using System;

class A 

{

    public string B { get; set; }

}

​

public class Program

{

    public static void Main()

    {

        var a = new A();

        a.B = "initial string";

        DoSmth(a);

        Console.WriteLine(a.B);

    }

    static void DoSmth(A a) 

    {

        a.B = "new string";

        a = null;

    }

}