How do I use a method with expression paramiters

Here is one potential way:

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

namespace EE_Q29131131
{
    class Program
    {
        static IEnumerable<Person> people = (from i in Enumerable.Range(0, 50)
                                             let now = DateTime.Now
                                             select new Person { ID = i, FirstName = $"FirstName{i}", LastName = $"LastName{i}", IsWorking = i % 2 == 0, Start = now.AddDays(-(i*9)), Finish = now.AddDays(-(i*9)).AddHours(8) });
        static void Main(string[] args)
        {
            Console.WriteLine($"First: {people.First()}");
            Console.WriteLine($"Last: {people.Last()}");
            var filtered = people.AsQueryable().Get(p => (p.IsWorking && (p.ID > 35 && p.ID < 40)), p => p.OrderBy(x => x.Start));
            Console.WriteLine($"First: {filtered.First()}");
            Console.WriteLine($"Last: {filtered.Last()}");
            Console.ReadLine();
        }
    }

    class Person
    {
        public int ID { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public bool IsWorking { get; set; }
        public DateTime Start { get; set; }
        public DateTime Finish { get; set; }
        public override string ToString()
        {
            return $"{{ {string.Join(", ", from property in GetType().GetProperties() select $"{property.Name}: {property.GetValueOrValues(this)}")} }}";
        }
    }

    static class Extensions
    {
        public static IEnumerable<T> Get<T>(this IQueryable<T> source, Expression<Func<T, bool>> filter = null, Func<IQueryable<T>, IOrderedQueryable<T>> orderBy = null)
        {
            IQueryable<T> results = source;
            if (filter != null)
            {
                results = results.Where(filter);
            }
            return orderBy != null ? orderBy(results) : results;
        }

        public static string GetValueOrValues(this PropertyInfo property, object source)
        {
            var value = property.GetValue(source, null);
            if (value is null)
            {
                return string.Empty;
            }
            else if (property.PropertyType.Equals(typeof(string)) && typeof(IEnumerable<>).IsAssignableFrom(property.PropertyType))
            {
                return $"{{ {string.Join(", ", from item in value as IEnumerable<object> select item)} }}";
            }
            else
            {
                return value.ToString();
            }
        }
    }
}

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Which produces the following output --saige-

How would I use Get<T>… in the context of a "GenericRepository" as opposed to an extension method. I mean how do I pass in a "Filter" and "OrderBy"?

I am trying to figure out how to use the bits on this page. https://tinyurl.com/ydca435z

T is what makes it generic.  Regardless of whether it is an extension method or a class method does not matter; e.g. -

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;

namespace EE_Q29131131
{
    class Program
    {
        static IEnumerable<Person> people = (from i in Enumerable.Range(0, 50)
                                             let now = DateTime.Now
                                             select new Person { ID = i, FirstName = $"FirstName{i}", LastName = $"LastName{i}", IsWorking = i % 2 == 0, Start = now.AddDays(-(i * 9)), Finish = now.AddDays(-(i * 9)).AddHours(8) });
        static void Main(string[] args)
        {
            Console.WriteLine($"First: {people.First()}");
            Console.WriteLine($"Last: {people.Last()}");
            var filtered = new QueryableReadOnlyRepository<Person>(people.AsQueryable()).Get<Person>(p => (p.IsWorking && (p.ID > 35 && p.ID < 40)), p => p.OrderBy(x => x.Start));
            Console.WriteLine($"First: {filtered.First()}");
            Console.WriteLine($"Last: {filtered.Last()}");
            Console.ReadLine();
        }
    }

    class Person
    {
        public int ID { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public bool IsWorking { get; set; }
        public DateTime Start { get; set; }
        public DateTime Finish { get; set; }
        public override string ToString()
        {
            return $"{{ {string.Join(", ", from property in GetType().GetProperties() select $"{property.Name}: {property.GetValueOrValues(this)}")} }}";
        }
    }

    public interface IReadOnlyRepository
    {
        IEnumerable<T> Get<T>(Expression<Func<T, bool>> filter = null, Func<IQueryable<T>, IOrderedQueryable<T>> orderBy = null) where T : class;
    }

    public class QueryableReadOnlyRepository<T> : IReadOnlyRepository
    {
        protected readonly IQueryable<T> source;

        public QueryableReadOnlyRepository(IQueryable<T> source)
        {
            this.source = source;
        }

        protected virtual IQueryable<T> GetQueryable<T>(Expression<Func<T, bool>> filter = null, Func<IQueryable<T>, IOrderedQueryable<T>> orderBy = null) where T : class
        {
            IQueryable<T> query = source as IQueryable<T>;

            if (filter != null)
            {
                query = query.Where(filter);
            }

            if (orderBy != null)
            {
                query = orderBy(query);
            }

            return query;
        }

        public virtual IEnumerable<T> Get<T>(Expression<Func<T, bool>> filter = null, Func<IQueryable<T>, IOrderedQueryable<T>> orderBy = null) where T : class
        {
            return GetQueryable<T>(filter, orderBy);
        }
    }

    static class Extensions
    {
        public static string GetValueOrValues(this PropertyInfo property, object source)
        {
            var value = property.GetValue(source, null);
            if (value is null)
            {
                return string.Empty;
            }
            else if (property.PropertyType.Equals(typeof(string)) && typeof(IEnumerable<>).IsAssignableFrom(property.PropertyType))
            {
                return $"{{ {string.Join(", ", from item in value as IEnumerable<object> select item)} }}";
            }
            else
            {
                return value.ToString();
            }
        }
    }
}

Select allOpen in new window

Produces the same output as above.

-saige-

ok, So the difference with your code and the code in the article is that the type is passed in at the method level and not the class level and I am still struggling with how to use those