VB.Net 中具有多个属性的 LINQ Group by

Question

我在这个问题上花了很多时间。我能够执行简单的 Group By LINQ 查询（在一个属性上），但对于多个字段我有点卡住了...... 这是我想要做的 LINQPad 示例：

dim lFinal={new with {.Year=2010, .Month=6, .Value1=0, .Value2=0}, 
            new with {.Year=2010, .Month=6, .Value1=2, .Value2=1},
            new with {.Year=2010, .Month=7, .Value1=3, .Value2=4},
            new with {.Year=2010, .Month=8, .Value1=0, .Value2=1},
            new with {.Year=2011, .Month=1, .Value1=2, .Value2=2},
            new with {.Year=2011, .Month=1, .Value1=0, .Value2=0}}

Dim lFinal2 = From el In lFinal
              Group el By Key = new with {el.Year,el.Month}
              Into Group
              Select New With {.Year = Key.Year, .Month=Key.Month, .Value1 = Group.Sum(Function(x) x.Value1), .Value2 = Group.Sum(Function(x) x.Value2)}

lFinal.Dump()
lFinal2.Dump()

lFinal 列表有 6 个项目，我希望 lFinal2 有 4 个项目：2010-6 和 2011-1 应该分组。

提前致谢。

Answer 1

使用Key关键字使匿名类型中的属性不可变，然后将它们用于比较

    Dim lFinal2 = From el In lFinal               
    Group el By Key = new with {key el.Year, key el.Month} 
    Into Group
    Select New With {
          .Year = Key.Year, 
          .Month = Key.Month, 
          .Value1 = Group.Sum(Function(x) x.Value1),
          .Value2 = Group.Sum(Function(x) x.Value2)
    } 

Answer 2

谢谢 ！
但我注意到我还需要编写 GetHashCode 函数才能使其正常工作。我提供最终类 + LINQ GroupBy 的 VB.Net 翻译：

类：

Public Class YearMonth
    Implements IEquatable(Of YearMonth)

    Public Property Year As Integer
    Public Property Month As Integer

    Public Function Equals1(other As YearMonth) As Boolean Implements System.IEquatable(Of YearMonth).Equals
        Return other.Year = Me.Year And other.Month = Me.Month
    End Function

    Public Overrides Function GetHashCode() As Integer
        Return Me.Year * 1000 + Me.Month * 100
    End Function
End Class

以及 LINQ 查询：

Dim lFinal2 = From el In lFinal
              Group el By Key = New YearMonth With {.Year = el.Year, .Month = el.Month}
              Into Group
              Select New ItemsByDates With {.Year = Key.Year,
                                            .Month = Key.Month,
                                            .Value1 = Group.Sum(Function(x) x.Value1),
                                            .Value2 = Group.Sum(Function(x) x.Value2)}

Answer 3

不是 100% 确定，但 group by 可能使用 Equals() 和/或 GetHashCode 实现，因此当您执行隐式创建时：

= Group el By Key = new with {el.Year,el.Month}

隐式对象不知道检查年份和月份（仅仅因为它具有属性不意味着它在与其他对象进行比较时检查它们）。

所以你可能需要做更多类似这样的事情：

= Group el By Key = new CustomKey() { Year = el.Year, Month = el.Month };

public class CustomKey{
    int Year { get; set; }
    int Month { get; set; }

    public override bool Equals(obj A) {
        var key (CustomKey)A;
        return key.Year == this.Year && key.Month == this.Month;
    }
}