MissingMethodException when modify record in solution with mixed TFMs

[MaceWindu]

Description

It is possible to get MissingMethodException exception for init-only properties in some build configurations (see below)

Ready-to-run code:
ConsoleApp48.zip

Reproduction Steps

To use records in code, built against pre-net5.0 TFMs we need to define IsExternalInit class manually in those projects.

To reproduce issue we need at least two TFMs:

• one that doesn't have IsExternalInit, we will use netstandard2.0 as example
• one with IsExternalInit, let's use net7.0

Solution configuration:

• project A with both TFMs, record and IsExternalInit wrapped in #if !NET5_0_OR_GREATER (that's important part!)
• project B with netstandard2.0 TFM, record modification code using r with { Prop = new value} and refence to project A (for record definition)
• project C with net7.0 TFM calling modification code from project B with references to both project A and project B (another importand part!)

When we build project C we also get:

• netstandard2.0 binary from project B
• net7.0 binary from project A (if we don't have direct reference to A, it will be nestandard2.0 transitive build)

Trying to run project C binary will result in MissingMethodException exception which looks to me like some unnecessary (or not?) strict modreqs verification

Expected behavior

It just works (C) Todd Howard

Actual behavior

Unhandled exception. System.MissingMethodException: Method not found: 'Void TestRecord.set_Test(System.String)'.
   at Test.TestRecord()
   at Program.Main()

Regression?

No response

Known Workarounds

Workaround is actually easy - don't use conditional compilation when adding IsExternalInit to project so all TFM artifacts will use same class. Fortunatelly because this is internal class, it is possible to do without conflicts if you don't use InternalsVisibleTo

Configuration

No response

Other information

No response

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[jkotas]

To use records in code, built against pre-net5.0 TFMs

Note this is not fully supported configuration. https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/configure-language-version . Some of the new C# 9 features work fine in this configuration, some of the new C# 9 features do not work at all, and some of the new C# 9 features work only with workarounds like in this case.

The workaround that you have suggested above (make IsExternalInit definition internal) is fine. If the init-only property is public, a more complex workaround that uses type forwarders is needed - #63472 is an example of the more complex workaround.