Question

我使用实体框架6.1.3和.net框架4.5.1与C＃lang。

我想做的是;我想将表达式与if-else语句结合起来这是我的表达

Expression<Func<Article, bool>> expression = 
    q => (!newsDayStart.HasValue || q.PublishedOn >= newsDayStart) &&
         (!newsDayEnd.HasValue || q.PublishedOn <= newsDayEnd) &&
         (!categoryId.HasValue || q.CategoryId == categoryId.Value) &&
         (string.IsNullOrEmpty(searchText) || q.Title.Contains(searchText) &&
         (!isActive != null || q.IsActive == isActive.Value));

到

Expression<Func<Article, bool>> expression = ......;

if ( newsDayStart.HasValue )
{
   //Obviosly += this statement will not work.
   expression += q => q.PublishedOn > = newsDayStart

}

//TODO write other if else statements...

//Send expression
_context.Articles.Where(expression).Count();

Answer 1

如果这是专门用于EF查询，那么您可能会发现链接Where（）调用更容易实现相同的效果。

Expression<Func<Article, bool>> expression = ......;

//Send expression
var query = _context.Articles.Where(expression)

if ( newsDayStart.HasValue )
{
    query = query.Where(q => q.PublishedOn > = newsDayStart);
}

query.Count();

<强> *修改

您可以尝试使用此第三方库PredicateBuilder http://www.albahari.com/nutshell/predicatebuilder.aspx

创建Linq表达式（组合表达式）

1 个答案: