Could not load file or assembly Microsoft.Extensions.DependencyInjection.Abstractions, Version=1.1.0.0

Since you're using the project in a .net framework library, there's an issue with auto-generated binding redirects (might be resolved in the upcoming 15.3 update / 2.0 .net core CLI). To work around it, add this in your cpsroj file (preferably before any <Import> element for a .targets file if present):

<PropertyGroup>
  <AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>
  <GenerateBindingRedirectsOutputType>true</GenerateBindingRedirectsOutputType>
</PropertyGroup>

This should force MSBuild to create / update a YourProject.dll.config file containing the necessary binding redirects.

I Googled my exception below, and it brought me to this stakoverflow post.

System.IO.FileNotFoundException: 'Could not load file or assembly 'Microsoft.Extensions.OptionsModel, Version=1.0.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60' or one of its dependencies. The system cannot find the file specified.'

I had the relevant Binding Redirects but if I Delete all bin/obj folders it worked fine afterwards.

This is an old thread but I had a similar same issue after I updated my Azure function from dotnet core version 3 to 3.1.

Error message: Could not load file or assembly 'Microsoft.Extensions.DependencyInjection.Abstractions, Version=3.1.9.0

In this case you need to update the Azure function version to 'v3' in .proj file.