Making Sure user enters a number or positive number

I'm trying to ask the user to enter a number higher than 0. When a negative number or a letter is entered, you get an error. I want the question to keep coming up until a correct input is entered. I tried doing that below, but I think I didn't lay it out correctly and missing some code.

double _heightVal = 0;
bool _userNotDone = true;

do
{
   Console.Write("Please enter the first part of your height in feet:");
   string height = Console.ReadLine();
   if (!double.TryParse(height, out _heightVal))
   {
      Console.WriteLine("Error. You must enter a number. Please try again.");
   }
   else if (_heightVal < 0)
   {
      Console.WriteLine("Error. Number must be greater than 0. Please try again.");
   }
   else
   {
       _userNotDone = false;
   }
}
while (_userNotDone == true || _heightVal < 0) ;