编辑代码

using System;

public class HelloWorld
{
    public static void Main()
    {   
        //null合并运算符
        string s1 = null;
        string s2 = s1 ?? "nothing";    // s2 evaluates to "nothing"
        //null条件运算符
        System.Text.StringBuilder sb = null;
        string s = sb? .ToString();  // No error; s instead evaluates to null
    }
}