c# - 如何使用 LINQ 按组合并/求和记录？

Question

例如，我如何使用 LINQ 按 GroupId 对以下记录进行分组，并对每个组中的所有其他列求和？ (从而将每组中的所有行合并为一个)

var list = new List<Foo>()
{ 
    new Foo() { GroupId = 0, ValueA = 10, ValueB = 100 },
    new Foo() { GroupId = 1, ValueA = 30, ValueB = 700 },
    new Foo() { GroupId = 1, ValueA = 40, ValueB = 500 },
    new Foo() { GroupId = 2, ValueA = 80, ValueB = 300 },
    new Foo() { GroupId = 2, ValueA = 20, ValueB = 200 },
    new Foo() { GroupId = 2, ValueA = 20, ValueB = 200 }
};

预期结果是:

| GroupId | ValueA | ValueB |
|---------|--------|--------|
|    0    |   10   |   100  |
|    1    |   70   |   1200 |
|    2    |   120  |   700  |

Answer 1

list.GroupBy(i => i.GroupId)
    .Select(g => new { GroupId = g.Key, 
                       ValueA = g.Sum(i => i.ValueA), 
                       ValueB = g.Sum(i => i.ValueB)});

或者只是为了好玩，您可以使用其重载在一次 GroupBy 调用中完成:

list.GroupBy(i => i.GroupId,
       (key, groupedItems) => new {
                                      GroupId = key,
                                      ValueA = groupedItems.Sum(i => i.ValueA),
                                      ValueB = groupedItems.Sum(i => i.ValueB),
                                  });

或者您可以使用 Aggregate 来避免多次迭代每个组:

list.GroupBy(i => i.GroupId)
    .Select(g => g.Aggregate((i1, i2) => new Foo{ GroupId = i1.GroupId,
                                                  ValueA = i1.ValueA + i2.ValueA,
                                                  ValueB = i1.ValueB + i2.ValueB,
                                                }));