public void Exists_WhenRepositoryIsNull_ThrowsArgumentNullException <TKey>(TKey defaultKey)
        {
            // Arrange & Act & Assert
            var exception = Assert.Throws <ArgumentNullException>(
                () => RepositoryExtensions.Exists <IEntity <TKey>, TKey>(null, x => x.Id.Equals(defaultKey))
                );

            Assert.Equal("repository", exception.ParamName);
        }