C#-Code

using System;

namespace ConsoleGrossKleinUmkehren
{
    static class Program
    {
        static void Main(string[] args)
        {
            while(true)
            {
                Console.WriteLine(Console.ReadLine().InvertCase());
            }
        }

        static string InvertCase(this string s)
        {
            string result = "";
            foreach (char c in s)
            {
                result += char.IsUpper(c) ? char.ToLower(c) : char.ToUpper(c);
            }

            return result;
        }
    }
}

C#-Code

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

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Bitte geben Sie einen Satz ein:");
            Console.WriteLine(invertChars(Console.ReadLine()));
            Console.ReadLine();

        }

        static string invertChars(string input)
        {
            string lower = "abcdefghijklmnopqrstuvwxyz";
            string upper = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
            string ret = "";

            for(int i = 0; i < input.Length; i++)
            {
                if (lower.Contains(input[i]))
                {
                    ret += input[i].ToString().ToUpper();
                }
                else if (upper.Contains(input[i]))
                {
                    ret += input[i].ToString().ToLower();
                }
                else
                {
                    ret += input[i].ToString();
                }
                
            }

            return ret;
        }

    }
}

C#-Code

  class Program {
    static void Main() {
      string input = Console.ReadLine();

      if(string.IsNullOrWhiteSpace(input)) {
        return;
      }

      StringBuilder sb = new StringBuilder();
      foreach(char c in input) {
        if(char.IsLetter(c)) {
          if(char.IsLower(c)) {
            sb.Append(char.ToUpper(c));
          }

          if(char.IsUpper(c)) {
            sb.Append(char.ToLower(c));
          }
        } else {
          sb.Append(c);
        }
      }

      Console.Write("\n" + sb.ToString());
      Console.ReadKey(true);
    }
  }

C#-Code

using System;

namespace Exercise_186
{
    public static class Program
    {
        private static void Main()
        {
            var text = Console.ReadLine();
            var invertedText = string.Empty;

            foreach (var c in text)
            {
                invertedText += char.IsUpper(c) ? char.ToLower(c) : char.ToUpper(c);
            }

            Console.WriteLine(invertedText);
            Console.Read();
        }
    }
}

C#-Code

using System;

namespace BuchstabenTausch
{
    class Program
    {
        static void Main(string[] args)
        {
            string text = "a B c D e F g H i J k L m N o P q R s T u V w X y Z ~ 1 2 3 4 5 6 7 8 9 0";
            string lowerUpperSwitch = "";

            Console.WriteLine("\nEingabe   :  " + text);

            foreach (char c in text)
            {
                if(char.IsUpper(c))
                {
                    lowerUpperSwitch += char.ToLower(c);
                }
                else if(char.IsLower(c))
                {
                    lowerUpperSwitch += char.ToUpper(c);
                }
                else
                {
                    lowerUpperSwitch += c;
                }
            }

            Console.WriteLine("\nErgebnis  :  " + lowerUpperSwitch);

            Console.ReadKey();
        }
    }
}

C#-Code

using System.Linq;
using static System.Console;

namespace GrossKleinbuchstabenUmkehren
{
    class Program
    {
        static void Main()
        {
            WriteLine(string.Concat(ReadLine().ToCharArray().Select(x => char.IsUpper(x) ? char.ToLower(x) : char.ToUpper(x))));
        }
    }
}