Question

我想用linq（对象列表）进行查询，我真的不知道怎么做，我可以做组和总和但不能选择其余的字段。例如：

ID  Value     Name   Category
1   5         Name1  Category1  
1   7         Name1  Category1
2   1         Name2  Category2
3   6         Name3  Category3
3   2         Name3  Category3

我希望按ID分组，按值按值分组并返回所有字段。

ID  Value     Name   Category
1   12        Name1  Category1  
2   1         Name2  Category2
3   8         Name3  Category3

Answer 1

更新：如果您尝试避免对所有字段进行分组，则可以按Id：

进行分组

data.GroupBy(d => d.Id)
    .Select(
        g => new
        {
            Key = g.Key,
            Value = g.Sum(s => s.Value),
            Name = g.First().Name,
            Category = g.First().Category 
        });

但是，此代码假定每个Id适用Name和Category。如果是这样，你应该考虑正常化，因为@Aron建议。这意味着将Id和Value保留在一个班级中并移动Name，Category（以及相同的Id中的任何其他字段都相同）另一个类，同时还有Id作为参考。规范化过程减少了数据冗余和依赖性。

Answer 2

void Main()
{
            //Me being lazy in init
    var foos = new []
    {
        new Foo { Id = 1, Value = 5},
        new Foo { Id = 1, Value = 7},
        new Foo { Id = 2, Value = 1},
        new Foo { Id = 3, Value = 6},
        new Foo { Id = 3, Value = 2},
    };
    foreach(var x in foos)
    {
        x.Name = "Name" + x.Id;
        x.Category = "Category" + x.Id;
    }
            //end init.

    var result = from x in foos
                group x.Value by new { x.Id, x.Name, x.Category}
                into g
                select new { g.Key.Id, g.Key.Name, g.Key.Category, Value = g.Sum()};
    Console.WriteLine(result);
}

// Define other methods and classes here
public class Foo
{
    public int Id {get;set;}
    public int Value {get;set;}

    public string Name {get;set;}
    public string Category {get;set;}   
}

Answer 3

试试这个：

var objList = new List<SampleObject>();

objList.Add(new SampleObject() { ID = 1, Value = 5, Name = "Name1", Category = "Catergory1"});
objList.Add(new SampleObject() { ID = 1, Value = 7, Name = "Name1", Category = "Catergory1"});
objList.Add(new SampleObject() { ID = 2, Value = 1, Name = "Name2", Category = "Catergory2"});
objList.Add(new SampleObject() { ID = 3, Value = 6, Name = "Name3", Category = "Catergory3"});
objList.Add(new SampleObject() { ID = 3, Value = 2, Name = "Name3", Category = "Catergory3"});

var newList = from val in objList
              group val by new { val.ID, val.Name, val.Category } into grouped
              select new SampleObject() { ID = grouped.ID, Value = grouped.Sum(), Name = grouped.Name, Category = grouped.Category };

用LINQPad检查：

newList.Dump();

Answer 4

如果您的课程真的很长，并且您不想复制所有内容，则可以尝试如下操作：

l.GroupBy(x => x.id).
  Select(x => {
    var ret = x.First();
    ret.value = x.Sum(xt => xt.value);
    return ret;
  }).ToList();

强大的力量带来巨大的责任。你要小心点第ret.value = x.Sum(xt => xt.value)行将更改，因为您传递的是参考文献，而不是新对象。如果要避免这种情况，则需要在类中添加Clone之类的MemberwiseClone方法（但是同样，这会创建浅表副本，所以要小心）。之后只需将行替换为：var ret = x.First().Clone();

选择多个字段group by和sum

4 个答案: