C# 菜鸟帮助:如果变量等于一个特定的字符串

最近启动了 C#，所以请原谅我，但我正在尝试将输入变量与 if 语句的字符串进行比较，但它似乎不起作用：

Console.WriteLine("Attack or Defend? (A/D): ");
choice = Console.ReadLine();
if (choice == "A")
{
    MonsterHealth = MonsterHealth - PlayerAttack;
    PlayerHealth = PlayerHealth - MonsterAttack;
}
if (choice == "D")
{
    PlayerHealth = PlayerHealth - MonsterAttack / 2;
}                    

if (choice.ToUpper() == "A")
{
...
}