WinForms show a modeless message dialog

ModelessMessage.cs

var f = new Form();
var b = new Button();
b.Text = "Show message";
b.AutoSize = true;
f.Controls.Add(b);
b.Click += (s, e) =>
{
  var p = new Form();
  p.Text = "Hello";
  p.Show(f);
};
f.ShowDialog();