[NET8] Bound external constants on static libraries are being linked as non-external on the final app.

[Flash3001]

Steps to Reproduce

Using NET 8

1. dotnet new ios
2. dotnet add package Xamarin.Google.iOS.MobileAds --version 8.13.0.3
3. add var adType = Google.MobileAds.AdLoaderAdType.GamBanner.GetConstant() anywhere in code and run.

Expected Behavior

Google.MobileAds.AdLoaderAdType.GamBanner.GetConstant() should return a valid string.

Actual Behavior

null is returned

Environment

dotnet --version 8.0.201
running on macOS 14.3.1 (23D60)
Xcode Version 15.3 (15E204a)

The Cause

external constants in the static library (contained in a xcframework) are being imported as non-external on the final app.

GoogleMobileAds.xcframework

My App

This is the relevant binding: https://github.com/xamarin/GoogleApisForiOSComponents/blob/c7ae591364f73a4e61d47d765a1e537bb509422e/source/Google/MobileAds/Enums.cs#L28

Last Version it was Working?

I don't know for certain. I jumped from Xamarin.iOS straight to NET8

Example Project

linking_error.zip

Workaround

1. Manually create a list of symbols that should be external, but aren't.
2. Add the list to the list _CustomLinkFlags using -exported_symbols_list my_list.list

  <ItemGroup>
    <_CustomLinkFlags Include="-exported_symbols_list" />
    <_CustomLinkFlags Include="google_ads_symbols.list" />
  </ItemGroup>

My App with workaround

Sample project with workaround implemented

linking_error_workaround.zip

[Flash3001]

Just in case someone else sees the same issue and is googling about it, these are signs it is happening:

<Google> Delegate does not conform to the required protocol, nil. Add nil to Loader's protocol list.

or an exception is thrown when accessing AdSizeCons.Banner

[Flash3001]

Update to this, the workaround works fine for debug builds, but on release builds the constants are completely missing.

Adding this extra workaround made the trick for my release builds:

        <_ReferencesLinkerFlags Include="-u_GADAdSizeFluid"  Visible="false" />
        <_ReferencesLinkerFlags Include="-u_GADAdSizeFullBanner"  Visible="false" />
        <_ReferencesLinkerFlags Include="-u_GADAdSizeInvalid"  Visible="false" />
        <_ReferencesLinkerFlags Include="-u_GADAdSizeLargeBanner"  Visible="false" />
        <_ReferencesLinkerFlags Include="-u_GADAdSizeLeaderboard"  Visible="false" />
        <_ReferencesLinkerFlags Include="-u_GADAdSizeMediumRectangle"  Visible="false" />
        <_ReferencesLinkerFlags Include="-u_GADAdSizeFromCGSize" Visible="false" />
        <_ReferencesLinkerFlags Include="-u_GADAdSizeSkyscraper"  Visible="false" />
        <_ReferencesLinkerFlags Include="-u_kGADAdSizeSmartBannerLandscape" Visible="false" />
        <_ReferencesLinkerFlags Include="-u_kGADAdSizeSmartBannerPortrait"  Visible="false" />
        <_ReferencesLinkerFlags Include="-u_GADAdLoaderAdTypeCustomNative"  Visible="false" />
        <_ReferencesLinkerFlags Include="-u_GADAdLoaderAdTypeGAMBanner"  Visible="false" />
        <_ReferencesLinkerFlags Include="-u_GADAdLoaderAdTypeNative" Visible="false" />```

[rolfbjarne]

This is a duplicate of #20061, which we've already fixed.

The fix should be included in the next .NET service release.

[rolfbjarne]

Great analysis of the issue btw!