Enum.IsDefined, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Microsoft (R) Visual C# Compiler version 2.9.0.63208 (958f2354)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public enum Units
    {
      Meter = 0,
      Centimeter = 1,
      Milimeter = 2
    }
    
    public class Program
    {
        public static void Main(string[] args)
        {
            
            string unitEnumStringOrIndex = null;
            
            unitEnumStringOrIndex = "Meter"; //Working
            unitEnumStringOrIndex = "1";// Not working

if(Enum.TryParse<Units>(unitEnumStringOrIndex, true, out Units unitEnum))
            {
                Console.WriteLine("TryParse \"{0}\" result: {1}.", unitEnumStringOrIndex, unitEnum);
                
                if(Enum.IsDefined(typeof(Units), unitEnumStringOrIndex))
                {
                    Console.WriteLine("IsDefined \"{0}\" returned true.", unitEnumStringOrIndex);
                }
                else 
                {
                    Console.WriteLine("IsDefined \"{0}\" returned false.", unitEnumStringOrIndex);
                    
                    if(Enum.IsDefined(typeof(Units), int.Parse(unitEnumStringOrIndex)))
                    {
                        Console.WriteLine("IsDefined {0} returned true.", unitEnumStringOrIndex);
                    }
                }
            }
            else 
            {
                Console.WriteLine("TryParse \"{0}\" returned false.", unitEnumStringOrIndex);
            }
            
        }

}
}

Show compiler warnings [ + ] Show input
Compilation time: 2,27 sec, absolute running time: 0,09 sec, cpu time: 0,11 sec, average memory usage: 15 Mb, average nr of threads: 3, absolute service time: 2,39 sec