Song Encryption - FinalExamStringProblem

using System;
using System.Linq;
using System.Collections.Generic;


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

			if (input == "end")
			{
				break;
			}

			string[]tokens = input.Split(':');
			string artist = tokens[0];
			string song = tokens[1];

			string validArtist = string.Empty;

			for (int i=0; i<artist.Length; i++)
			{
				char curr = artist[i];

				if (i==0 && char.IsUpper(curr))
				{
					validArtist += curr;
				}
				else if (i==0 && (char.IsUpper(curr)==false))
				{
					Console.WriteLine("Invalid input!");
					validArtist = string.Empty;
					break;
				}

				if (i>0 && (char.IsLower(curr) || curr=='\'' || curr==' '))
				{
					validArtist += curr;
				}
				else if (i>0 && ((char.IsLower(curr) || curr=='\'' || curr==' ')==false))
				{
					Console.WriteLine("Invalid input!");
					validArtist = string.Empty;
					break;
				}
			}
			string encryptedSinger = string.Empty;

			if (validArtist!=string.Empty)
			{
				int len = validArtist.Length;


				for (int i=0; i<validArtist.Length; i++)
				{
					char curr = validArtist[i];

					if (char.IsLetter(curr))
					{
						if (char.IsUpper(curr))
						{
							int currAsNum = (int)curr;
							currAsNum += len;
							if (currAsNum > 90)
							{
								currAsNum -=26;
							}

							char numAsSymbol = (char)currAsNum;
							encryptedSinger += numAsSymbol;
						}
						else
						{
							int currAsNum = (int)curr;
							currAsNum += len;
							if (currAsNum > 122)
							{
								currAsNum -=26;
							}

							char numAsSymbol = (char)currAsNum;
							encryptedSinger += numAsSymbol;
						}
					}
					else
					{
						encryptedSinger += curr;
					}
				}
			}
			else
			{
				continue;
			}

			encryptedSinger += '@';

			string validSong = string.Empty;

			for (int i=0; i<song.Length; i++)
			{
				char curr = song[i];

				if (char.IsUpper(curr) || curr==' ')
				{
					validSong += curr;
				}
				else
				{
					Console.WriteLine("Invalid input!");
					validSong = string.Empty;
					break;
				}
			}

			string encryptedSong = string.Empty;

			if (validSong != string.Empty)
			{
				int len = validArtist.Length;

				for (int i=0; i<validSong.Length; i++)
				{
					char curr = validSong[i];

					if (char.IsUpper(curr))
					{
						int currAsNum = (int)curr;
						currAsNum += len;

						if (currAsNum > 90)
						{
							currAsNum -=26;
						}

						char numAsSymbol = (char)currAsNum;
						encryptedSong += numAsSymbol;
					}
					else
					{
						encryptedSong += curr;
					}
				}
			}
			else
			{
				continue;
			}

			string result = encryptedSinger+encryptedSong;
			Console.WriteLine("Successful encryption: {0}",result);
		}
	}
}