Question

我有要动态生成的LINQ查询：

var groupData =
    from l in data
    group l by l.Field1 into field1Group
    select new MenuItem()
    {
        Key = field1Group.Key,
        Count = field1Group.Count(),
        Items = (from k in field1Group
                 group k by k.Field2 into field2Group
                 select new MenuItem()
                 {
                     Key = field2Group.Key,
                     Count = field2Group.Count()
                 }).ToList()
    };

最终目标是能够通过字段的任意组合对数据进行动态分组，而对嵌套查询没有限制。

我可以达到第一级，但是我在嵌套子查询中苦苦挣扎：

string field1 = "Field1";
string field2 = "Field2";

var groupDataD =
    data.
    GroupBy(field1, "it").
    Select("new ( it.Key, it.Count() as Count )");

使用链式动态LINQ可能吗？还是有更好的方法来实现这一目标？

Answer 1

以下内容应该可以工作（尽管我个人宁愿避免使用此类代码）：

跟随this answer在ParseAggregate中添加以下内容：

Expression ParseAggregate(Expression instance, Type elementType, string methodName, int errorPos)
{
   // Change starts here
   var originalIt = it;
   var originalOuterIt = outerIt;
   // Change ends here

   outerIt = it;
   ParameterExpression innerIt = Expression.Parameter(elementType, elementType.Name);
   it = innerIt;
   Expression[] args = ParseArgumentList();

   // Change starts here
   it = originalIt;
   outerIt = originalOuterIt;
   // Change ends here

   ...
}

将Select，GroupBy，ToList添加到IEnumerableSignatures中，并在ParseAggregate中添加相应的条件，如this answer中所述：

interface IEnumerableSignatures
{
  ...
  void GroupBy(object selector);
  void Select(object selector);
  void ToList();
  ...
}

Expression ParseAggregate(Expression instance, Type elementType, string methodName, int errorPos)
{
   ...
   if (signature.Name == "Min" || 
       signature.Name == "Max" || 
       signature.Name == "GroupBy" || 
       signature.Name == "Select")
   ...
}

最后，您的查询将是：

string field1 = "Field1";
string field2 = "Field2";

var result =
    data
    .GroupBy(field1, "it")
    .Select($@"new ( 
       it.Key,
       it.Count() as Count,
       it.GroupBy({field2})
         .Select(new (it.Key, it.Count() as Count))
         .ToList() as Items
    )");

请注意，"it"在父查询和子查询中使用的实例不同。我试图利用"outerIt"来克服这种混淆，但不幸的是没有成功（但是也许您会成功？也许1，2会有所帮助）

一个可供将来参考的简单示例：

public class Person
{
    public string State { get; set; }
    public int Age { get; set; }
}
public static Main()
{
    var persons = new List<Person>
    {
       new Person { State = "CA", Age = 20 },
       new Person { State = "CA", Age = 20 },
       new Person { State = "CA", Age = 30 },
       new Person { State = "WA", Age = 60 },
       new Person { State = "WA", Age = 70 },
    };
    var result = persons
        .GroupBy("State", "it")
        .Select(@"new ( 
           it.Key,
           it.Count() as Count,
           it.GroupBy(Age)
             .Select(new (it.Key, it.Count() as Count))
             .ToList() as Items
        )");
     foreach (dynamic group in result)
     {
         Console.WriteLine($"Group.Key: {group.Key}");
         foreach (dynamic subGroup in group.Items)
         {
             Console.WriteLine($"SubGroup.Key: {subGroup.Key}");
             Console.WriteLine($"SubGroup.Count: {subGroup.Count}");
         }
     }
}

动态LINQ子查询

1 个答案: