01_ReverseString.cs

using System;

// Write a program that reads a string from the console, reverses it 
// and prints the result back at the console.

class ReverseString
{
    static void Main(string[] args)
    {
        char[] charArr = Console.ReadLine().ToCharArray();
        Array.Reverse(charArr);
        string reversed = new string(charArr);
        Console.WriteLine(reversed);
    }
}

02_StringLength.cs

using System;
using System.Text;

/*
 * Write a program that reads from the console a string of maximum 20 characters. 
 * If the length of the string is less than 20, the rest of the characters 
 * should be filled with *. Print the resulting string on the console. 
 */

class StringLength
{
    static void Main()
    {
        string input = Console.ReadLine();
        StringBuilder sb;
        if(input.Length>20){
            sb = new StringBuilder(input.Substring(0,20));
        }else{
            sb = new StringBuilder(input);
            while(sb.Length<20){
                sb.Append('*');
            }
        }
        Console.WriteLine(sb.ToString());   
    }
}

03_CountSubstringOccurences.cs

using System;

/*
 * Write a program to find how many times a given string appears in a given text as substring.
 * The text is given at the first input line. The search string is given at the second input line.
 * The output is an integer number. Please ignore the character casing. Overlapping between 
 * occurrences is allowed.
 */

class CountSubstringOccurences
{
    static void Main()
    {
        string input = Console.ReadLine();
        string substring = Console.ReadLine();
        int numOccurences = 0;
        int startIndex = 0;
        
        while (input.IndexOf(substring, startIndex, StringComparison.OrdinalIgnoreCase) >= 0)
        {
            numOccurences++;
            startIndex = input.IndexOf(substring, startIndex, StringComparison.OrdinalIgnoreCase) + 1;
        }
        Console.WriteLine(numOccurences);
    }
}

04_TextFilter.cs

using System;
using System.Linq;
using System.Text;

class TextFilter
{
    static void Main()
    {
        string[] bannedWords = Console.ReadLine().Split(',').Select(p => p.Trim()).ToArray();
        string input = Console.ReadLine();
        StringBuilder textBuilder = new StringBuilder(input);

        foreach (string bannedWord in bannedWords)
        {
            string mask = new string('*', bannedWord.Length);
            textBuilder.Replace(bannedWord, mask);
        }

        Console.WriteLine(textBuilder.ToString());
    }
}

05_UnicodeCharacters.cs

using System;

/*
 * Write a program that converts a string to a sequence of C# Unicode character literals.
 */
 
class UnicodeCharacters
{
    static void Main()
    {
        string input = Console.ReadLine();
        foreach (char ch in input)
            Console.Write(@"\u{0}", ((int)ch).ToString("x4"));
        Console.WriteLine();
    }
}

06_Palindromes.cs

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

/*
 * Write a program that extracts from a given text all palindromes, e.g. ABBA, lamal, exe 
 * and prints them on the console on a single line, separated by comma and space. 
 * Use spaces, commas, dots, question marks and exclamation marks as word delimiters. 
 * Print only unique palindromes, sorted lexicographically.
 */

class Palindromes
{
    private static bool IsPalyndrome(string word)
    {
        if (word.Length == 1)
            return true;
        int len = word.Length;
        for(int i=0; i<len/2; i++){
            if(word[i] != word[len-i-1])
                return false;
        }
        return true;
    }

    static void Main()
    {
        char[] delimiters = {' ', ',', '.', '?', '!' };
        SortedSet<string> palyndromes = new SortedSet<string>();
        List<string> words = Console.ReadLine().
            Split(delimiters, StringSplitOptions.RemoveEmptyEntries).
            Select(p => p.Trim()).ToList();

        foreach(string word in words){              
            if (IsPalyndrome(word))
                palyndromes.Add(word);
        }
        Console.WriteLine(string.Join(", ", palyndromes));
    }   
}

07_LettersChangeNumbers.cs

using System;
using System.Linq;
using System.Threading;

//https://judge.softuni.bg/Contests/Practice/DownloadResource/452

class LettersChangeNumbers
{
    static void Main()
    {
        Thread.CurrentThread.CurrentCulture = 
            System.Globalization.CultureInfo.InvariantCulture;
        char[] separators = {' ','\t' };
        string[] words = Console.ReadLine().Split(separators, 
            StringSplitOptions.RemoveEmptyEntries).Select(p => p.Trim()).ToArray();
        
        double sum=0.0;
        foreach (string word in words)
        {
            char prefix = word[0];
            char suffix = word[word.Length - 1];
            double number = double.Parse(word.Substring(1, word.Length - 2));

            if (Char.IsUpper(prefix))
                number /= 1.00*((int)prefix - (int)'A' + 1);
            else
                number *= 1.00 * ((int)prefix - (int)'a' + 1);

            if (Char.IsUpper(suffix))
                number -= ((int)suffix - (int)'A' + 1);
            else
                number += ((int)suffix - (int)'a' + 1);

            sum += number;
        }
        Console.WriteLine("{0:F2}", sum);
    }
}