MsalCacheHelper.UserRootDirectory returns empty string when run in Azure function environment

[ghost]

When running the following code in an azure function environment:

var storageProperties = new StorageCreationPropertiesBuilder(_cacheFileName, MsalCacheHelper.UserRootDirectory).Build();
MsalCacheHelper cache = await MsalCacheHelper.CreateAsync(storageProperties).ConfigureAwait(false);

I hit the following exception:

Exception: System.ArgumentException: Path cannot be the empty string or all whitespace. (Parameter 'path')
   at System.IO.Directory.CreateDirectory(String path)
   at Microsoft.Identity.Client.Extensions.Msal.CrossPlatLock..ctor(String lockfilePath, Int32 lockFileRetryDelay, Int32 lockFileRetryCount)
   at Microsoft.Identity.Client.Extensions.Msal.MsalCacheHelper.CreateCrossPlatLock(StorageCreationProperties storageCreationProperties)
   at Microsoft.Identity.Client.Extensions.Msal.MsalCacheHelper.CreateAsync(StorageCreationProperties storageCreationProperties, TraceSource logger)

While this code had run fine locally, we hit exceptions 100% of the time after deploying this code to our azure function. After some investigations, we realized that MsalCacheHelper.UserRootDirectory returns an empty string when in an azure function environment.

The MsalCacheHelper.UserRootDirectory code mentions that this value "Gets the user's root directory across platforms." It would be great if the azure function platform was included here!

[bgavrilMS]

Good point, this helper was only ever tested on Windows, Mac and Linux.

We'd appreciate a contribution to get this to work on Azure Functions, I'm not even sure if "root dir" concept exists in this case.