using System;

using AutoMapper;

public class Program

{   

    public static void Main()

    {

        var config = new MapperConfiguration(cfg => cfg.CreateMap<ClassA, ClassB>().ForMember(src => src.B, opt => opt.MapFrom(dest => dest.A)));

        var mapper = new Mapper(config);

        var one = new ClassA() { A = null };

        var two = new ClassB() { B = "two" };

        // instance populated, maps as expected

        Console.WriteLine("output1");

        var output1 = mapper.Map(source: one, destination: two);

        Console.WriteLine(string.IsNullOrWhiteSpace(one.A));

        Console.WriteLine(string.IsNullOrWhiteSpace(output1.B));    

    }

}

​

public class ClassA 

{

    public string A {get; set;}

}

​

public class ClassB

{

    public string B {get; set;}

}