Question

How to return Anonymous Types List

Asked by: techsuppoprt

Hi Experts.

Need a piece of advice here please.

What's the best way to return the Anonymous type List ?
Lets say I have this list:

            var usersList = (from u in usersTable.GetAll()
                         select new
                         {
                             Id = u.Id,
                             Login = u.UserLoginId,
                             Name = u.Name
                         }).ToList();

What would be the return type for the method that returns usersList ?
Yes, I could just return "object" but I would like to avoid boxing if possible as its not very memory sufficient.
Thank you in advance.

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Asked On
2008-12-19 at 12:41:38ID23999534
Tags

C#

,

LINQ

,

Anonymous Types

Topics

Language Integrated Query - LINQ

,

Microsoft Visual C#.Net

,

C# Programming Language

Participating Experts
2
Points
400
Comments
9

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Answers

 

by: FernandoSotoPosted on 2008-12-19 at 14:26:42ID: 23216042

Hi techsuppoprt;

To your question, "What would be the return type for the method that returns usersList ?", A List of an anonymous type. Anonymous types are given a name at compile time that can not be used in your code.

Fernando

 

by: techsuppoprtPosted on 2008-12-19 at 16:28:05ID: 23216517

Sorry .. just to confirm..
Are you saying "object" is the only way to return my Anonymous Types List ?

 

by: FernandoSotoPosted on 2008-12-19 at 18:13:00ID: 23216733

No, If you use the ToList() as you did in the question you will return a List(Of Anonymous) type.

 

by: PockyMasterPosted on 2008-12-20 at 14:08:23ID: 23219899

You might consider wrapping it in a class instead.

As far as I know you can not return anonymous types from methods. But in your case, you could have returned IEnumerable instead of object e.g.

class User
{
  public int ID {get;set;}
  public int Login {get;set;}
  public string Name {get;set;}
 
}
.....
public List<User> GetUsers()
{
return  (from u in usersTable.GetAll()
                         select new User
                         {
                             ID = u.Id,
                             Login = u.UserLoginId,
                             Name = u.Name
                         }).ToList();
}

                                              
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by: techsuppoprtPosted on 2008-12-22 at 20:12:59ID: 23231322

Fernando, could you provide an example of "you will return a List(Of Anonymous) type." please ?

 

by: FernandoSotoPosted on 2008-12-22 at 21:34:04ID: 23231537

Hi techsuppoprt;

Below is a code snippet using the Northwind DB. The button1_Click event show how to return a List of Anonymous type from a query. As stated by PockyMaster you can not return a List of Anonymous types so button2_Click event show a alternative to returning an Anonymous type.

Fernando

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
 
namespace WindowsFormsApplication35
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            NorthwindDataContext nw = new NorthwindDataContext();
 
            // In the following statement employees is an Anonymous type of IEnumerable
            var employees = from emp in nw.Employees
                            select new
                            {
                                ID = emp.EmployeeID,
                                Title = emp.Title,
                                Name = emp.LastName + ", " + emp.FirstName
                            };
 
            // When you apply the method ToList() the return of employees.ToList()
            // is a List<AnonymousType> as seen from the Watch window figure 1 below
            foreach (var emp in employees.ToList())
            {
                Console.WriteLine("{0,-6}\t{1,-30}\t{2,-32}", emp.ID, emp.Title, emp.Name);
            }
        }
 
        // Because Anonymous types can only be accessed from the method they were created in with
        // the local variable you can not pass a List of Anonymous type out of a methed As stated by
        // PockyMaster in his solution. 
        public class empInfo
        {
            public int ID { get; set; }
            public String Title { get; set; }
            public String Name { get; set; }
        }
 
        // This method will return a List<empInfo> to the caller
        public List<empInfo> GetEmployeeInfo()
        {
            NorthwindDataContext nw = new NorthwindDataContext();
 
            // In the following statement employees is a List<empInfo>. In the select clause the query
            // will create a empInfo for each record selected from the query and placing that empInfo
            // into the List.
            List<empInfo> employees = (from emp in nw.Employees
                                       select new empInfo
                                       {
                                           ID = emp.EmployeeID,
                                           Title = emp.Title,
                                           Name = emp.LastName + ", " + emp.FirstName
                                       }).ToList();
            return employees;
        }       
        
        
        private void button2_Click(object sender, EventArgs e)
        {
            // Getting a List<empInfo>
            List<empInfo> employees = GetEmployeeInfo();
            foreach (var emp in employees.ToList())
            {
                Console.WriteLine("{0,-6}\t{1,-30}\t{2,-32}", emp.ID, emp.Title, emp.Name);
            }
        }
    }
}

                                              
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by: techsuppoprtPosted on 2008-12-22 at 22:57:47ID: 23231765

I"m actually well aware of that method stated by you and Poke ( thanks to you Fernando for educating me in previous posts ). I guess I was curious to find out if there is a way to return the anonymous type without doing extra work of creating that extra anonymous type class:

        public class empInfo
        {
            public int ID { get; set; }
            public String Title { get; set; }
            public String Name { get; set; }
        }

 

by: FernandoSotoPosted on 2008-12-23 at 07:04:32ID: 23233982

Hi techsuppoprt;

The class empInfo that gets created by the query in my code snippet is not an anonymous type class but a concrete class that can be passed around the program without any problem.

If you were to execute this code:

// The only way to return an Anonymous type from a function
// is by an object as in this method
public object GetEmployeeInfo()
{
    NorthwindDataContext nw = new NorthwindDataContext();
    // employees is an Anonymous type
    var employees = from emp in nw.Employees
                    select new
                    {
                        ID = emp.EmployeeID,
                        Title = emp.Title,
                        Name = emp.LastName + ", " + emp.FirstName
                    };
    // Return an Anonymous type to the caller
    return employees;
}

private void button2_Click(object sender, EventArgs e)
{
    // Anonymous type returned from function
    object employees = GetEmployeeInfo();
    // This statement will only print to the console windoe/output window
    // the SQL statement that was executed on the server. To get access to
    // the data in the object you will need to use .Net Reflection class
    // to determine what is in the object and its values.
    Console.WriteLine(employees.ToString());
}

Fernando

 

by: techsuppoprtPosted on 2008-12-24 at 12:11:07ID: 23241733

Alright.. so "object" is the way....

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