使用字符串字段名称动态构建带层次结构的linq查询？

Question

I'm stumped.. I need to take a flat list of objects, a couple of arrays of field names as string (which to group by, which to sort by, and which to select) that are provided at runtime and somehow generate a LINQ query to return an object for JSON serialization.

I've built a sample repro below of the setup. OrderBy is easy enough, I just use GetType().GetProperty() to find the right property, and iterate the orderby fields to daisy-chain .OrderBy calls. That all falls apart on the GroupBy because each one wraps the results in IEnumerable>..

Is there a good way to do this? Everything I'm finding on the web has people resorting to good old recursive procedural code. LINQ must have a way to do this, but I'm lost. Everything I try won't even compile, the types just won't match up..

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Text;
using System.Threading.Tasks;
using System.Linq.Dynamic;

namespace linqtest2
{
    class Program
    {
        static void Main(string[] args)
        {
            var products = new List<Product>
                {
                    new Product("Food", "Fruit", "Apple"),
                    new Product("Food", "Fruit", "Banana"),
                    new Product("Food", "Fruit", "Orange"),
                    new Product("Food", "Vegetables", "Carrot"),
                    new Product("Food", "Vegetables", "Pea"),
                    new Product("Drink", "Soft Drink", "Orange Juice"),
                    new Product("Drink", "Soft Drink", "Lemonade"),
                    new Product("Drink", "Alcoholic", "Bitter"),
                    new Product("Drink", "Alcoholic", "Lager"),
                    new Product("Drink", "Alcoholic", "Vodka")
                };

            string[] select = new[] { "Category", "Name" };
            string[] orderBy = new[] { "Name", "Category" };
            string[] groupBy = new[] { "Category", "Subcategory" };
        }
    }

    public class Product
    {
        public string Name { get; set; }
        public string Category { get; set; }
        public string Subcategory { get; set; }

        public Product(string category, string subcategory, string name)
        {
            Category = category;
            Subcategory = subcategory;
            Name = name;
        }
    }
}

Answer 1

Please check the Dynamic Linq library which provide a way to use string instead of lambdas.

With dynamic linq you will can write:

products.OrderBy("Category").ThenBy("Name")

And a lot of other possibilities.

EDIT:

Updating the solution to be more dynamic .

IQueryable<Product> query = products;
bool firstOrderBy = true;
foreach(var orderExpression in orderBy)
{
    if (firstOrderBy)
    {
        query = query.OrderBy(orderExpression);
        firstOrderBy = false;
    }
    else
    {
        query = query.ThenBy(orderExpression);
    }
}

As I suggest, verify the library, explore it and adapt it to your needs. You can also do GroupBy and Select , please with it.

Check also the solution proposed by @dkackman and see which solution is better for your needs, either of the solutions you will need to do a little work to adapt to your needs.

Answer 2

Linq syntax is really built around knowing the static structure of the data at compile time. The underlying System.Linq.Expressions namespace has enormous capability to create runtime, as opposed to compile time, bindings but it is a fairly labor intensive under taking.

In order to support dynamic typing of inputs and outputs you're best served by resorting to some libraries.

Dynamic Linq as @Sergio suggests is one option.

SqlLinq is another. With SqlLinq you'd build up the whole query dynamically as a string and execute it something like this:

var products = new List<Product>
    {
        new Product("Food", "Fruit", "Apple"),
        new Product("Food", "Fruit", "Banana"),
        new Product("Food", "Fruit", "Orange"),
        new Product("Food", "Vegetables", "Carrot"),
        new Product("Food", "Vegetables", "Pea"),
        new Product("Drink", "Soft Drink", "Orange Juice"),
        new Product("Drink", "Soft Drink", "Lemonade"),
        new Product("Drink", "Alcoholic", "Bitter"),
        new Product("Drink", "Alcoholic", "Lager"),
        new Product("Drink", "Alcoholic", "Vodka")
    };

var results = products.Query<Product, dynamic>("SELECT category, subcategory, count(*) AS cnt FROM this GROUP BY category, subcategory ORDER BY category, subcategory");

foreach(dynamic d in results)
{
    Debug.WriteLine("{0} {1} {2}", (string)d.category, (string)d.subcategory, (int)d.cnt);
}