Queryable

What is the IQueryable interface used for? We find a Queryable class, an IQueryable interface, and an AsQueryable extension method.

The IQueryable class is used to develop LINQ query providers. This feature is used to develop the .NET Framework itself.

Queryable example

Here we have an int array and use AsQueryable (an extension method) on it. This gives us an IQueryable reference.

using System;
using System.Linq;

var values = new int[] { 5, 10, 20 };
// We can convert an int array to IQueryable.
// ... And then we can pass it to Queryable and methods like Average.
double result = Queryable.Average(values.AsQueryable());
Console.WriteLine(result);
11.6666666666667

IQueryable

The IQueryable interface inherits from IEnumerable. We can use IQueryable in a similar way as IEnumerable to act upon collections of elements with a unified type.

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

class Program
{
    static void Main()
    {
        // List and array can be converted to IQueryable.
        List<int> list = new List<int>();
        list.Add(0);
        list.Add(1);

        int[] array = new int[2];
        array[0] = 0;
        array[1] = 1;

        // We can use IQueryable to treat collections with one type.
        Test(list.AsQueryable());
        Test(array.AsQueryable());
    }

    static void Test(IQueryable<int> items)
    {
        Console.WriteLine($"Average: {items.Average()}");
        Console.WriteLine($"Sum: {items.Sum()}");
    }
}
Average: 0.5
Sum: 1
Average: 0.5
Sum: 1

Development of an IQueryable class appears to be both complicated and difficult. It is probably best left to the implementors of the .NET Framework.

The .NET Framework contains features that are used for the development of the .NET Framework itself. Other features like "dynamic" are also focused on framework development.