ChooseTheTypeMap.cs

void Main()
{
	var config = new AutoMapper.MapperConfiguration(c =>
	{
		c.CreateMap<B, A>();
		c.CreateMap<A, AB>();
		c.CreateMap<B, AB>();
	});
	var mapper = config.CreateMapper();
	var runtimeMapper = (IRuntimeMapper) mapper;
	var b = new B() { a = 1, b = 2 };
	var typeMap = config.ResolveTypeMap(typeof(A), typeof(AB));
	var options = new MappingOperationOptions(mapper.ServiceCtor);
	var context = new ResolutionContext(b, null, typeof(A), typeof(AB), typeMap, options, runtimeMapper);
	runtimeMapper.Map(context).Dump();
}

class A
{
	public int a { get; set; }
}

class B : A
{
	public int b { get; set; }
}

class AB
{
	public int a { get; set; }
	public int b { get; set; }
}