Mapping and Projecting enum strange behavior

[brenovieira]

It's quite a question father than an issue, because I don't know the right behavior. I'm using AutoMapper 4.0.4.

I'm trying to map and project (ProjectTo<>) some enum properties and the only working configuration I found was something like:

Mapper.CreateMap<Source, Destination>();
var m = Mapper.CreateMap<SourceType, DestinationType>();
m.ProjectUsing(s => (DestinationType)s);
m.ConvertUsing(s => (DestinationType)s);

//Source and Destination are classes
//SourceType and DestinationType are enums and properties of Source and Destination

• Using Mapper.Initialize(cfg => ...) or Mapper.AddProfile<Profile>() have the same behavior

Is there another way (simpler) to achieve that (mapping and projecting enums)?
I'm asking this because I have several enums and if I just map them, there is no need of creating enum mapping, but projecting them doesn't work "automatically".

Please see a gist I created here: https://gist.github.com/brenovieira/106b904f3899713e090d

[jbogard]

Well, to use projection, the LINQ provider needs to understand that ProjectUsing piece. The trick is to find the correct expression that EF I'm assuming will understand.

What behavior are you seeing?

[brenovieira]

The behavior I didn't except is based on 2 points:

1. To map an enum SourceType to another enum DestinationType, I don't need to call CreateMap<SourceType, DestinationType>(). AutoMapper "knows" this mapping by default.
   You said that long time ago at StackOverflow, so I didn't created it at first.
   In case of mapping a class Sourceto another class Destination, we must call CreateMap<Source, Destination>().
   But to use projection the behavior is the opposite:
   To enums, I must call CreateMap<SourceType, DestinationType>().ProjectUsing(s => ...), but to project classes I don't need to call .ProjectUsing(s => ...), only CreateMap<Source, Destination>() works.
2. To just map, I can configure AutoMapper like this:

Mapper.CreateMap<Source, Destination>();

And both mappings work: Mapper.Map<Source, Destination>(source);and Mapper.Map<SourceType, DestinationType>(sourceType);.

To just project, as I said in 1., I can configure it like this:

Mapper.CreateMap<Source, Destination>();
Mapper.CreateMap<SourceType, DestinationType>().ProjectUsing(s => (DestinationType)s);

And both projections work: (IQueryable<Source>).ProjectTo<Destination>(); and (IQueryable<SourceType>).ProjectTo<DestinationType>();.

The problem happens when I wanna map and project, because the configuration above doesn't work, so I need to configure it differently:

Mapper.CreateMap<Source, Destination>();
var m = Mapper.CreateMap<SourceType, DestinationType>();
m.ProjectUsing(s => (DestinationType)s);
m.ConvertUsing(s => (DestinationType)s);

In other words, I don't need to create enum mapping, but if I create it (to project, for instance), I also need to call another method (.ConvertUsing(...)) to tell AutoMapper how to map enum.

The gist I created has unit tests of the 3 use cases above.
What do you think, @jbogard ?

I believe that if it is the expected behavior, it should be explained in documentation at least. I searched on, but I found nothing about enum specifically. So I spent some time trying different configurations to find a working one :( hahaha

[jbogard]

AutoMapper has two modes, projection and mapping. Projection uses only
expressions to map, and mapping can do waaaaaaaay more things. Though your
two code snippets look the same, one uses Func and one uses Expression. The
piece that executes the mapping are also completely separate. The
projection doesn't actually map anything, it creates an expression tree for
some instructions to map, then the LINQ provider does the actual mapping.

Hence the difference.

If you want to map and project with low level mappings, you need to use
both HOWEVER I could see ProjectUsing only being needed for simple cases.

