Resolving the Package Error After Migrating from .NET Core 2.2 to 3.0

Описание: Learn how to fix the `Package Error` encountered during .NET Core migration, specifically the issue with `Microsoft.AspNetCore.Mvc.Formatters.JsonOutputFormatter`.
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This video is based on the question https://stackoverflow.com/q/64681480/ asked by the user 'Serlok' ( https://stackoverflow.com/u/9213093/ ) and on the answer https://stackoverflow.com/a/64693945/ provided by the user 'Yinqiu' ( https://stackoverflow.com/u/13973037/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Fixing the Package Error After Migrating from .NET Core 2.2 to 3.0

Migrating from one version of .NET Core to another can often lead to unexpected hurdles, especially when it comes to package compatibility. A common issue developers face during this migration is the package error stating: "Could not load type 'Microsoft.AspNetCore.Mvc.Formatters.JsonOutputFormatter' from assembly 'Microsoft.AspNetCore.Mvc.Formatters.Json'." This guide will help you navigate through the error and ensure a smooth upgrade to .NET Core 3.0.

Understanding the Issue

Upon upgrading from .NET Core 2.2 to 3.0, various packages need to be updated as the architecture and features may have undergone significant changes. The error mentioned usually arises due to outdated or incompatible package references in your project file (.csproj). In particular, using older versions of libraries that are not aligned with the new runtime can cause your application to crash during startup, specifically during the execution of MapControllers().

Steps to Resolve the Package Error

Here, we will break down the solution into clear sections to help you address the package error systematically.

1. Analyze Your Current Packages

Firstly, take a look at your project file and find the package references. The ones causing the issue are typically related to outdated libraries. In this case, check the following:

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2. Update Package Versions

Once you identify the packages that are outdated, it's time to upgrade them to a supported version for .NET Core 3.0. You should comment out the current package versions and replace them with the latest versions as follows:

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3. Using Package Management Tools

To ensure that you are always using the latest version of a library, consider:

Visual Studio's NuGet Package Manager: This tool shows you the listed packages and alerts you about updates.

Command Line Interface (CLI): Use commands to check for outdated packages. For instance, using dotnet list package --outdated can provide a quick overview of what needs to be updated.

4. Removing Unnecessary References

Sometimes you may have references that are no longer necessary or are causing conflicts. In this scenario, consider removing Microsoft.AspNetCore.Mvc.Core if it is not required, especially if you are already including Microsoft.AspNetCore.Mvc.NewtonsoftJson in version 3.0.0.

Conclusion

Migrating to a new version of .NET Core requires careful attention to the packages in your project. Outdated dependencies can lead to errors such as the one discussed, causing your applications to crash during startup. By updating your package references as outlined in this guide, you should be able to resolve the error and enjoy the advancements of .NET Core 3.0.

If you run into further issues, always consult the package documentation or the community forums. Happy coding!