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When working with AutoMapper, you may encounter errors when trying to map properties from a source object to a target object, especially if the source contains properties that don't exist in the target. This is a common pitfall, but it’s easy to resolve once you understand the root cause.

The Issue:

Consider the scenario where we have a PostNoteRequest class containing two new fields: PublishDateTime and TimezoneId. These fields are necessary for calculating the CreatedAt property of the target Note class, but the Note class doesn’t have PublishDateTime or TimezoneId at all.

Here’s a typical AutoMapper mapping that results in an error:

CreateMap<PostNoteRequest, Note>()
    .ForMember(m => m.CreatedAt, _ => _.MapFrom((src,dst) =>
    {
        // Logic to calculate CreatedAt based on PublishDateTime and TimezoneId
    }));

Even though we're calculating CreatedAt based on the source properties, AutoMapper will try to map PublishDateTime and TimezoneId directly from the source to the target, resulting in an error like this:

Error mapping types.
Mapping types:
PostNoteRequest -> Note
HappyNotes.Models.PostNoteRequest -> HappyNotes.Entities.Note

The issue arises because AutoMapper expects all properties mentioned in the MapFrom expression to exist in both the source and target objects.

The Cause:

AutoMapper doesn't know how to handle fields (PublishDateTime and TimezoneId) that don’t exist in the target class (Note). The moment we reference these fields directly in the delegate passed to MapFrom, AutoMapper assumes they need to be mapped.

The Solution:

To solve this, we have two main approaches:

1. Use a Custom Value Resolver

A custom value resolver allows us to decouple the logic of calculating CreatedAt from the mapping process. It ensures that we don't reference non-existent fields in the target object.

public class CreatedAtResolver : IValueResolver<PostNoteRequest, Note, long>
{
    public long Resolve(PostNoteRequest source, Note destination, long member, ResolutionContext context)
    {
        if (!string.IsNullOrWhiteSpace(source.PublishDateTime) && !string.IsNullOrWhiteSpace(source.TimezoneId))
        {
            return DateTime.UtcNow.ToUnixTimeSeconds();
        }
        
        var dateStr = source.PublishDateTime;
        if (dateStr.Length == 10) dateStr += " 20:00:00";

        DateTime date = DateTime.ParseExact(dateStr, "yyyy-MM-dd HH:mm:ss", CultureInfo.InvariantCulture);
        TimeZoneInfo timeZone = TimeZoneInfo.FindSystemTimeZoneById(source.TimezoneId);

        return TimeZoneInfo.ConvertTime(new DateTime(date.Year, date.Month, date.Day, date.Hour, date.Minute, date.Second, DateTimeKind.Unspecified), timeZone).ToUnixTimeSeconds();
    }
}

// In your mapping configuration
CreateMap<PostNoteRequest, Note>()
    .ForMember(m => m.CreatedAt, opt => opt.MapFrom<CreatedAtResolver>());

This approach encapsulates the logic for CreatedAt calculation in a resolver, which avoids directly referencing PublishDateTime or TimezoneId in the target object.

2. Use ConstructUsing for Manual Construction

Another way to handle this is by manually constructing the target object in the ConstructUsing method, ensuring CreatedAt is set correctly without relying on AutoMapper’s default property mapping behavior.

CreateMap<PostNoteRequest, Note>()
    .ForMember(m => m.CreatedAt, opt => opt.MapFrom((src, dst) =>
    {
        var dateStr = src.PublishDateTime;
        if (dateStr.Length == 10) dateStr += " 20:00:00";
        
        DateTime date = DateTime.ParseExact(dateStr, "yyyy-MM-dd HH:mm:ss", CultureInfo.InvariantCulture);
        TimeZoneInfo timeZone = TimeZoneInfo.FindSystemTimeZoneById(src.TimezoneId);
        
        return TimeZoneInfo.ConvertTime(new DateTime(date.Year, date.Month, date.Day, date.Hour, date.Minute, date.Second, DateTimeKind.Unspecified), timeZone).ToUnixTimeSeconds();
    }))
    .AfterMap((src, dst) =>
    {
        dst.Tags = string.Join(" ", dst.TagList);
        dst.Content = dst.IsLong ? src.Content.GetShort() : src.Content;
    });

This method allows you to explicitly handle the CreatedAt logic while avoiding AutoMapper’s attempt to map non-existent properties from the source object.

Conclusion:

AutoMapper is a powerful tool, but it expects the properties referenced in its mapping expressions to exist in both the source and target objects. When you need to perform custom logic (like calculating CreatedAt based on other properties), using custom value resolvers or manual construction can help you avoid errors and maintain clear and maintainable code.

By understanding how AutoMapper expects the mapping to work, you can prevent issues related to missing properties and ensure your mappings are both efficient and correct.