按动态汇总分组 [英] Group by Dynamic Aggregate

问题描述

在下面的代码中，我想用其名称存储在feildtoretrive中的任何其他属性替换x.BaseSalary:

In the code below I want to replace x.BaseSalary with any other property whose name is stored in feildtoretrive:

var feildtoretrive  = "BaseSalary"
var groupcal = db.Employees.GroupBy(x=>x.Region)
                           .Select(group => new { 
                                    Key = group.Key, 
                                    Avg = group.Average(x => x.BaseSalary) 
                                                })
                           .ToList();

推荐答案

您需要

• 创建一个访问实例成员的MemberExpression
• 编译一个lambda表达式，该表达式为传递的实例参数返回该成员

(我假设Employees中的元素属于Employee类型)

(I assume that the elements in Employees are of type Employee)

// the parameter expression for the lambda (your 'x')
var parameter = Expression.Parameter(typeof(Employee));
// the property access expression (your 'x.BaseSalary' or 'x.<feildtoretrive')
var propAccess = Expression.PropertyOrField(parameter, feildtoretrive);
// the build-together and compiled lambda
 var expression = (Expression<Func<Employee, int?>>)Expression.Lambda(propAccess, parameter);

您现在可以将lambda用于x.Average呼叫:

You can now use lambda for the x.Average call:

new { Key = group.Key, Avg = group.Average(lambda) }

一个警告:现在仅适用于int?类型的成员.我缺乏一些如何独立于类型的经验，但是您可以计算出哪些类型的平均值呢?但是，如果有int或double成员，则可能需要另一个强制转换表达式.

A caveat: This only works now for members of type int?. I'm lacking a little experience on how to do this more type independent, but what types can you calculate an average over? But if there are int or double members, another cast expression maybe necessary.

(将返回类型更改为int?).根据 Ivan Stoev的评论我的后续问题，您可以尝试以下方法:

(changed the return type to int?). According to Ivan Stoev's comment on my follow up question you could try this:

new { Key = group.Key, Avg = group.AsQueryable().Average(expression) }

EF6 应该识别对AsQueryable()的调用，然后使用正确的Average方法(请注意，我将expression用作参数，而不是lambda). EF Core和linq2Sql 不会一起使用.

EF6 should recognize the call to AsQueryable() and then use the correct Average method (note that I use the expression as argument instead of the lambda). EF Core and linq2Sql won't work with that.

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