实现效果:

知识运用:
TextBox控件的Multiline属性
public overrider bool Multiline{ get;set;}
实现代码:
private void button1_Click(object sender, EventArgs e)
{
textBox1.Location = new Point(0,0);
textBox1.Multiline = true;
textBox1.Width = this.Width;
textBox1.Height = ClientRectangle.Height - button1.Height - 3;
}