float a = 162.00001f;
textBox1.Text = a.ToString("0.#####");

просто 162

using System;
using System.Globalization;

namespace SingleToStringApplication
{
    static class Extensions
    {
        static readonly CultureInfo ci = new CultureInfo("en-us");

        public static string AsString(this float value)
        {
            var integerPart = (int)value;
            var fractionalPart = value - integerPart;

            return String.Format(ci, "{0}{1:.#####}", integerPart, fractionalPart);
        }
    }

    class Program
    {
        static void Main()
        {

            float a = 0.00001f;
            float b = 0.01f;
            float c = 162.00001f;

            Console.WriteLine(a.AsString());
            Console.WriteLine(b.AsString());
            Console.WriteLine(c.AsString());
            Console.ReadKey();
        }
    }
}