LINQ select property by name

Question

I am trying to use a variable inside the LINQ select statement.

Here is an example of what I'm doing now.

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

namespace ConsoleTesting
{
internal class Program
{
    private static void Main(string[] args)
    {
        List<Person> listOfPersons = new List<Person>
        {
            new Person(),
            new Person(),
            new Person(),
            new Person(),
            new Person(),
            new Person(),
            new Person(),
            new Person(),
            new Person(),
            new Person(),
            new Person()
        };

        var firstNames = Person.GetListOfAFirstNames(listOfPersons);

        foreach (var item in listOfPersons)
        {
            Console.WriteLine(item);
        }

        Console.WriteLine();
        Console.ReadKey();
    }


    public class Person
    {
        public string City { get; set; }
        public string CountryName { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }

        public Person()
        {
            FirstName = NameFaker.Name();
            LastName = NameFaker.LastName();
            City = LocationFaker.City();
            CountryName = LocationFaker.Country();
        }

        public static List<string> GetListOfAFirstNames(IEnumerable<Person> listOfPersons)
        {
            return listOfPersons.Select(x => x.FirstName).Distinct().OrderBy(x => x).ToList();
        }

        public static List<string> GetListOfCities(IEnumerable<Person> listOfPersons)
        {
            return listOfPersons.Select(x => x.FirstName).Distinct().OrderBy(x => x).ToList();
        }

        public static List<string> GetListOfCountries(IEnumerable<Person> listOfPersons)
        {
            return listOfPersons.Select(x => x.FirstName).Distinct().OrderBy(x => x).ToList();
        }

        public static List<string> GetListOfLastNames(IEnumerable<Person> listOfPersons)
        {
            return listOfPersons.Select(x => x.FirstName).Distinct().OrderBy(x => x).ToList();
        }
    }
}
}

I have some very not DRY code with GetListOf ... Methods

I feel that I should be able to do something like this

public static List<string> GetListOfProperty(
IEnumerable<Person> listOfPersons, string property)
        {
            return listOfPersons.Select(x =>x.property).Distinct().OrderBy(x=> x).ToList();
        }

but this is not vaild code. I think the key may be related to making func

If this is the answer, how should I do it?

Here is the second retry attempt. But this is also not an option.

        public static List<string> GetListOfProperty(IEnumerable<Person> 
listOfPersons, string property)
        {
            Person person = new Person();
            Type t = person.GetType();
            PropertyInfo prop = t.GetProperty(property);
            return listOfPersons.Select(prop).Distinct().OrderBy(x => 
x).ToList();
}

I think it might be DeadEnd / red herring, but I thought I would show my work.

. , datalist AJAX . 20+, , 20+, , . , , .

:

, , - select Statement???

.

Answer 1

.Select(x =>x.property).

. , , , (string), :

var x = Expression.Parameter(typeof(Person), "x");
var body = Expression.PropertyOrField(x, property);
var lambda = Expression.Lambda<Func<Person,string>>(body, x);

Select:

.Select(lambda).

( LINQ IQueryable<T>)

.Select(lambda.Compile()).

( LINQ IEnumerable<T>).

, , property .

Answer 2

, , :

public static List<string> GetListOfProperty(IEnumerable<Person> 
    listOfPersons, string property)
{
    Type t = typeof(Person);         
    PropertyInfo prop = t.GetProperty(property);
    return listOfPersons
        .Select(person => (string)prop.GetValue(person))
        .Distinct()
        .OrderBy(x => x)
        .ToList();

}

typeof #, "" , Type. , , , .

PropertyInfo GetValue, . . static, null .

GetValue object, .

person => (string)prop.GetValue(person) lamba, ​​:

string Foo(Person person) { ... }

, , string.

public static List<T> GetListOfProperty<T>(IEnumerable<Person> 
    listOfPersons, string property)
{
    Type t = typeof(Person);         
    PropertyInfo prop = t.GetProperty(property);
    return listOfPersons
        .Select(person => (T)prop.GetValue(person))
        .Distinct()
        .OrderBy(x => x)
        .ToList();
}

Answer 3

, ...

, T,

public static List<T> Query<T>(this IEnumerable<Person> instance, Func<Person, T> selector)
{
    return instance
        .Select(selector)
        .Distinct()
        .OrderBy(x => x)
        .ToList();
}

, person, id int, ,

public class Person
{
    public int Id { get; set; }
    public string City { get; set; }
    public string CountryName { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

, , -

var ids = listOfPersons.Query(p => p.Id);
var firstNames = listOfPersons.Query(p => p.FirstName);
var lastNames = listOfPersons.Query(p => p.LastName);
var cityNames = listOfPersons.Query(p => p.City);
var countryNames = listOfPersons.Query(p => p.CountryName);

Edit

, , ,

public static List<string> Query(this IEnumerable<Person> instance, string property)
{
    switch (property)
    {
        case "ids": return instance.Query(p => p.Id.ToString());
        case "firstName": return instance.Query(p => p.FirstName);
        case "lastName": return instance.Query(p => p.LastName);
        case "countryName": return instance.Query(p => p.CountryName);
        case "cityName": return instance.Query(p => p.City);
        default: throw new Exception($"{property} is not supported");
    }
}

var cityNames = listOfPersons.Query("cityName");

Answer 4

Reflection. - .

:

public static List<string> GetListOfValues(IEnumerable<Person> listOfPersons, string propertyName)
{
    var ret = new List<string>();

    PropertyInfo prop = typeof(Person).GetProperty(propertyName);
    if (prop != null)
        ret = listOfPersons.Select(p => prop.GetValue(p).ToString()).Distinct().OrderBy(x => x).ToList();

    return ret;
}

Hope this helps.

It is based on C # 6

Answer 5

You can also use this. works for me.

public static class ObjectReflectionExtensions
{
    public static  object GetValueByName<T>(this T thisObject,  string propertyName)
    {
        PropertyInfo prop = typeof(T).GetProperty(propertyName);
        return prop.GetValue(thisObject);

    }
}

And call like that.

public static List<string> GetListOfProperty(IEnumerable<Person> listOfPersons, string propertyName)
    {
        return listOfPersons.Select(x =>(string)x.GetValueByName(propertyName)).Distinct().OrderBy(x=> x).ToList();
    }