using System;

​

public static class Program

{

    public static void Main()

    {

        var obj = new Foo();

        ref var alias = ref obj.CreationTime;

        alias = DateTime.Now; // OK

        ref readonly var alias2 = ref obj.GetCreationTime();

        alias2 = DateTime.Now; // 編譯錯誤!

    }

}

​

public class Foo

{

    public DateTime CreationTime = DateTime.Now;

    public ref readonly DateTime GetCreationTime() => ref CreationTime;

}