On Wednesday, August 19, 2015, Breno Vieira notifications@github.com
wrote:

The behavior I didn't except is based on 2 points:

To map an enum SourceType to another enum DestinationType, I don't
need to call CreateMap<SourceType, DestinationType>(). AutoMapper
"knows" this mapping by default.
You said that long time ago at StackOverflow
http://stackoverflow.com/questions/11265829/how-can-i-map-between-two-enums-using-automapper/11268711#11268711,
so I didn't created it at first.
In case of mapping a class Sourceto another class Destination, we must
call CreateMap<Source, Destination>().
But to use projection the behavior is the opposite:
To enums, I must call CreateMap<SourceType,
DestinationType>().ProjectUsing(s => ...), but to project classes I
don't need to call .ProjectUsing(s => ...), only CreateMap<Source,
Destination>() works.
2.

To just map, I can configure AutoMapper like this:

Mapper.CreateMap<Source, Destination>();

And both mappings work: Mapper.Map<Source, Destination>(source);and Mapper.Map<SourceType,
DestinationType>(sourceType);.

To just project, as I said in 1., I can configure it like this:

Mapper.CreateMap<Source, Destination>();
Mapper.CreateMap<SourceType, DestinationType>().ProjectUsing(s => (DestinationType)s);

And both projections work: (IQueryable).ProjectTo();
and (IQueryable).ProjectTo();.

The problem happens when I wanna map and project, because the
configuration above doesn't work, so I need to configure it differently:

Mapper.CreateMap<Source, Destination>();
var m = Mapper.CreateMap<SourceType, DestinationType>();
m.ProjectUsing(s => (DestinationType)s);
m.ConvertUsing(s => (DestinationType)s);

In other words, I don't need to create enum mapping, but if I create it
(to project, for instance), I also need to call another method
(.ConvertUsing(...)) to tell AutoMapper how to map enum.

The gist I created has unit tests of the 3 use cases above.
What do you think, @jbogard https://github.com/jbogard ?

I believe that if it is the expected behavior, it should be explained in
documentation at least. I searched on, but I found nothing about enum
specifically. So I spent some time trying different configurations to find
a working one :( hahaha

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[brenovieira]

@jbogard, I'm sorry for the delay.

AutoMapper has two modes, projection and mapping. Projection uses only expressions to map, and mapping can do waaaaaaaay more things. Though your two code snippets look the same, one uses Func and one uses Expression.

That's absolutelly correct.
Indeed, as I have like 30 enums, I made this method to create my enum mappings:

protected void CreateEnumMap<TEnum1, TEnum2>(Expression<System.Func<TEnum1, TEnum2>> exp,
    Expression<System.Func<TEnum2, TEnum1>> reverseExp, bool createReverse = true)
{
    var map = CreateMap<TEnum1, TEnum2>();
    map.ConvertUsing(exp.Compile());
    map.ProjectUsing(exp);

    if (createReverse) CreateEnumMap(reverseExp, exp, false);
}

The piece that executes the mapping are also completely separate. The projection doesn't actually map anything

That I'm not sure. I didn't dig into this part of the AutoMapper code, but as an AutoMapper user perspective I see that:

Mapper.CreateMap<Source, Destination>();
Mapper.Map<Source, Destination>(source); //this works

Mapper.CreateMap<Source, Destination>();
Mapper.CreateMap<SourceType, DestinationType>().ProjectUsing(s => (DestinationType)s);
Mapper.Map<Source, Destination>(source); //this doesn't work correctly

What I'm suggesting is AutoMapper creates the expression tree to enum-enum projections.
Then creating enum-enum mapping wouldn't be necessary to map and project.

In my large AutoMapper config, I just call .ProjectUsing() and .ConvertUsing() with enums.
That's a problem to maintenance, because I need treat class and enum separetely.

I map and project two POCO classes (with same property names), just callling CreateMap<Source, Destination>().

On the other hand, to enum I don't need to call CreateMap<> to use mapping.
But if I project it, I need to call CreateMap<> and ProjectUsing
Even worse, If I map and project it, I need to call CreateMap<>, ProjectUsing and ConvertUsing.

It's not a clear behavior to me and the documentation doesn't explain that.
Do you agree? hehe

[jbogard]

Yeah, in fact MapFrom does work on both sides which kinda invalidated my argument.

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