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Upgrade assistant - overview

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Note: This documentation is archived because it was the preview version of the release of CMS 12/Commerce 14/Search & Navigation 14. See Get started developing with CMS.

See Installing and running for step-by-step instructions on how to install and run the upgrade assistant.

The Upgrade assistant automates the transition from an ASP.NET Standard project to an ASP.NET Core project. The tool employs the following steps. (You could plug in your own conversion steps.)

  1. Creates a backup
    Upgrade assistant creates a backup because it is going to change important things. You can disable this step if you already have a backup, such as in a source code repository. (Is it possible to choose backup location?)
  2. Identifies solution conversion order
    Upgrade assistant determines and recommends the order to migrate projects within a solution. You can accept the recommendation or decide on another order. See Analyzers.
  3. Runs try-convert
    Upgrade assistant runs the try-convert tool to convert the project file from a cs-style project to an SDK-style project. (are there any configuration options?) The try-convert CLI tool is a standard Microsoft package and is found on GitHub.
  4. Removes transitive NuGet package dependencies
    Old NuGet package dependencies that may have been present in packages.config are removed.
  5. Re-targets project to .NET 5.0
  6. Updates NuGet packages
    Upgrade assistant updates NuGet packages to versions compatible with .NET 5.0. A config file uses a list with old package names and versions to map to new package names and versions. If a package is not needed, it is removed. NuGet package references should be .NET Core standard compatible.
    Upgrade assistant removes old packages and transitive dependencies,  and then adds new packages. The tool then makes an extra restore to rebuild the lock files. After that, it runs a second pass to look for new transitive dependencies and removes those, just in case new unnecessary transitive dependencies were added by the new NuGet packages. This lets the tool remove redundant packages that were once needed but are unnecessary after the conversion.
  7. Adds template files
    Upgrade assistant adds template files that NET.Core needs for startup and hosting code that is typically used in Optimizely environments. You can tweak the template files later. See Startup code.
  8. Migrates app config files
    Upgrade assistant migrates non-C# files, like web.config, using an interface called IConfigUpdater that takes an array of config files and updates them to follow ASP.NET Core standard. You can also implement your own IConfigUpdater by dropping your libraries into a specific folder; they are picked up at runtime and converted. See  Application configuration files.
  9. Updates source code
    Upgrade assistant updates the source code by removing unnecessary namespace declarations that are not supported by ASP.NET Core, and adding namespace references that are supported instead. 
    Some cases cannot be handled automatically because there may be too many differences between modules, such as converting HTTP modules to middleware.  you must manually convert such cases.

When you have run the tool on your solution, the solution will most probably not build until you have manually completed some migration steps. Analyzers are added to help you identify some of the remaining changes needed after the tool runs.


Analyzer functionality is included in the Upgrade assistant to analyze your solution and determine the components that need to be migrated to ASP.NET Core standard. After the analyzers evaluate your code, you can let the Upgrade assistant automatically convert your solution to ASP.NET Core.

Some tasks are not fully automated. For example, the Upgrade assistant does not handle Razor views or CSHTML, but the analyzer functionality does. The analyzer functionality flags non-ASP.NET Core standard with green squiggles directly in the Visual Studio project. You can fix these errors with code quick fix functionality in Visual Studio.

The analyzer also works on an already-converted solution, so when you open your updated ASP.NET Core site in Visual Studio and pull in more code that was not converted to ASP.NET Core standard, the analyzer flags any code that does not follow .NET Core standard (for example, occurrances of System.Web or Owin).

The analyzer tool also works for non-framework libraries, such as business logic libraries.

  • If you have a library that targets ASP.NET Standard and you convert this into ASP.NET Core, it cannot be consumed by ASP.NET Standard anymore.
  • If you have a library with multiple solutions depending on it, other apps targeting this library cannot use it, so in this case you should have a full framework library and then a library specifically for ASP.NET Core code on the side, to make sure shared libraries can still be used by apps not on ASP.NET Core.

Startup code

One challenge with going from ASP.NET Standard to ASP.NET Core is that you no longer have a global.asax file, which allowed you to write code that would run in response to higher "system level" events, such as Application_Start, Application_End, Session_Start, Session_End.

Instead, you have two methods, Configure and ConfigureServices, and a Startup class, and those are significantly different. These cannot be migrated automatically because there are too many app-specific things in global.asax.

At step 5 in the migration process (adding templates), you need four startup files: Program.cs, Startup.cs, appsettings.json, and appsettings.Development.json. If they are missing, Upgrade assistant creates them. It pulls some template files, changes some namespaces (System.Web and Owin are no longer used, and are removed) and then adds them to the project. Your old Startup.cs is saved under the name Startup.old.cs.

In Startup.cs, you can configure services required by the app and define the app's request handling pipeline as a series of middleware components.

Program.cs is often very similar from project-to-project and should not require many changes.

Application configuration files replacing web.config

In previous versions of ASP.NET, web.config stored application configuration settings such as database connection strings, global variables, and so on. In ASP.NET Core, the application configuration settings can come from different configurations sources such as files, user secrets, environment variables, or command-line arguments.

The appsettings.json file stores configuration settings previously found in web.config. If you do not already have appsettings.json in your solution, the Upgrade assistant creates one for you (by adding a template file in step 5) and migrates your web.config settings to this file.

If you had a list of namespaces in web.config that were automatically imported into Razor views, (such as System.Web.Mvc.Html, System.Web.Routing, AlloyTemplates.Models.Pages, and AlloyTemplates.Models.Blocks), you cannot automatically import these namespaces in the views using ASP.NET Core. In ASP.NET Core, you must import them by the _ViewImports.cshtml file instead. Upgrade assistant creates _ViewImports.cshtml and moves the namespaces from web.config to this new file, along with TagHelpers because they can be useful in ASP.NET Core projects. Also, Upgrade assistant does not migrate namespaces starting with System.Web because they are not supported by ASP.NET Core; you should replace them with other namespaces.

The appsettings.{Environment}.json file is a configuration file where {Environment} equals the application's current hosting environments, such as Development, Staging, or Production. This is valuable as you can use different appsettings for different environments.


The Upgrade assistant is divided into two repositories. The first one, [], contains the general Upgrade assistant that handles general code conversions from ASP.NET standard to ASP.NET Core. The second repository, [], contains code that is Optimizely specific.

In this second repository, there is an ExtensionManifest.json file which contains the configuration settings for migrating Optimizely-specific code to ASP.NET Core. From the start, it looks something like this, with four sections for different configuration options for the migration steps.

  "ConfigUpdater": {
    "ConfigUpdaterPath": "ConfigUpdaters"
  "PackageUpdater": {
    "PackageMapPath": "PackageMaps"
  "TemplateInserter": {
    "TemplateConfigFiles": [
  • ConfigUpdater. For any configuration updaters.
  • PackageUpdater. Contains the map file that maps old NuGet packages and versions to new packages and versions.
  • SourceUpdater. For analyzers and code fix providers.
  • TemplateInserter. Contains template files for the Optimizely-specific code, such as Program.cs and Startup.cs.

Related information

If you are just starting to look at .NET Core and want to understand more about potential challenges in migrating any particular project .NET Core, start by looking at .NET Framework dependencies the project has, and third-party libraries or NuGet packages it depends on, and understand whether those dependencies are likely to work on .NET Core. Resources that can help with that analysis include:

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Last updated: Jul 02, 2021

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