Which Version of MSAL are you using ?
MSAL 4.23.0
MSAL Extensions 2.16.5
Platform
Xamarin.macOS
Is this a new or existing app?
a. The app is in production, and I have upgraded to a new version of MSAL
Repro
var storageProperties = new StorageCreationPropertiesBuilder("msal_cache.dat", Environment.GetFolderPath(Environment.SpecialFolder.Personal), ClientId)
.WithMacKeyChain(Foundation.NSBundle.MainBundle.BundleIdentifier, "msal_account")
.Build();
var cacheHelper = await MsalCacheHelper.CreateAsync(storageProperties).ConfigureAwait(false);
cacheHelper.RegisterCache(pca.UserTokenCache);
Expected behavior
After updating MSAL to 4.2.30 I get a PlatformNotSupportedException when trying to add a token cache which is being thrown here: https://github.com/AzureAD/microsoft-authentication-library-for-dotnet/blob/79b57039a076391fe350a736fe9c951322fdd422/src/client/Microsoft.Identity.Client/TokenCache.Notifications.cs#L156-L164
Actual behavior
No exception when adding custom token cache
Additional context/ Logs / Screenshots

I have the same issue.
Keeping it open until we ship 4.24
For reference, we're also using this on Xamarin.Mac in order to use the macOS keychain as the token cache.
Without this, iOS uses the keychain, but macOS does not.
Ack, please stay on MSAL 4.22 for now. Will get this shipped in 1-2 weeks.
Fixed in Msal release 4.24.0
Now that this issue is resolved, it would be great to add the macOS target to the existing Xamarin.Forms project as well.
@amirvenus : which existing Xamarin.Forms project? do you mean the test app in this repo? or a sample? (AAD? B2C?)
@amirvenus : which existing Xamarin.Forms project? do you mean the test app in this repo? or a sample? (AAD? B2C?)
I meant this repo:
https://github.com/Azure-Samples/active-directory-b2c-xamarin-native
Most helpful comment
Ack, please stay on MSAL 4.22 for now. Will get this shipped in 1-2 weeks.