Issues with Bidirectional Mapping Configuration and Generic 'Count' Method Error in AutoMapper v6

[jordidalmauokay]

When I want to map an expression on TSource to TTarget, it makes sense that my mapping configuration defines a map from TSource to TTarget. However, it also needs a map from TTarget to TSource. Why is that?

Also, mapping a certain expression, that worked on AM v6, now gives me this error, but I have no idea how to fix this:

No generic method 'Count' on type 'System.Linq.Enumerable' is compatible with the supplied type arguments and arguments. No type arguments should be provided if the method is non-generic.

[palonza]

Hi @jordidalmauokay,

Due to the error 'generic method 'Count' on type 'System.Linq.Enumerable':
This error commonly occurs when you try to count elements in a sequence without providing a predicate or a specific element type.
Do you have the block code? (Without seeing the specific code or expression causing the error, it's a bit challenging to provide a precise solution.)

Reverse mapping from TTarget to TSource:
CreateMap<TTarget, TSource>();

Without the reverse mapping, AutoMapper wouldn't know how to map from TTarget back to TSource, causing issues in scenarios where bidirectional mapping is necessary.

Declarative programming language in certain aspects. LINQ enables declarative queries on collections of data and various data sources. This approach makes it easier to write queries in a declarative style, where you specify what you want to achieve and not how to do it.

[jordidalmauokay]

Hi @palonza, Thanks for the quick reply,

It took me all day, but I managed to extract a repro project that demonstrates the issue.
The problem seems to be related to reusing a source type for a second target type. I would think this should be supported by AutoMapper, as mapping the objects directly works fine. Mapping an expression on the source is what's not working.

Hopefully my repro will be clear. You can find the code here:
https://github.com/jordidalmauokay/automapper-bidirectionalissue

[palonza]

Hi @jordidalmauokay,

Please check my solution and let me know if it helped you.

RESOLUTION 01

In Program.cs, in your original code, please replace this line 57...:

Expression<Func<TargetType, bool>> target1sWithListItemsExpr = mapper.MapExpression<Expression<Func<TargetType, bool>>>(sourcesWithListItemsExpr);

...with the following:

Expression<Func<TargetType, bool>> target1sWithListItemsExpr = target => target.Id != 0 && target.ItemList != null && target.ItemList.Any();

Now run your code, and it should work fine.

git checkout -b solution-02

CAUSE

The issue lies in the expression; the mapping configuration (which also includes the reverse) is designed to convert a source entity to a target entity and vice-versa, a target entity to a source entity.

You are attempting to reverse an expression that expects one type Expression<Func<TargetType, bool>> and you try to pass another type Expression<Func<SourceType, bool>>.

Important: in your map configuration, reverse map works to convert from entities to entities or from entities to DTOs, in fact, the expressions have delegates.

RESOLUTION 02

To solve it, you should modify 3 lines in your code.

In the file SourceChildType.cs, you should add the SourceChildListItemType class starting after line 7. Additionally, you should change the return type of ItemList in line 6 to IEnumerable<SourceChildListItemType>.

Then, in the Program.cs class, you should change line 19, replacing SourceListItemType with SourceChildListItemType.

Replace line 19 in Program.cs for this line:

cfg.CreateMap<SourceChildListItemType, TargetChildListItemType>().ReverseMap();

Replace SourceChildType.cs with this (note that line 6 is modified, and the new class SourceChildListItemType is added at the end:

namespace ExpressionMappingIssue
{
    internal class SourceChildType
    {
        public int Id { get; set; }
        public IEnumerable<SourceChildListItemType> ItemList { get; set; } // Uses same type (SourceListItemType) for its itemlist as SourceType //Automapper cannot infer type SourceListItemType
    }

    internal class SourceChildListItemType
    {
        public virtual int Id { get; set; }
    }
}

Now run your code, and it should work fine.

git checkout -b solution-03

CAUSES

Also, in the SourceChildType class, it is slightly different from its counterpart TargetChildType. Take note of the return of ItemList is of type SourceListItemType, while its counterpart in TargetChildType is TargetChildListItemType. What happens is that AutoMapper cannot automatically infer that there is a different type in SourceChildType in the ItemList. When executing the reverse expression (line 57), specifically in target1sWithListItemsExpr, it fails because it expects SourceChildListItemType, and in the reverse, it encounters SourceListItemType. When using Any(), it doesn't find any, and I realized this because I temporarily replaced Any with Count()>0 there, and it also didn't work. AutoMapper, in its documentation, states that 'AutoMapper cannot “guess” which specific child destination mapping to use.'

Visit AutoMapper Documentation.

[jordidalmauokay]

Hi @palonza,

Thank you for your help! I checked your solution, and it worked perfectly!

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