How to resolve "Could not load type 'Microsoft.AspNetCore.Mvc.MvcJsonOptions' " in .Net core 3.1?

Question

when I am running my application then it's showing me error like Could not load type 'Could not load type 'Microsoft.AspNetCore.Mvc.MvcJsonOptions' from assembly 'Microsoft.AspNetCore.Mvc.Formatters.Json, Version=3.1.1.0, Culture=neutral, PublicKeyToken=adb9793829ddae60.

My csproj file contains code like below :

<Project Sdk="Microsoft.NET.Sdk.Web">

  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <RootNamespace>Insurance_Customer_Portal_API</RootNamespace>
    <AddRazorSupportForMvc>true</AddRazorSupportForMvc>
  </PropertyGroup>

  <ItemGroup>
    <FrameworkReference Include="Microsoft.AspNetCore.App" />
    <PackageReference Include="Microsoft.AspNetCore.Cors" Version="2.2.0" />
    <PackageReference Include="Microsoft.AspNetCore.Mvc.Versioning.ApiExplorer" Version="4.1.1" />
    <PackageReference Include="SwashBuckle.AspNetCore.MicrosoftExtensions" Version="0.5.0" />
    <PackageReference Include="Swashbuckle.AspNetCore.SwaggerUI" Version="5.0.0" />
  </ItemGroup>

</Project>

Is there any things which I need to change in code ?

alsami · Accepted Answer · 2020-03-09 13:10:35Z

2

You have some dependencies in your project that are not required anymore, which are causing a mismatch of actually used DLLs that end up in the build-output.

When your project uses Microsoft.NET.Sdk.Web and >= netcoreapp3.0, a reference to Microsoft.AspNetCore.App is not required so that line can be safely removed.
Microsoft.AspNetCore.Cors is also part of the framework now so you don't need an explicit reference.
Last but not least, add a NuGet reference to Microsoft.AspNetCore.Mvc.NewtonsoftJson which can be found here.

edited Mar 9, 2020 at 13:10

answered Mar 9, 2020 at 12:49

alsami

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4 Comments

Harry R Over a year ago

So should I add <FrameworkReference Include="Microsoft.AspNetCore.App" Version="3.1.0" /> in csproj file ?

alsami Over a year ago

No, you have to remove it, as stated before.

Harry R Over a year ago

I am using 1. Microsoft.AspNetCore.Mvc.NewtonsoftJson, 2. Microsoft.AspNetCore.Mvc.Versioning.ApiExplorer, 3. SwashBuckle.AspNetCore.MicrosoftExtensions, 4. Swashbuckle.AspNetCore.SwaggerUI. But still it's showing me same error. Still am I wrong on any page ?

alsami Over a year ago

I just saw that SwashBuckle.AspNetCore.MicrosoftExtensions has very old dependencies and has not been updated since 2018. Can you try removing the dependency?

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