private void button_Click(object sender, EventArgs e)
        {
           if(this.Controls.Contains(this.userControl1))
                 this.Controls.Remove(this.userControl1);
           if(!this.Controls.Contains(this.userControl2))
            this.Control.Add(this.userControl2);
        }

        private void button2_Click(object sender, EventArgs e)
        {
            if(!this.Controls.Contains(this.userControl1))
                 this.Control.Add(this.userControl1);
           if(this.Controls.Contains(this.userControl2))
                this.Controls.Remove(this.userControl2);
        }