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Learn how to effectively use Automapper to conditionally map properties based on whether they are in their default state or have changed.
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Handling Mapping in Automapper: Only Update When Properties Change

When working with C# and Automapper, you might find yourself needing a way to map properties only if they have changed from their default values. This is especially useful for scenarios where you want to avoid overriding existing data in a destination object with default values from a source object. In this guide, we will explore a practical solution to this problem and show you how to implement it in your projects.

The Problem

Let's consider a scenario involving two classes: Source and Destination. Both classes contain properties that start with preset default values. For example:

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The challenge arises when you need to map values from Source to Destination. You want the mapping to automatically replace Destination properties only when the corresponding properties in Source do not equal their default values. For example:

If Source.Foo is still "Foo" (its default), you want to simply keep the original default value in Destination.Foo.

If Source.Foo has been changed to something else, it should reflect that new value in Destination.Foo.

This requires a conditional mapping logic in Automapper, which we will detail below.

The Solution

To achieve this conditional mapping in Automapper, we can utilize the opt.Condition() method. Here's a refined, generic approach that you can easily implement without needing to configure each property manually:

Step 1: Create a Custom Profile

Begin by defining a profile that sets up the mappings between your source and destination classes. Here's how you can implement the FooBarProfile:

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Step 2: Implement OnlyIfChanged Method

The next step is to create a method called OnlyIfChanged, which will check if the properties in Source differ from their default values:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Create a Helper Method to Get Property Values

You will also need a method to fetch the value of a member from the source object:

[[See Video to Reveal this Text or Code Snippet]]

Summary

By implementing the above solution, you've created a robust mechanism using Automapper that will only map properties from Source to Destination if those properties have changed from their default values. This allows for cleaner data handling and avoids overwriting important values with defaults.

This pattern not only improves your data integrity but also leads to cleaner, more maintainable code.

Now, you can confidently use Automapper to perform conditional mappings based on changes rather than simply default values, keeping your application's data flow logical and effective.

Feel free to refer back to this guide whenever you encounter similar mapping scenarios in your C# projects!