Скачать или смотреть Resolving Automapper UseDestinationValue Issues for Lists

Discover how to successfully map a `List Sensor ` to `List Beer ` using Automapper without overriding existing values, including a simple fix!
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This video is based on the question https://stackoverflow.com/q/68296639/ asked by the user 'Eduardo Preuss Schmitz' ( https://stackoverflow.com/u/8904037/ ) and on the answer https://stackoverflow.com/a/68326702/ provided by the user 'Andrii Litvinov' ( https://stackoverflow.com/u/2138959/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Automapper and UseDestinationValue with Lists

When developing applications in C# , you often need to map one type of object to another. This is where Automapper comes in handy, providing a straightforward way to perform object-to-object mapping. However, issues can arise when trying to map collections or lists, especially if you wish to retain existing values in your destination objects. In today's guide, we will address a specific challenge: mapping a List<Sensor> to a List<Beer> without overriding the Name property of Beer with the Name property from Sensor.

The Problem

You have a scenario where you want to map values from sensor objects to beer objects. Here’s a brief look at your original mapping setup:

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The intention was clear: when mapping, if the Name in Sensor is null, it should not override the existing name in Beer. However, the mapping for collections wasn't performing as expected. Despite the correct setup for single objects, the list mapping was failing.

The Solution

Understanding the Missing Element: AddCollectionMappers()

The reason your mapping was not preserving the Name values in the Beer list boils down to a missing line of code.

Fix: To make the collection mapping behave correctly, you simply need to add cfg.AddCollectionMappers(); to your configuration. This line ensures that all mappings for collections are handled properly, including the setup for UseDestinationValue().

Updated Mapping Code

Here’s how your updated configuration will look with the necessary addition:

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How It Works

Here’s what happens with the updated code:

AddCollectionMappers(): This method configures Automapper to handle lists/collections by ensuring correct mapping behavior across the entire list.

ForMember and UseDestinationValue(): When mapping each Sensor to Beer, if the Name from Sensor is null, Automapper recognizes the existing value in Beer and retains it. Consequently, this prevents unintentional overwrites.

By utilizing the right configuration, you can now successfully maintain your intended values during list mapping.

Conclusion

Working with collections in Automapper might seem tricky at times, but understanding how to properly configure your mappings—especially with the AddCollectionMappers() method—can make all the difference. Now, you can efficiently convert a List<Sensor> to List<Beer>, all while retaining the existing data as you intended.

Key Takeaway: Don't forget to include AddCollectionMappers() when you're working with Automapper and lists. It ensures your mapping logic behaves as expected, saving you from unexpected data loss in your applications.

Thank you for joining us today! Stay tuned for more insights into streamlining your C# programming practices.