如何正确执行“分组依据"?在LINQ上的VB代码里面的DataTable? [英] How to perform correctly "group by" in linq on DataTable inside vb code?

问题描述

为什么以下group by子句不起作用? 最初的问题是如何在LINQ中使用DataTable在vb代码(dot.net v4.0)中执行分组并在分组上求和?这是示例，但未产生所需的输出. 它返回2行而不是1行.

Why isn't the following group by clause working? The original question was how do I perform group in LINQ inside vb code (dot.net v4.0) with DataTable and sum on the group? This was the sample, but it's not producing the desired output. It returns 2 lines instead of one line.

    Dim table As New DataTable()
    table.Columns.Add("GroupName", GetType(String))
    table.Columns.Add("ProductName", GetType(String))
    table.Columns.Add("QTY", GetType(Integer))

    table.Rows.Add("a", "b", 1)
    table.Rows.Add("a", "b", 1)


    Dim testQuery =
                     (From e In table
                      Group e By Key = New With {
                        .GroupName = e("GroupName"),
                        .ProductName = e("ProductName")
                      } Into Group
                      Select New With {
                        .ProductName = Key.ProductName,
                        .QTY = Group.Sum(Function(x) Convert.ToInt32(x("QTY"))),
                        .GroupName = Key.GroupName
                     })

推荐答案

在VB.NET中，按组合键分组的语法是逗号分隔的键，而不是匿名类型:

In VB.NET, the syntax for grouping by composite keys is comma-separated keys, not an anonymous type:

                 (From e In table
                  Group e By
                    GroupName = e("GroupName"),
                    ProductName = e("ProductName")
                  Into Group
                  Select New With {
                    .ProductName = ProductName,
                    .QTY = Group.Sum(Function(x) Convert.ToInt32(x("QTY"))),
                    .GroupName = GroupName
                 })

使用您的语法，它仅对匿名对象使用默认的相等比较器，而不会比较字段.

With your syntax, it just uses the default equality comparer on the anonymous objects, which doesn't compare the fields.

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