public class Demo
{
    public string name { get; set; }
}
string value = nameof(Demo.name); //"name"

public class Demo
{
    public string name { get; set; }
　　 public int age { get; set; }
    public void doSomeThing()
    {

    }
}

Demo demo = null;
//before       
if (demo != null) demo.doSomeThing();           
//after
demo?.doSomeThing();    
                                     
//before                       
string valuex = (demo != null) ? demo.name : "defaultValue";
//after
string valuey = demo?.name ?? "defaultValue";
// for int 
int? age = demo?.age; 
int age = (demo?age).GetValueOrDefault();

namespace Project
{
    public class Demo
    {
        public static void method()
        {

        }
    }
}

using static Project.Demo;

//before
Demo.method();
//after
method();

string value1 = "kelly";
string value2 = "penny";
//before
string value4 = string.Format("i love {0}, i have {1}", value1, value2); 
//after
string value3 = $"i love {value1}, i hate {value2}"; //i love kelly, i hate penny

public class Demo
{ 
    //before
    public string getString1(string value)
    {
        return $"i love {value}";
    }
    //after
    public string getString2(string value) => $"i love {value}";

    //before
    public string fullname1 {
        get {
            return getString1("kelly");
        }
    }
    //after
    public string fullname => getString1("kelly"); //this feature only support get 
}

//before
public class Demo1
{
    public Demo1()
    {
        this.name = "value";
    }
    public string name { get; set; }
}
//after
public class Demo2
{
    public string name { get; set; } = "value";
}

//before
Dictionary<string, string> data1 = new Dictionary<string, string>
{
    { "key1" , "value" },
    { "key2" , "value" }
};
//after
Dictionary<string, string> data2 = new Dictionary<string, string>
{
    ["key1"] = "value",
    ["key2"] = "value",
};