C#中检查null的语法糖，非常实用

int? a = null;
int b = a ?? -1;
Console.WriteLine(b);  // output: -1

int? a = null;
a ??= -1;
Console.WriteLine(a);  // output: -1

using System;
public class C {
    public static void Main() {
        string i = null;
        int? length = i?.Length;
        Console.WriteLine(length ?? -1); //output: -1
    }
}

using System;
public class C {
    public static void Main() {
        string[] i = null;
        string result = i?[1];
        Console.WriteLine(result ?? "null"); // output:null
    }
}

using System;
public class C {
    public static void Main() {
        string[] i = null;
        Console.WriteLine(i?[1]?.Substring(0).Length); //不弹错误
        Console.WriteLine((i?[1]?.Substring(0)).Length) // System.NullReferenceException: Object reference not set to an instance of an object.
    }
}

//参数给予默认值
if(x == null) x = "str";
//替换
x ??= "str";


//条件判断
string x;
if(i<3) 
    x = y;
else 
{  
    if(z != null) x = z; 
    else z = "notnull";
}
//替换
var x = i < 3 ? y : z ?? "notnull"


//防止对象为null的时候，依然执行代码
if(obj != null) 
    obj.Act();
//替换
obj?.Act();

//Dictionary取值与赋值
string result;
if(dict.ContainKey(key))
{
    if(dict[key] == null) result = "有结果为null";
    else result = dict[key];
}
else 
    result = "无结果为null";
//替换
var result= dict.TryGetValue(key, out var value) ? value ?? "有结果为null" : "无结果为null";