Help please (I dont get it)

Question

https://edabit.com/challenge/99oN5igrbXddAjHEL

why does it work this way:

public static string ReverseCase(string str)

{
str.ToCharArray();
for (int i = 0; i < str.Length; i++)
{
///////////this part is the important//////////////
if (char.IsUpper(str[i]))
{
char.ToLower(str[i]);
}
else
{
char.ToUpper(str[i]);
}
/////////////////////////////////
}
return str.ToString();
}
}
but not this way:
char.IsUpper(str[i]) ? char.ToLower(str[i]) : char.ToUpper(str[i]);

Answer 1

static string ReverseCase(string str) {
Char[] chars = str.ToCharArray();
for (int i = 0; i < chars.Length; ++i) {
chars[i] = Char.IsUpper(chars[i])? Char.ToLower(chars[i]):Char.ToUpper(chars[i]);
}
return new String(chars);
}

static void Main(string[] args) {
string str = "HelLo, WoRlD";
str = ReverseCase(str);
Console.WriteLine(str);
}

Ternary operator needs to be used as an rvalue.