using System;

public class Program

{

    public static void Main()

    {

        char[] vals = [ '0', '1', '2', '3', '4', '5', 'a', ' ' ];

        foreach(var v in vals)

        {

            var val = ParseToEnum<TestEnum>(v);

            Console.WriteLine(val == null ? "[NULL]" : val);

        }

    }

    public static TEnum? ParseToEnum<TEnum>(char input) 

        where TEnum : struct, Enum

    {

        if (int.TryParse(input.ToString(), out int val) 

            && Enum.IsDefined(typeof(TEnum), val))

        {

            return (TEnum)Enum.ToObject(typeof(TEnum), val);

        }

​

        return null;

    }

    public enum TestEnum

    {

        Zero = 0,

        One = 1,

        Two = 2,

        Three = 3

    }

​

}