c# - 是否可以重构此 nHibernate Linq 查询？

Question

目前我有以下代码:

switch (publicationType)
{
    case PublicationType.Book:
        return Session.Query<Publication>()
            .Where(p => p.PublicationType == PublicationType.Book)
            .OrderByDescending(p => p.DateApproved)                        
            .Take(10)
            .Select(p => new PublicationViewModel
            {
                ...
            });
    case PublicationType.Magazine:
        return Session.Query<Publication>()
            .Where(p => p.PublicationType == PublicationType.Magazine)
            .OrderByDescending(p => p.DateApproved)                        
            .Take(10)
            .Select(p => new PublicationViewModel
            {
                ...
            });
    case PublicationType.Newspaper
    .... 
}

如您所见，除了 publicationType 条件外，每次查询都是相同的。我试图通过创建一个采用 Func 的方法来重构它，例如

private IEnumerable<PublicationViewModel> GetPublicationItems(Func<PublicationType, bool>> pubQuery)
{
    return Session.Query<Publication>()
        .Where(pubQuery)                
        .OrderByDescending(p => p.DateApproved)                        
        .Take(10)
        .Select(p => new PublicationViewModel
        {
            ...
        });                
}

private bool IsBook(PublicationType publicationType)
{
    return publicationType == PublicationType.Book;
}

然后像这样调用这个方法

GetPublicationItems(IsBook);

但是当我这样做时，我得到了错误:InvalidCastException:无法将“NHibernate.Hql.Ast.HqlParameter”类型的对象转换为类型“NHibernate.Hql.Ast.HqlBooleanExpression”。

还有其他方法吗？

Answer 1

听起来你并不真的需要一个函数——你只需要一个PublicationType:

private IEnumerable<PublicationViewModel>
    GetPublicationItems(PublicationType type)
{
    return Session.Query<Publication>()
        .Where(p => p.PublicationType == type)
        .OrderByDescending(p => p.DateApproved)                        
        .Take(10)
        .Select(p => new PublicationViewModel
        {
            ...
        });                
}

如果您真的需要它比这更通用，您可能只需要更改代码以使用表达式树而不是委托(delegate)(并更改输入类型):

private IEnumerable<PublicationViewModel> GetPublicationItems(
    Expression<Func<Publication, bool>> pubQuery)
{
    return Session.Query<Publication>()
        .Where(pubQuery)                
        .OrderByDescending(p => p.DateApproved)                        
        .Take(10)
        .Select(p => new PublicationViewModel
        {
            ...
        });                
}

不过此时您将无法使用 GetPublicationItems(IsBook) 调用它。你可以这样做:

GetPublicationItems(p => p.PublicationType == PublicationType.Book)

或者:

private static readonly Expression<Func<Publication, bool>> IsBook =
    p => p.PublicationType == PublicationType.Book;


...

GetPublicationItems(IsBook)