// Write a C# method that counts the number of unique characters in a given string, ignoring case and any whitespace characters.

//  1.  The method should ignore case (e.g., ‘A’ and ‘a’ should be considered the same character).

//  2.  The method should ignore any whitespace characters (spaces).

//  3.  The method should return the count of unique characters in the string.

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// EXAMPLES

// CountUniqueCharacters("Hello World") // should return 7 (H, E, L, O, W, R, D)

// CountUniqueCharacters("C# is Fun!")  // should return 8 (C, #, I, S, F, U, N, !)

// CountUniqueCharacters("aAaA")        // should return 1 (A)

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// Constraints:

//  • You may assume that the input string is not null.

//  • Special characters and numbers (like ‘#’ or ‘1’) should be counted as unique characters if present.

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using System;

using System.Collections.Generic;

using System.Linq;

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public class Program

{

    public static void Main()

    {

        Console.WriteLine(CountUniqueCharacters("aAaA"));

    }

    public static int CountUniqueCharacters(string input) 

    {

        var grouped = input.Where(c => c != ' ').GroupBy(c => c.ToString().ToUpper());

​

        return grouped.Where(g => g.Count() >= 1).Count();  

    }

}

​

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