c# - linq中的switch case

Question

我有一个像这样的 linq 查询

 var tmp = (from t in db.sometable
            where (bunch of conditions)
            group t by new { t.somefield} into g
            select new
            {                               
                Name = g.Key.someobject.Name,
                xColor = g.Sum(a => (int?)a.LineItems.Where(m => m.Product == "x" && m.Color == true)
                                                     .Sum(m => m.Quantity)),
                xBW = g.Sum(a => (int?)a.LineItems.Where(m => m.Product == "x" && m.Color == false)
                                                  .Sum(m => m.Quantity))    
            })

现在我想优化这个查询。如果我必须写一个存储过程，我会写这样的东西:

if(a.LineItems.Product == 'x')
{
   if(m.Color == true)
     xColor++;
   else
     xBW++;
}

这样我就可以在一次扫描中获得聚合值。基本上我想优化这个查询以避免多次扫描。

Answer 1

在 Linqpad 中查看这个简单的代码片段，它按照您期望的方式在 visual studio 中运行，删除 Dump方法调用。我为一个简单的演示简化了类结构，它只是一个 POCO , 不是 complex type包含另一个 list里面

void Main()
{
    var testList = Test.CreateList();

    var tmp = (from t in testList
               group t by new { t.Name } into g
               select new
               {
                   Name = g.Key.Name,
                   xColor = g.Sum(a =>
                   {
                       if (a.Product == "x" && a.Color == true)
                           return a.Quantity;
                       return 0;
                   }),
                    xBW = g.Sum(a =>
                   {
                       if (a.Product == "x" && a.Color == false)
                           return a.Quantity;
                       return 0;
                   })
               });

    tmp.Dump();
}

public class Test
{
    public string Name { get; set; }

    public string Product { get; set;}

    public bool Color { get; set;}

    public int? Quantity { get; set;}

    public static List<Test> CreateList()
    {
        return new List<Test>()
        {
          new Test {Name="A",Color = true,Product="x",Quantity=5},
          new Test {Name="A",Color = true,Product="x",Quantity=5},
          new Test {Name="A",Color = true,Product="x",Quantity=5},
          new Test {Name="B",Color = true,Product="x",Quantity=5},
          new Test {Name="B",Color = true,Product="x",Quantity=5},
          new Test {Name="B",Color = true,Product="x",Quantity=5}         
        };
    }
}

但是这是否有效还有待商榷