First non-repeated character C# LINQ

I've tried some answers from Stackoverflow but they don't count register of the symbols. For example

sTreSS => T (not 's'), stress => t

Here is what i tried

public static char FirstNonRepeatedCharacter(string s)
        {
            var output = s.GroupBy(item => item).First(x => x.Count() == 1).Key;
            return output;
        }

I need to edit the code with case-insensitive and return the correct register

sEVeraL -> s; - SomeBody - S

Solution

This is exactly what you need, but not fully in LINQ. In my opinion, you don't have to strictly rely on LINQ...

Console.WriteLine(FirstNonRepeatedCharacter("sTreSS"));
Console.WriteLine(FirstNonRepeatedCharacter("stress"));
Console.WriteLine(FirstNonRepeatedCharacter("sEVeraL"));
Console.WriteLine(FirstNonRepeatedCharacter("SomeBody"));
Console.WriteLine(FirstNonRepeatedCharacter("AaBbCc"));
Console.ReadKey();

static char? FirstNonRepeatedCharacter(string s)
{
    // Gather the count for each character (case insensitive, example: 's' and 'S' is in the same group).
    var counts = new Dictionary<char, int>();
    foreach (var ch in s.ToLower())
    {
        counts[ch] = counts.TryGetValue(ch, out var count)
            ? count + 1
            : 1;
    }

    // Return first character with count 1.
    return s.FirstOrDefault(ch => counts[char.ToLower(ch)] == 1);
}

Output is:

nothing (null)