Question

我有一个LINQ to Entity查询，它使用select将db中的原始数据转换为对象，在此过程中执行一些计算。计算重复了几次，所以我尝试重构LINQ来使用let。但是，这样做会严重影响性能。是否有替代LET的方法，我只能用于代码重用而不影响性能？

var defaultHours = 40;
var defaultOldPersonPoints = 100;
var defaultYoungPersonPoints = 50;

// Example with no let but lots of ugly, unreadable, redundant code
var exampleNoLet =
    (from p in people
     join op in otherPeople
         on p.PersonId equals op.PersonId into inner
     from outer in inner.DefaultIfEmpty(null)
     select new
     {
         AllocatedPoints = (p.PersonTypeId == (int)PersonType.Old
                            ? defaultOldPersonPoints
                            : p.PersonTypeId == (int)PersonType.Young
                            ? defaultYoungPersonPoints : 0)
                            + (int)(
                            (p.PersonTypeId == (int)PersonType.Old
                            ? defaultOldPersonPoints
                            : p.PersonTypeId == (int)PersonType.Young
                            ? defaultYoungPersonPoints : 0)
                            * (p.ContractedHours.HasValue
                            ? (p.ContractedHours.Value - defaultHours) / defaultHours : 0))
     });


// Using the LET allows me to clean up the code somewhat but causes a massive performance hit
var exampleUsingLet =
    (from p in people
     join op in otherPeople
         on p.PersonId equals op.PersonId into inner
     from outer in inner.DefaultIfEmpty(null)
     let defaultPoints = p.PersonTypeId == (int)PersonType.Old
                            ? defaultOldPersonPoints
                            : p.PersonTypeId == (int)PersonType.Young
                            ? defaultYoungPersonPoints : 0
     let contractedHourRatio = p.ContractedHours.HasValue 
                            ? (p.ContractedHours.Value - defaultHours) / defaultHours : 0
     select new
     {
         AllocatedPoints = defaultPoints + (int)(defaultPoints * contractedHourRatio)
     });

Answer 1

如果查询性能是问题所在，请考虑将其移至LINQ-to-Objects。例如，目前，您在投影中没有使用p以外的任何内容（我不清楚为什么您正在做似乎是左外 - 实际上加入，所以你可以做一些像：

var queryBase = from p in people
                join op in otherPeople
                on p.PersonId equals op.PersonId into inner
                from outer in inner.DefaultIfEmpty(null)
                select p;

var query = from p in queryBase.AsEnumerable() // <=== switch to LINQ-to-Objects
            let defaultPoints = p.PersonTypeId == (int)PersonType.Old
                        ? defaultOldPersonPoints
                        : p.PersonTypeId == (int)PersonType.Young
                        ? defaultYoungPersonPoints : 0
            let contractedHourRatio = p.ContractedHours.HasValue 
                        ? (p.ContractedHours.Value - defaultHours) / defaultHours : 0
            select new
            {
                AllocatedPoints = defaultPoints + (int)(defaultPoints * contractedHourRatio)
            });

这里，SQL查询应该非常简单;所有更复杂的工作都是在LINQ-to-Objects中完成的。

有没有替代LINQ让运算符没有达到性能？

1 个答案: