TimeSpan is a type, which is not valid in TimeSpan - C#

  public void DisplayTime()
        {
            if (this.startTime.Year != 1)
            {
                this.interval = new TimeSpan();
                DateTime startTime = new DateTime();
                string str = "";
                if (this.inPlayTime.Year == 1)
                {
                    startTime = this.startTime;
                }
                else
                {
                    startTime = this.inPlayTime;
                }
                if (this.inPlayGo)
                {
                    this.lbTime.ForeColor = Color.DarkGreen;
                    this.interval = (TimeSpan) (DateTime.Now - startTime);
                }
                else
                {
                    this.lbTime.ForeColor = Color.Black;
                    this.interval = (TimeSpan) -(DateTime.Now - this.startTime);
                                             ^^^^^^^^
                                             тут ошибка 
Severity    Code    Description Project File    Line
Error   CS0119  'TimeSpan' is a type, which is not valid in the given context   botbeetle   

                    if ((Convert.ToInt32(this.interval.TotalMinutes) == ClassSound.time) && this.bool_0)
                    {
                        this.bool_0 = false;
                        if ((ClassSound.spTimeStart != null) && ClassSetting.SoundOn)
                        {
                            try
                            {
                                ClassSound.spTimeStart.Play();
                                base.Show();
                                base.BringToFront();
                            }
                            catch
                            {
                            }
                        }
                    }
                }
                str = str + this.interval.ToString();
                int index = str.IndexOf('.');
                if (index < 0)
                {
                    index = str.Length;
                }
                this.lbTime.Text = str.Substring(0, index);
            }
        }