LINQ LAMBDA,通过列表组(LINQ Lambda, Group by with list)

2019-08-03 15:22发布

我遇到一些麻烦找到正确的语法来完成以下任务:

是否有可能与LINQ(Lambda表达式),以.GroupBy数据,而不是使用通常的.SUM()或.Count之间的()我想要得到的数据为int的List。

我定义我命名自己的类:Filter_IDs。 它的构造函数需要两个参数:

public int? type; // Represents the object_type column from my database
public List<int?> objects; // Represents the object_id column from my database

我想从我的数据库将数据加载到该对象。 下面的LINQ查询应导致Filter_IDs的列表:

下面的LINQ查询应导致Filter_IDs的列表:

List<Filter_IDs> filterids = ef.filterLine
        .GroupBy(fl => fl.objectType)
    .Select(fl => new Filter_IDs { type = fl.Key, objects = fl.Select(x => x.object_id).ToList() })
    .ToList();

使用此查询没有给出建筑物错误,但给出了运行时“NotSupportedException异常”。

该数据库是这样的,给你一个更好地了解数据:

http://d.pr/i/mnhq+ (droplr图像)

在此先感谢,戈

Answer 1:

我认为这个问题是DB不能够调用ToList的选择,也不是为了创建一个新的过滤器_id。

尝试是这样的:

List<Filter_IDs> filterids = ef.filterLine.Select(o => new { objectType = o.objectType, object_id=o.object_id})
    .GroupBy(fl => fl.objectType).ToList()
    .Select(fl => new Filter_IDs { type = fl.Key, objects = fl.Select(x => x.object_id).ToList() })
    .ToList();


Answer 2:

也许你想

IList<Filter_IDs> filterIds = ef.filterline
    .Select(fl => fl.objectType).Distinct()
    .Select(ot => new Filter_IDs
        {
            type = ot,
            objects = ef.filterline
                          .Where(fl => fl.objectType == ot)
                          .Select(fl =>objectType)
                          .ToList()
        }).ToList();

获取不同名单objectType ,并用它来子查询的每个列表object_id

然而,它似乎更高效的我只是列举,以便值,

var results = new List<Filter_IDs>();
var ids = new List<int>();
var first = true;
int thisType;

foreach (var fl in ef.filterLines
                       .OrderBy(fl => fl.objectType)
                       .ThenBy(fl => fl.object_Id))
{
    if (first)
    {
        thisType = fl.objectType;
        first = false;
    }
    else
    {
        if (fl.objectType == thisType)
        {
            ids.Add(fl.object_Id);
        }
        else
        {
           results.Add(new Filter_IDs
                {
                    Type = thisType,
                    objects = ids
                });
           thisType = fl.objectType;
           ids = new List<int>();   
        }
    }    
}


Answer 3:

您可以在客户端使用的GroupBy:

List<Filter_IDs> filterids = ef.filterLine
        .Select(fl=>new {fl.ObjectType, fl.object_id})
        .AsEnumerable()
        .GroupBy(fl => fl.objectType)
    .Select(fl => new Filter_IDs { type = fl.Key, objects = fl.Select(x => x.object_id).ToList() })
    .ToList();


文章来源: LINQ Lambda, Group by with list