Vowel_letter_class

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace Let
{
    public partial class MainForm : Form
    {
        List<char> VOWELS = new List<char>();
        public  bool Is_Vowel(char a)
        {
            if (char.IsUpper(a))
                a = char.ToLower(a);
            return VOWELS.Contains(a);
        }
        public MainForm()
        {
            InitializeComponent();
            VOWELS.Add('a');
            VOWELS.Add('e');
            VOWELS.Add('i');
            VOWELS.Add('u');
            VOWELS.Add('o');
        }

        private void btn_ex_Click(object sender, EventArgs e)
        {
            this.Close();
        }

        private void MainForm_Load(object sender, EventArgs e)
        {

        }


        private void btn_dis_Freq_Click(object sender, EventArgs e)
        {

            if (textBox1.Text.Length < 1) return; /// return if text is empty
            string sentence = textBox1.Text;
            string res="";
            foreach (char a in sentence)

                if(Is_Vowel(a))
                    res+=("'"+a.ToString()+"'  >>>>> "+Letter.Frequency(a)+"%"+Environment.NewLine);

            MessageBox.Show(res);
             }

        private void conv_to_up_Click(object sender, EventArgs e)
        {
            string res="";
            foreach (char a in textBox1.Text)
            {
                if (Is_Vowel(a))
                    res += (char.ToUpper(a)).ToString();
                else
                res += a.ToString();
            }
            textBox1.Text = res;
        }

        private void conv_to_low_Click(object sender, EventArgs e)
        {
            string res = "";
            foreach (char a in textBox1.Text)
            {
                if (Is_Vowel(a))
                    res += (char.ToLower(a)).ToString();
                else
                    res += a.ToString();
            }
            textBox1.Text = res;
        }

        private void btn_start_end_Click(object sender, EventArgs e)
        {
            if (textBox1.Text.Length < 1) return; /// return if text is empty

            bool is_start_with_vowel=false;
            bool is_End_with_vowel=false;
            int last_index = textBox1.Text.Length-1;

            is_start_with_vowel= Is_Vowel(textBox1.Text[0]);
            is_End_with_vowel=Is_Vowel(textBox1.Text[last_index]);

            if (is_start_with_vowel && is_End_with_vowel)
                MessageBox.Show("Start and end with Vowel ");
            else if (is_start_with_vowel && !is_End_with_vowel)
                MessageBox.Show("Start With Vowel ");
            else if (!is_start_with_vowel &&  is_End_with_vowel)
            MessageBox.Show("End With Vowel ");
            else MessageBox.Show("No Vowel in start or end");
        }

        private void btn_vowel_Loc_Click(object sender, EventArgs e)
        {
            if (textBox1.Text.Length < 1) return; /// return if text is empty

            string res = "";
            string v = textBox1.Text;

            for (int i = 0; i < v.Length; i++)

                if (Is_Vowel(v[i]))
                    res += (v[i].ToString() + " at index of " + i.ToString() + Environment.NewLine);
            MessageBox.Show(v+Environment.NewLine+res,"Location Of Vowel");
        }

        private void linkLabel1_LinkClicked(object sender, LinkLabelLinkClickedEventArgs e)
        {
            System.Diagnostics.Process.Start("http://en.wikipedia.org/wiki/Letter_frequency");
        }

    }
}