How to Fix the AutoMapper Mapping Exception in ASP.NET Core

Learn how to resolve the `AutoMapperMappingException` in your ASP.NET Core application by understanding the common pitfalls of mapping data types.
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How to Fix the AutoMapper Mapping Exception in ASP.NET Core

If you've encountered the AutoMapperMappingException error while building an ASP.NET Core application, you're not alone. This exception often arises when AutoMapper cannot find the necessary type mapping configuration or supports the mapping you are trying to perform. In this guide, we will walk you through understanding this error and how to resolve it effectively.

The Problem: Mapping Exception

While developing a simple inventory application to store and retrieve details about devices and their users, a common issue was faced when attempting to display a list of devices tied to their owners. When the data retrieval process was triggered, the application threw the following error:

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

This issue usually indicates that AutoMapper lacks a defined mapping for the types you are attempting to convert, or your existing mappings are not set up correctly. A closer inspection of the code can help pinpoint the problem.

Analyzing the Code

Let's take a look at the relevant parts of the code, particularly the mapping configurations and the method responsible for retrieving assets.

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

This indicates you are trying to create two bidirectional mappings: one between the Asset model and the AssetVM view model, and another between AppUser and AppUsersVM.

2. Data Retrieval in GetAssets Method

Here’s how you initially attempted to retrieve and map the data:

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

In the above snippet, you are trying to map a list of Asset to a single AssetVM, which leads to the AutoMapperMappingException.

Solution: Adjusting the Code

1. Mapping Lists Correctly

Since you are retrieving a list of assets, the mapping should reflect this. We need to change the mapping from a single AssetVM to a list of AssetVM. Here’s the corrected line of code:

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2. Updating the Method Signature

In line with mapping a list, you must also update your method return type. Change the method declaration from:

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to:

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

3. Fixing the User Details Mapping

Lastly, ensure that the AppUser in the AssetVM is named correctly to avoid any mapping mishaps. Instead of public AppUsersVM AppUsers { get; set; }, ensure it's done like:

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

Summary of Changes Made

To summarize, here’s what you need to change in your code:

Mapping Correction: Change the line to properly map a list.

Return Type Update: Modify the async method interface to return a list instead of a single object.

Typo Fix: Ensure proper naming conventions for all properties to prevent mapping errors.

Final Thoughts

By following the steps outlined in this post, you can effectively resolve the AutoMapperMappingException in ASP.NET Core. It’s crucial to ensure that your mappings are configured to handle collections properly and that the property names match correctly between your source and destination objects.

As you continue learning and growing as a programmer, don't hesitate to seek feedback or recommendations from peers. Understanding why certain errors occur is a key part of building resilient applications.

Happy Coding!