button theButton; ...etc.
  
  public ModalForm
  {
    theButton.Click+= new EventHandler(ClickHandler)
  }

  public void ClickHandler(object sender, Eventargs e)
  {    
    objParenForm.myProperty = 1;
    Dispose();
  }    
}

public partial class ParentForm : Form
{
  ModalForm mForm
  public int myProperty;

  public void ParentForm_Load(object sender, Eventargs e)
  {
    mForm = new ModalForm();
    mForm.Show(this);
  }
}