c# - 如何在仅 EF4.1 代码场景中重用部分 Linq to Entity Select 表达式

Question

这是我拥有的非常简化的代码版本:

class PrintJob : IEntity
{
    public string UserName { get; set; }
    public string Departmen { get; set; }
    public int PagesPrinted { get; set; }
}

class PrintJobReportItem
{
    public int TotalPagesPrinted { get; set; }
    public int AveragePagesPrinted { get; set; }
    public int PercentOfSinglePagePrintJobs { get; set; }
}

class PrintJobByUserReportItem : PrintJobReportItem
{
    public string UserName { get; set; }
}

class PrintJobByDepartmenReportItem : PrintJobReportItem
{
    public string DepartmentName { get; set; }
    public int NumberOfUsers { get; set; }
}

然后我有两个问题:

var repo = new Repository(...);

var q1 = repo.GetQuery<PrintJob>()
    .GroupBy(pj => pj.UserName)
    .Select(g => new PrintJobByUserReportItem
    {
    #region this is PrintJobReportItem properties
        TotalPagesPrinted = g.Sum(p => p.PagesPrinted),
        AveragePagesPrinted = g.Average(p => p.PagesPrinted),
        PercentOfSinglePagePrintJobs = g.Count(p => p.PagesPrinted == 1) / (g.Count(p => p.PagesPrinted) != 0 ? g.Count(p => p.PagesPrinted) : 1) * 100,
    #endregion    
        UserName = g.Key
    });

var q2 = repo.GetQuery<PrintJob>()
    .GroupBy(pj => pj.Departmen)
    .Select(g => new PrintJobByDepartmenReportItem
    {
    #region this is PrintJobReportItem properties
        TotalPagesPrinted = g.Sum(p => p.PagesPrinted),
        AveragePagesPrinted = g.Average(p => p.PagesPrinted),
        PercentOfSinglePagePrintJobs = g.Count(p => p.PagesPrinted == 1) / (g.Count(p => p.PagesPrinted) != 0 ? g.Count(p => p.PagesPrinted) : 1) * 100,
    #endregion    
        DepartmentName = g.Key,
        NumberOfUsers = g.Select(u => u.UserName).Distinct().Count()
    });

从这 2 个查询中提取我为 TotalPagesPrinted、AveragePagesPrinted 和 PercentOfSinglePagePrintJobs 赋值的部分有什么建议，以便它可以重复使用并遵循 DRY 原则。

我只使用 EF 4.1 代码方法，无法切换到另一种技术或方法。此外，我无法具体化该数据，我需要将其保留为查询，因为我的网格组件稍后会添加更多内容进行查询，因此我无法切换到 Linq to Object。

Answer 1

我将创建一个具有两个属性的新类 CLASSNAME

• PrintJobReportItem 类型
• 分组 IEnumerable<IGrouping<TKey, TSource>>

然后创建一个扩展方法

public static IQueryable<CLASSNAME> EXTENSIONNAME<TKey, TSource>(this IEnumerable<IGrouping<TKey, TSource>> source)
{
  return from g in source
         select new CLASSNAME
         {
           PrintJobReportItem = new PrintJobReportItem
                                {
                                  TotalPagesPrinted = g.Sum(p => p.PagesPrinted),
                                  AveragePagesPrinted = etc...,
                                  PercentOfSinglePagePrintJobs = etc...,
                                },
           GROUPING = g
         };
}

然后像这样使用，我没有测试过，但我认为它会起作用

var q1 = repo.GetQuery<PrintJob>()
    .GroupBy(pj => pj.UserName)
    .EXTENSIONNAME()
    .Select(g => new PrintJobByDepartmenReportItem
                 {
                    PrintJobReportItem = g.PrintJobReportItem,
                    DepartmentName = g.GROUPING.Key,
                    NumberOfUsers = g.GROUPING.Select(u => u.UserName).Distinct().Count()

                 });