How not to lose properties that are present in my

I have a following class structure:

public class A
{
    public bool Property1 { get; set; }
    public bool Property2 { get; set; }
}

public class ContainerForA
{
    public A A { get; set; }
}

public class A1
{
    public bool Property1 { get; set; }
}

public class ContainerForA1
{
    public A1 A { get; set; }
}

I create a mapping for this set of classes:

Mapper.CreateMap<A1, A>();
Mapper.CreateMap<ContainerForA1, ContainerForA>();

I create an instance of this set of classes:

        var cnt_a = new ContainerForA()
                        {
                            A = new A()
                                    {
                                        Property1 = false,
                                        Property2 = true
                                    }
                        };

        var cnt_a1 = new ContainerForA1()
                         {
                             A = new A1()
                                     {
                                         Property1 = true
                                     }
                         };

If I call Mapper.Map(cnt_a1.A, cnt_a.A) I'm getting the result I was expecting: both properties (Property1 and Property2) of object cnt_a are true

But if I call Mapper.Map(cnt_a1, cnt_a) I'm getting true for Property1 and false for Property2. Could someone explain me why? And is there any option for me to declare my mappings in the way so I won't lose properties that are present in my destination object but are NOT in my source object?

I would guess that when you map from ContainerForA1 to ContainerForA, that when mapping the property A, it creates a new instance of A for ContainerForA rather than using the existing one. This will use the default values for all of the properties, which is false for a bool.

How do we work around this?

First, we need to tell AutoMapper to not overwrite the property A on ContainerForA. To do that, we will tell AutoMapper to ignore the property.

Mapper.CreateMap<ContainerForA1, ContainerForA>()
    .ForMember(cForA => cForA.A, option => option.Ignore());

Now we need to update A manually, using AfterMap

Mapper.CreateMap<ContainerForA1, ContainerForA>()
    .ForMember(cForA => cForA.A, option => option.Ignore())
    .AfterMap((cForA1, cForA) => Mapper.Map(cForA1.A, cForA.A));

You will probably want to add some checks to the AfterMap method to ensure that cForA.A is not null. I'll leave that for you.