FileLoadException: Could not load file or assembly System.Runtime, Version=4.2.0.0

This can happen if you create an ASP.NET Core 2.0 Web Application that targets the [.NET Core] platform, deploy to Azure (or deploy locally), and then change it to target the [.NET Framework] instead.

e.g. if you change

<PropertyGroup>
    <TargetFramework>netcoreapp2.0</TargetFramework>
</PropertyGroup>

to

<PropertyGroup>
    <TargetFramework>net461</TargetFramework>
</PropertyGroup>

but if you do not delete the existing files from Azure before redeploying you will get this error.

It can also happen in the following scenario:

• First create a .NET Core project with Visual Studio 2017 with:

  File -> New Project -> Visual C# > Web > Asp.NET Core Application`

• Choose the [.NET Core] Platform on the Second Screen.

• Deploy the web app to Azure.

• Then create a new Project with :

  File -> New Project -> Visual C# > Web > Asp.NET Core Application`

• Choose the .NET Framework setting on the Second Screen.

Redeploy without deleting the existing files from Azure will cause this error.

Hope that helps.

What worked for me was:

Simply, empty the bin folder in your project and rebuild. I had to manually delete the contents via windows explorer as 'clean' left files kicking around.

Job done.