如何让AutoMapper不缓存映射对象？

Question

When AutoMapper encounters an object that's already been mapped, it seems to use that object again, instead of trying to re-map it. I believe it does this based on .Equals() .

I have a tree that's being mapped. So, a node with some properties, and children. More than one of the nodes have the same value of .Equals() , because it's based off an Id property. The children of the nodes are different and I need those re-mapped, but it's using a cached map value.

Is there a way to turn the cached mapping off? All I can think of is implementing a new converter, but that totally defeats the purpose of using AutoMapper.

Here is an example on how to reproduce.

void Main()
{
    var source = new List<Tag>
    {
        new Tag 
        { 
            Id = 1, 
            Name = "Tag 1", 
            ChildTags = new List<Tag>
            {
                new Tag 
                { 
                    Id = 2, 
                    Name = "Tag 2", 
                    ChildTags = new List<Tag> 
                    {
                        new Tag {Id = 3, Name = "Tag 3"},
                        new Tag {Id = 4, Name = "Tag 4"}
                    }
                }
            }
        },
        new Tag { Id = 1, Name = "Tag 1" },
        new Tag 
        {
            Id = 3, Name = "Tag 3", ChildTags = new List<Tag>
            {
                new Tag {Id = 4, Name = "Tag 4"}
            }
        }
    };

    Mapper.CreateMap<Tag, Tag>();
    var results = Mapper.Map<IList<Tag>, IList<Tag>>(source);

    results.Dump();
}

public class Tag
{
    public int Id { get; set; }
    public string Name { get; set; }
    public IEnumerable<Tag> ChildTags { get; set; }

    public override bool Equals(Object obj)
    {
        if (obj == null)
        {
            return false;
        }

        var x = this;
        var y = (Tag)obj;

        return x.Id.Equals(y.Id);
    }

    public override int GetHashCode()
    {
        return Id.GetHashCode();
    }
}

Answer 1

There is now an option to disable the cache.

Mapper.CreateMap<Tag, Tag>();
var results = Mapper.Map<IList<Tag>, IList<Tag>>(source, opt => opt.DisableCache = true);

Answer 2

I've faced the same issue with the mapper, looking around i found that a solution for it, by adding

Mapper.Reset();

Source blog (corrected URL)

Answer 3

I also get the same issue. It doesn't happen when you map the same object twice - it happens when you have a tree heirarcy of objects, and the same value exists in two places of the tree (but with different child values) When mapping the second instance of the item - it uses the child values of the first instance, instead of re-evaluating what the child values should be.

Here is my example:

class Tag { 
  int Id {get; set;}
  string Name {get; set;}
  IEnumerable<Tag> ChildTags  {get; set;}
}

public void Test()
{
var source =  new List<Tag>
            {
                new Tag { Id = 1, Name = "Tag 1", ChildTags = new List<Tag>
                            {
                                new Tag { Id = 2, Name = "Tag 2", ChildTags = new List<Tag> 
                                            {
                                                new Tag {Id = 3, Name = "Tag 3"},
                                                new Tag {Id = 4, Name = "Tag 4"}
                                            }
                                    }
                            }
                    },
                new Tag { Id = 1, Name = "Tag 1" },
                new Tag {
                        Id = 3, Name = "Tag 3", ChildTags = new List<Tag>
                            {
                                new Tag {Id = 4, Name = "Tag 4"}
                            }
                    }
            };

Mapper.CreateMap<Tag, Tag>()
    .ForMember(dest => dest.ChildTags,
        opt => opt.MapFrom(src => src.ChildTags));
var result = Mapper.Map<IList<Tag>, IList<Tag>>(tags);
}

In the result

• the first instance of Tag 1 (ie source[0]) and all of its children are perfect

• the second instance of Tag 1 (ie source[1]) has all the children of the first instance - it should not have any children

• the second instance of Tag 3 (ie source[2]) does not have any children - it should have Tag 4 as a child

Answer 4

When AutoMapper encounters an object that's already been mapped, it seems to use that object again, instead of trying to re-map it. I believe it does this based on .Equals()

Can you explain why and when you see that ?

After a quick look in the source code, I'm sure there is no cache for objects. Here is a test that illustrate this :

   public class CustomerSource
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public DateTime DateOfBirth { get; set; }

        public int NumberOfOrders { get; set; }
    }

    public class CustomerTarget
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public DateTime DateOfBirth { get; set; }

        public int NumberOfOrders { get; set; }
    }

    [TestMethod]
    public void Test_AutoMapper()
    {
        Mapper.CreateMap<CustomerSource, CustomerTarget>();

        var source = new CustomerSource() { DateOfBirth = DateTime.Now, FirstName = "FirstName", LastName = "LastName", NumberOfOrders = int.MaxValue };

        var res1 = Mapper.Map<CustomerSource, CustomerTarget>(source);
        Console.WriteLine(res1.FirstName); // PRINT FirstName

        source.FirstName += "[UPDATED]";
        source.LastName += "[UPDATED]";

        var res2 = Mapper.Map<CustomerSource, CustomerTarget>(source);
        Console.WriteLine(res1.FirstName); // PRINT FirstName[UPDATED]

    }

Without your code, it is difficult to go more deeply. There is also a method Mapper.Reset() that clears the MapperEngine and the MapingConfiguration (all internal mapping expressions will be lost)

Answer 5

It seems that the Equals behaviour of the tree object you're mapping is inappropriate.

The method should only return true if "the specified object is equal to the current object." - http://msdn.microsoft.com/en-us/library/bsc2ak47.aspx

In your case, you have two tree objects sharing the same id but clearly, they are not "equal" since they have different children.

I suggest looking at why the Equals method has been abused in this way and whether you could get the behaviour you need by not overriding the Equals method, instead using a different method to check the tree id field with a more appropriate name eg. TreeIdsAreEqual.