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Yup, will use it instead of the StringBuilder.

@rzikm, could you help me with that weird error happening only for some builds?

src/libraries/System.Net.Mail/src/System/Net/Mail/MailAddressCollection.cs(54,40): error CS0122: (NETCORE_ENGINEERING_TELEMETRY=Build) 'ValueStringBuilder' is inaccessible due to its protection level

ValueStringBuilder is an internal type shared by linking the source file. You need to add it to the csproj like for example here:

That's exactly what I did:

And it's in the same block where MailAddressCollection.cs is, no more groups where the collection is mentioned. For whatever reason it's not included in some cases while I see no directives which might control that.

@YohDeadfall The build failure is in unit test project, you need to include the ValueStringBuilder.cs in src\libraries\System.Net.Mail\tests\Unit\System.Net.Mail .Unit.Tests.csproj as well

This is a gimmick of some unit test projects in this repo, we compile the same source codes again as part of unit test projects in order to be able to use internal APIs for testing.

@rzikm, now it looks fine, thank you!

CI is passing, the one failure is unrelated. looks good now.

Thanks a lot for the patience with the contribution!