Question

我有一个创建列表的linq查询。我只想在where使用第二个affiliationID != 0子句。有什么提示吗？

var associations = (from a in ftjent.Associations
                    join ap in ftjent.AssociationProducts on a.AssociationID equals ap.AssociationID
                    where ap.Product.Name == productName
                    where a.AffiliationID == affiliationID
                    select new
                        {
                            a.Acronym,
                            a.AssociationID,
                            a.Name
                         }
                     ).Distinct().OrderBy(assoc => assoc.Acronym);

Answer 1

将affiliationId与0进行比较，如果它没有值，则where将始终评估为true，并且不会进行第二次比较：

var associations = (from a in ftjent.Associations
                join ap in ftjent.AssociationProducts on a.AssociationID equals ap.AssociationID
                where ap.Product.Name == productName
                where (affiliationId == 0) ||  a.AffiliationID == affiliationID
                select new
                    {
                        a.Acronym,
                        a.AssociationID,
                        a.Name
                     }
                 ).Distinct().OrderBy(assoc => assoc.Acronym);

Answer 2

var associations = (from a in ftjent.Associations
                join ap in ftjent.AssociationProducts on a.AssociationID equals ap.AssociationID
                where ap.Product.Name == productName
                && (a.AffiliationID == affiliationID
                   || affiliationID == null || affiliationID == 0)
                select new
                    {
                        a.Acronym,
                        a.AssociationID,
                        a.Name
                     }
                 ).Distinct().OrderBy(assoc => assoc.Acronym);

基本上，不是试图只在一个条件下附加条件，这些类型问题的常见解决方案（这是常见的，相信我）是将条件否定为“或”，给出查询替代;这里，记录的affiliationID必须与条件匹配，或者条件值没有意义，因此所有记录的完整条件都为真。

Answer 3

另一种选择：

var associations = (from a in ftjent.Associations
                join ap in ftjent.AssociationProducts on a.AssociationID equals   ap.AssociationID
                where ap.Product.Name == productName               
                select new
                    {
                        a.Acronym,
                        a.AssociationID,
                        a.Name,
                        a.AffiliationID //add AffilationID into the unknown type

                     }
                 ).Distinct().OrderBy(assoc => assoc.Acronym);

if(affilationID != 0)
     associations = associations.Where(a=>a.AffiliationID == affiliationID);

这不是最短的代码，但它清楚地表明了流程的分离。

选择就是你。

Answer 4

使用查询理解语法很难做到这一点，但是在使用查询理解语法进行查询之前，您总是可以提取逻辑并创建另一个IQueryable。

var products = ftjent.AssociationProducts;
if (affiliationID != null)
    products = products.Where(x => x.AffiliationID = affiliationID);

var associations = (
    from a in ftjent.Associations
    join ap in products on a.AssociationID equals ap.AssociationID
    where ap.Product.Name == productName
    select new
    {
        a.Acronym,
        a.AssociationID,
        a.Name
    }
)
.Distinct()
.OrderBy(assoc => assoc.Acronym);

仅当变量具有值时，才在Linq中使用“where”语句

4 个答案: