Converting hh:mm:ss to hours decimal in C#

I have a textbox that displays a stopwatch in the format hh:mm:ss with the following:

Stopwatch stopwatch1 = new Stopwatch();

 private void timer1_Tick(object sender, EventArgs e)
        {
            //Set Textbox text equal to elapsed time 
            TimerTextBox1.Text = stopwatch1.Elapsed.ToString("hh\\:mm\\:ss");
      }

Works great. What I am looking for:
After clicking a button,  I want to convert TimerTextBox1.Text to a hour decimal of the time elapsed. Example..  01:30:00 will convert to 1.5 hours.

Thank you for your assistance!
Shannon