Do i need to create automapper createmap both ways?

For the info of the people who stumble upon this question. There appears to be now a built-in way to achieve a reverse mapping by adding a .ReverseMap() call at the end of your CreateMap() configuration chain.

In AutoMapper you have a Source type and a Destination type. So you will be able to map between this Source type and Destination type only if you have a corresponding CreateMap. So to answer your questions:

1. You don't need to define the reverse mapping. You have to do it only if you intend to map back.
2. Yes, you need to call CreateMap to indicate that those types are mappable otherwise an exception will be thrown when you call Map<TSource, TDest> telling you that a mapping doesn't exist between the source and destination type.

I've used an extension method do mapping both ways

    public static IMappingExpression<TDestination, TSource> BothWays<TSource, TDestination>
        (this IMappingExpression<TSource, TDestination> mappingExpression)
    {
        return Mapper.CreateMap<TDestination, TSource>();
    }

usage:

 CreateMap<Source, Dest>().BothWays();

1. Yes, or you can call CreateMap<ModelClass, ViewModelClass>().ReverseMap().
2. If two classes have same Member(Property,Field,GetMethod()), you needn't call CreateMap<TSrc,TDest>. Actually, if every member in TDest are all exist in TSrc, you needn't call CreateMap<TSrc,TDest>. The following code works.

class Person
{
    public string Name { get; set; }
    public int Age { get; set; }  
}
class Person2
{
   public string Name { get; set; }
   public int? Age { get; set; }
   public DateTime BirthTime { get; set; }
}
public class NormalProfile : Profile
{
    public NormalProfile()
    {
       //CreateMap<Person2, Person>();//
    }
}
   
var cfg = new MapperConfiguration(c => 
{ 
    c.AddProfile<NormalProfile>();
});
//cfg.AssertConfigurationIsValid();
var mapper = cfg.CreateMapper();
var s3 = mapper.Map<Person>(new Person2 { Name = "Person2" });