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);
}