dotNetAdvcdGrammer/ExtensionMethod/ExClass.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ExtensionMethod
{
    // static 클래스 선언 필요
    public static class ExClass
    {
        public static string ToChangeCase(this string str)
        {
            StringBuilder sb = new StringBuilder();
            foreach (char ch in str)
            {
                if (ch >= 'A' && ch <= 'Z')
                    sb.Append((char)('a' + ch - 'A'));
                else if (ch >= 'a' && ch <= 'z')
                    sb.Append((char)('A' + ch - 'a'));
                else
                    sb.Append(ch);
            }

            return sb.ToString();
        }

        public static bool Found(this string str, char ch)
        {
            int position = str.IndexOf(ch);

            return position >= 0;
        }
    }
}