How would you count occurrences of a char within a string?

In many interviews, i faces a problem with the getting count of occurrences of a char within a string. Mostly i ignored these question and i replied like "Sorry, I don't know &  Sorry, i cannot reply".

Finally, i mind it and write a simple code as below :

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

namespace CharOccuranceInString

{

    class Program

    {

        static void Main(string[] args)

        {

            #region Occurance of char in a string using c#

            

            string source = "abcaadbcdbbcd";

            int countA = 0;

            int countB = 0;

            int countC = 0;

            int countD = 0;

            foreach (char c in source)

            {

                if (c == 'a') countA++;

                if (c == 'b') countB++;

                if (c == 'c') countC++;

                if (c == 'd') countD++;

            }

            Console.WriteLine("Occurance of a=" + countA.ToString() +

                                   "\n Occurance of b=" + countB.ToString() +

                                   "\n Occurance of c=" + countC.ToString() +

                                   "\n Occurance of d=" + countD.ToString());

            Console.ReadKey();

            #endregion

        }

    }

}