Is there a simple way that I can sort characters in a string in alphabetical order

You can use LINQ:

String.Concat(str.OrderBy(c => c))

If you want to remove duplicates, add .Distinct().

Yes; copy the string to a char array, sort the char array, then copy that back into a string.

static string SortString(string input)
{
    char[] characters = input.ToArray();
    Array.Sort(characters);
    return new string(characters);
}

new string (str.OrderBy(c => c).ToArray())

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