Programa que detecta la cantidad de veces que una letra, pueda aparecer en una oración.

Alphabet.cs

using System;

class Alphabet
{
	private char[]abc = new char[26];
	private int[] countUpper = new int[26];
	private int[] countLower = new int[26];
	
	public Alphabet()
	{
  //Para rellenar el abecedario en mayuscula en "char[] abc"
		for(int i = 0; i != 26; ++i)
			abc[i] = (char)(i + 'A');
	}
	
	public void DetectUpperOrLower(char charx)
	{
		int pos;
		bool convertion = false;
		if(charx >= 'a' && charx <= 'z')
		{
			convertion = true;
			charx -= (char)32;
		}
    //La clase SearchBinary está en: https://gist.github.com/MrDave1999/843654eb567b730bd9af8755ec1a6fa0
		pos = SearchBinary.BBIterative<char>(abc, charx);
		if(convertion)
			++countLower[pos];
		else
			++countUpper[pos];
	}
	
	public void ShowChars()
	{
		for(int i = 0; i != 26; ++i)
		{
			if(countUpper[i] != 0)
				Console.WriteLine("-> {0} : {1}\n", abc[i], countUpper[i]);
			else if(countLower[i] != 0)
				Console.WriteLine("-> {0} : {1}\n", (char)(abc[i] + 32), countLower[i]);
				
		}
		for(int i = 0; i != 26; ++i)
		{
			countUpper[i] = 0;
			countLower[i] = 0;
		}
	}
}	

sentence.cs

using System;

class Program
{
	static void Main()
	{
		string sentence;
		Alphabet abc = new Alphabet();
		
		while(true)
		{
			Console.Write("Ingrese una oracion:\n");
			sentence = Console.ReadLine();
	
			for(int i = 0, len = sentence.Length; i != len; ++i)
			{
				if((sentence[i] >= 'a' && sentence[i] <= 'z') || (sentence[i] >= 'A' && sentence[i] <= 'Z'))
					abc.DetectUpperOrLower(sentence[i]);
			}
			abc.ShowChars();
			Console.ReadLine();
		}
		
	}
}