Dynamic LINQ Sub Query

问题

I have LINQ query that I want to generate dynamically:

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()
    };

The ultimate goal is to be able to dynamically group the data by any combination of fields with no limit on the nested queries.

I can get as far as the first level but I'm struggling with the nested sub queries:

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

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

Is this possible with chained dynamic LINQ? Or is there a better way to achieve this?

回答1:

The following should work (though personally I would rather avoid using such code):

1. Follow this answer to add the following in ParseAggregate, :

   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
   
      ...
   }
   

2. Add Select, GroupBy, ToList into IEnumerableSignatures, and respective conditions in ParseAggregate, as explained in this answer:

   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")
      ...
   }
   

3. Finally, Your query would be:

   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
       )");
   

   Note that "it" holds a different instance when used in the parent query vs. the subquery. I tried to take advantage of "outerIt" to overcome this conflation, but unfortunately without success (but maybe you'd succeed? maybe 1, 2 would help)

4. A simple example for future reference:

   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}");
            }
        }
   }
   

来源：https://stackoverflow.com/questions/54401313/dynamic-linq-sub-query