TypeLoadException: Could not load type ‘Microsoft.AspNetCore.Mvc.MvcJsonOptions’ from assembly ‘Microsoft.AspNetCore.Mvc.Formatters.Json

Issue Description

While adding Swagger to .NET Core 3.0 gives below error,

TypeLoadException: Could not load type ‘Microsoft.AspNetCore.Mvc.MvcJsonOptions’ from assembly ‘Microsoft.AspNetCore.Mvc.Formatters.Json

Resolution:

The issue I found to be due to encompatiblity of existing swagger/swashbuckle packges with latest .NET Core i.e 3.0.

Below is how I was able to fix the issue.

• Please uninstall any old version of Swagger / Swashbuckle packages.

• Please install below Swashbuckle Nuget package using package manager console,

PM> Install-Package Swashbuckle.AspNetCore -Version 5.0.0

[Note: Please use latest available version]

Please see step by step on enabling Swagger(Open API) to.NET Core 3.0 web API.

Above package will install everything else required for swagger like,

• Swashbuckle.AspNetCore.SwaggerUI 5.0.0
• Swashbuckle.AspNetCore.Swagger 5.0.0
• Swashbuckle.AspNetCore.SwaggerGen 5.0.0

That’s all, You are all set with a fix, now you can run and test your swagger documentation.

Please make a note of a few breaking changes from .NET Core 2.2 to 3.0 in Swagger. I have detailed them in the article Swagger API Documentation in .NET Core 3.0.

Other useful References:

• Swagger API Documentation in .NET Core 3.0
• ASP.NET Core 3.0 – Use JWT Authorization Token in Swagger(OpenAPI)
• JWT Authentication in ASP.NET Core 3.0 with examples

Did the above steps resolve your issue?

Please sound off in the comments below!

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