[release/3.1] Tools: Fix up App.config path on .NET Framework

[bricelam]

Fixes #19760

@ajcvickers Should we Servicing-consider this?

Description

For EF Core 2.x projects, we use AppDomainSetup.ConfigurationFile to specify which *.config file to use at design time.

In EF Core 3.0, we removed support for .NET Framework and simplified some design-time hooks since we no longer needed to support our AppDomain strategy for invoking user code.

In EF Core 3.1, we re-added support for .NET Framework but not our AppDomain strategy. Instead, we use a Reflection strategy to invoke user code. There is no API for specifying the *.config file without creating a new AppDomain. (see dotnet/runtime#931)

Customer Impact

APIs used to read the connection string from the *.config file (like ConfigurationManager) stopped working at design-time after upgrading to EF Core 3.1. For example, the Migrations command Update-Database started throwing NullReferenceException.

How found

We discussed this limitation as a team prior to releasing EF Core 3.1 and decided it was acceptable given that most EF Core users on .NET Framework were creating ASP.NET Core applications and, thus, using Microsoft.Extensions.Configuration and not System.Configuration.

Since releasing, several users have hit this limitation and filed issues requesting that we fix it.

Test coverage

Manually verified. Automated testing in this area has been very flakey in the past proving to not be very useful.

Regression?

Yes, from EF Core 2.2 to 3.1. EF Core 3.0 did not support .NET Framework.

Risk

Very low. If the added code fails (e.g. on Mono) execution continues with the same behavior that exists today.

[ajcvickers]

@bricelam If you're confident the fix is safe, then I'll take it to Tactics. It does make sense to fix this in 3.1 where .NET Framework is still supported.

[ajcvickers]

@bricelam Approved by Tactics for 3.1.7.

[bricelam]

Blocked waiting for branch to open.

[jklaus]

Nudge

[ajcvickers]

@jklaus This is approved for 3.1.7. It now needs to go through the patch build and validation process, which will start soon.

[jklaus]

@ajcvickers Thanks for the update.