title: How to obtain form's inner width and height
body: <p>As per title.</p>
  <p>I am writing a program that all components can adjust size automatically by form's size.</p>
  <p>When I using <code>this.Width</code> and <code>this.Height</code> properties, its return value is the width and height of whole form (red frame).</p>
  <p>How to get the form's width and height of its inner area (green frame)?</p>
  <p><img src="https://i.stack.imgur.com/4RuOn.jpg" alt="" /></p>