using System;

    using System.Text.RegularExpressions;

    public class Test

    {            

        public static void Main ()

        {

            var isNumeric1 = IsNumeric("1");

            Console.WriteLine(isNumeric1);

​

            var isNumeric2 = IsNumeric("HelloWorld");

            Console.WriteLine(isNumeric2);

​

            //call IsNuermic with the value of this.txtIdGrupo.Text like this

            //var isNumeric = IsNumeric(this.txtIdGrupo.Text);

        }

​

        private static bool IsNumeric(string str)

        {

            string id = str;

            if (!Regex.IsMatch(id, @"^\d+$"))   

                return false;

​

            return true;

        }

    }