src/Microsoft.Data.SqlClient/add-ons/AzureKeyVaultProvider/AzureSqlKeyCryptographer.cs:31

• [nitpick] The _keyFetchTaskDictionary now stores KeyVaultKey instances rather than tasks. Consider renaming it to better reflect its contents, e.g. _keyCacheDictionary or _fetchedKeys.

        private readonly ConcurrentDictionary<string, KeyVaultKey> _keyFetchTaskDictionary = new();

src/Microsoft.Data.SqlClient/add-ons/AzureKeyVaultProvider/AzureSqlKeyCryptographer.cs:175

• FetchKeyFromKeyVault retrieves keyClient but never initializes it. You need to call CreateKeyClient(vaultUri) before accessing _keyClientDictionary or handle the case where keyClient is null.

            return keyClient?.GetKey(keyName, keyVersion);

src/Microsoft.Data.SqlClient/add-ons/AzureKeyVaultProvider/AzureSqlKeyCryptographer.cs:81

• With the async/task-based guard removed, parallel calls for a missing key will all invoke FetchKeyFromKeyVault. Consider adding a lock or double-checked locking to prevent N+1 fetches under concurrency.

            if (_keyFetchTaskDictionary.TryGetValue(keyIdentifierUri, out KeyVaultKey keyVaultKey))

src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/SqlConnection.cs:279

• The method signature returns List<string>, but the collection expression [...] produces an array by default. This will cause a type mismatch. Either construct a List<string> explicitly or verify that C# 12 collection expressions target the declared return type.

                return [.. s_systemColumnEncryptionKeyStoreProviders.Keys];

src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/SqlConnection.cs:294

• Same as above: returning a collection expression for a method declared to return List<string> may not compile. Consider new List<string>(...) or confirm the collection expression creates a List<string>.

                return [.. _customColumnEncryptionKeyStoreProviders.Keys];