changing string dynamically in C#

Question

When I'm putting the following code specifically in the immediate window in Visual studio, it returns correctly:

whatToMatch.Remove((whatToMatch.IndexOf(input[i])), 1)

But when I put it in a program as shown below, it fails:-

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

namespace IsPangram
{
    class Program
    {
        static void Main(string[] args)
        {
            string whatToMatch = "abcdefghijklmnopqrstuvwxyz";
            string input = Console.ReadLine().ToLower();
            for (int i = 0; i < input.Length; i++)
            {
                if (whatToMatch.Contains(input[i]))
                {
                    whatToMatch.Remove((whatToMatch.IndexOf(input[i])), 1);
                }
                if (whatToMatch.Length == 0)
                    Console.WriteLine("pangram");
            }
            Console.WriteLine("not pangram");

        }
    }
}

I was expecting "whatToMatch" to change dynamically as it is correct code, but it's not changing. Why? And how to resolve this issue?

Answer 1

From msdn about String.Remove Method (Int32, Int32)

It returns a new string in which a specified number of characters in the current instance beginning at a specified position have been deleted.

So it doesn't modify the string that call it, it return a new string. So you should use

whatToMatch = whatToMatch.Remove((whatToMatch.IndexOf(input[i])), 1)

Answer 2

As already mentioned, strings in .NET are immutable, therefore you cannot expect your string to change dynamically.

Here is a concise solution to your problem using LINQ:

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

namespace IsPangram
{
    static class Program
    {
        public static bool IsPangram(this string input)
        {
            return
                !input.ToLower()
                      .Aggregate("abcdefghijklmnopqrstuvwxyz".ToList(),
                                 (ts, c) => ts.Where(x => x != c).ToList())
                      .Any();
        }

        public static void Main(string[] args)
        {
            Console.WriteLine(Console.ReadLine().IsPangram() ?
                              "Is pangram" :
                              "Is not pangram");
        }
    }
}