Question

我正在尝试找到一种更有效的方法，以便从查询中向列表中添加其他信息。例如，如果我有一个ObjectID和StringA集的对象列表，我想基于ObjectID查询数据库以检索StringB和StringC：

public class SomeObject {
    public int ObjectID { get; set; }
    public string StringA { get; set; }
    public string StringB { get; set; }
    public string StringC { get; set; }
}

public void AddInformationToSomeObjects(List<SomeObject> someObjects)
{
    var listOfIDs = someObjects.Select(so => so.ObjectID).ToList();
    var informationToAdd = db.Table.Where(t => listOfIDs.Contains(t.ObjectID)).Select(t => new { ObjectID = t.ObjectID, StringB = t.StringB, StringC = t.StringC }).ToList();

    foreach (var someObject in someObjects)
    {
        var information = informationToAdd.Where(i => i.ObjectID == someObject.ObjectID).FirstOrDefault();
        someObject.StringB = information.StringB;
        someObject.StringC = information.StringC;
    }
}

有没有办法将查询和赋值组合成一个语句？

Answer 1

只需更新整个List ...

public void AddInformationToSomeObjects(IEnumerable<SomeObject> someObjects)
{
    someObjects =
        from obj in someObjects
        join dbObj in db.Table
        on obj.ObjectID equals dbObj.ObjectID
        select new SomeObject
        {
            ObjectID = obj.ObjectID,
            StringA = obj.StringA,
            StringB = dbObj.StringB,
            StringC = dbObj.StringC
        }
}

Answer 2

正如Jan P.所说，重新获取新名单可能不是什么大问题。但它获得第一个列表涉及一些昂贵的操作（如获取blob）你的想法一点也不差。简单的连接就是：

public List<SomeObject> AddInformationToSomeObjects(List<SomeObject> someObjects)
{
    var listOfIDs = someObjects.Select(so => so.ObjectID).ToList();
    var informationToAdd = db.Table
        .Where(t => listOfIDs.Contains(t.ObjectID))
        .Select(t => new { 
                            ObjectID = t.ObjectID,
                            StringB = t.StringB,
                            StringC = t.StringC
                        }).ToList();

    return(
        from org in someObjects
        join a in informationToAdd on org.ObjectID equals a.ObjectID
        select new SomeObject {
                                ObjectID = org.ObjectID,
                                StringA = org.StringA,
                                StringB = a.StringB,
                                StringC = a.StringC
                            }).ToList();
}

返回新列表而不更改原始列表只是个人偏好（最小化方法/功能的副作用）。

从linq-to-entities查询中向列表添加其他信息

2 个答案: