C# 无奖竞猜第二弹：输出结果是什么？

🏷️ C#

static void Main(string args)
{
    try
    {
        StringBuilder builder = new StringBuilder("world, ");

        AppendHello(ref builder);
        Console.WriteLine(builder.ToString());

        ClearBuilder(ref builder);
        Console.WriteLine(builder.ToString());
    }
    catch (Exception e)
    {
        Console.WriteLine(e.Message);
    }
    finally
    {
        Console.ReadLine();
    }
}

static void AppendHello(ref StringBuilder builder)
{
    builder.Append("hello!");
}

static void ClearBuilder(ref StringBuilder builder)
{
    builder = null;
}