C# List.AsQueryable() 返回 System.Collections.Generic.List 而不是 IQueryable

Question

I have a list of objects List<Student> that I want return as an IQueryable<Student> .

When I try to use .AsQueryable() on the list to convert it to IQueryable<Student> I got a System.Collections.Generic.List`1[ConsoleApp.Student] .

How I can convert list to IQueryable() ?

See below I minimum reproducible example:

using System.Collections.Generic;
using System.Linq;

namespace ConsoleApp
{
    public class Student
    {
        public int Id { get; set; }
        public string Name { get; set; }
    }

    internal class Program
    {
        static void Main(string[] args)
        {
            var students = new List<Student>() ;
            students.Add(new Student() { Id = 1, Name = "J" });
            students.Add(new Student() { Id = 2, Name = "X" });

            IQueryable<Student> queryable = students.AsQueryable();

            Console.WriteLine("Hello, World!");
        }
    }
}

Answer 1

.AsQueryable() returns a EnumerableQuery<T> that wraps the list (or IEnumerable<T> to be precise) and provides the IQueryable interface. So if you just want an IQueryable<T> your don't need to do anything more. Your example already demonstrates this perfectly well.

Note that, the .ToString() -method on EnumerableQuery just seem to call the underlying collection, so it will still print {System.Collections.Generic.List1[ConsoleApp.Student]} in the debugger, or when converting it to string. I suspect this is why you are confused. You can call .GetType().Name to get the actual typename of an object.

The larger question is, why do you want a IQueryable? This is mostly intended to query databases. If you have objects in memory you usually just use IEnumerable<T> . If you just want to print your students, Overload.ToString on the student object and call string.Join to produce a string for all students.