Reuse Select Expression in LINQ

Question

I have three methods to get the user details based on attributes:

• Find user details by id,
• Find user details by username,
• Find user details by email

Here is the program class:

using System;
using System.Data;
using System.Linq;
using System.Data.DataSetExtensions;
public class Program
{
 public static void Main()
 {
 Console.WriteLine("Hello World");
 var user = GetUserById(1);
 Console.WriteLine(user.Name);
 var user1 = GetUserByUsername("user3");
 Console.WriteLine(user1.Name);
 var user2 = GetUserByEmail("[email protected]");
 Console.WriteLine(user2.Name);
 }
 public static User GetUserById(int id)
 {
 var users = GetUsers().AsEnumerable().Where(row => row.Field<int>("Id") == id);
 var userInfo = users.Select(s => new User{
 Id = s.Field<int>("Id"),
 Username = s.Field<string>("Username"),
 Name = s.Field<string>("Name"),
 Email = s.Field<string>("Email"),
 CreatedDate = s.Field<DateTime>("CreatedDate"),
 Phone = s.Field<string>("Phone"),
 City = s.Field<string>("City"),
 State = s.Field<string>("State"),
 Country = s.Field<string>("Country")
 }).FirstOrDefault();
 return userInfo;
 }
 public static User GetUserByUsername(string username)
 {
 var users = GetUsers().AsEnumerable().Where(row => row.Field<string>("Username") == username);
 var userInfo = users.Select(s => new User{
 Id = s.Field<int>("Id"),
 Username = s.Field<string>("Username"),
 Name = s.Field<string>("Name"),
 Email = s.Field<string>("Email"),
 CreatedDate = s.Field<DateTime>("CreatedDate"),
 Phone = s.Field<string>("Phone"),
 City = s.Field<string>("City"),
 State = s.Field<string>("State"),
 Country = s.Field<string>("Country")
 }).FirstOrDefault();
 return userInfo;
 }
 public static User GetUserByEmail(string email)
 {
 var users = GetUsers().AsEnumerable().Where(row => row.Field<string>("Email") == email);
 var userInfo = users.Select(s => new User{
 Id = s.Field<int>("Id"),
 Username = s.Field<string>("Username"),
 Name = s.Field<string>("Name"),
 Email = s.Field<string>("Email"),
 CreatedDate = s.Field<DateTime>("CreatedDate"),
 Phone = s.Field<string>("Phone"),
 City = s.Field<string>("City"),
 State = s.Field<string>("State"),
 Country = s.Field<string>("Country")
 }).FirstOrDefault();
 return userInfo;
 }
 static DataTable GetUsers()
 {
 // Consider these are the data from the SQL table.
 DataTable table = new DataTable();
 table.Columns.Add("Id", typeof(int));
 table.Columns.Add("Username", typeof(string));
 table.Columns.Add("Name", typeof(string));
 table.Columns.Add("Email", typeof(string));
 table.Columns.Add("CreatedDate", typeof(DateTime));
 table.Columns.Add("Phone", typeof(string));
 table.Columns.Add("City", typeof(string));
 table.Columns.Add("State", typeof(string));
 table.Columns.Add("Country", typeof(string));
 // Here we add five DataRows.
 table.Rows.Add(1, "user1", "David", "[email protected]", DateTime.Now, "9999999999", "City 1", "State 1", "India");
 table.Rows.Add(2, "user2", "Sam", "[email protected]", DateTime.Now, "8888888888", "City 2", "State 2", "USA");
 table.Rows.Add(3, "user3", "Christoff", "[email protected]", DateTime.Now, "7777777777", "City 3", "State 3", "UK");
 table.Rows.Add(4, "user4", "Janet", "[email protected]", DateTime.Now, "6666666666", "City 4", "State 4", "Germany");
 table.Rows.Add(5, "user5", "Melanie", "[email protected]", DateTime.Now, "5555555555", "City 5", "State 5", "France");
 return table;
 }
}
public class User
{
 public int Id {get;set;}
 public string Username {get;set;}
 public string Name {get;set;}
 public string Email {get;set;}
 public DateTime CreatedDate {get;set;}
 public string Phone {get;set;}
 public string City {get;set;}
 public string State {get;set;}
 public string Country {get;set;}
}

Is there any possibility to reuse the below object in LINQ using predicate?

new User{
 Id = s.Field<int>("Id"),
 Username = s.Field<string>("Username"),
 Name = s.Field<string>("Name"),
 Email = s.Field<string>("Email"),
 CreatedDate = s.Field<DateTime>("CreatedDate"),
 Phone = s.Field<string>("Phone"),
 City = s.Field<string>("City"),
 State = s.Field<string>("State"),
 Country = s.Field<string>("Country")
 }

DotNet Fiddle: https://dotnetfiddle.net/psiBtv

• Olivier Jacot-Descombes: Please post real code that does compile. This code is incomplete and does not compile. This question seems more suited for stackoverflow. Btw., a solution would be to add a constructor to User accepting the a parameter and doing the assignments internally, so you could write new User(a).
• C.M.: return Data.Select(SelectFunc).FirstOrDefault(); ... then write SelectFunc as a function that takes a single parameter of type 'a' and User as the return type.
• Ramesh Kannan: I've edited the code sample @OlivierJacot-Descombes

Answer 1

I've used Func<> to reuse the object in Linq. Here is the working sample.

using System;
using System.Data;
using System.Linq;
using System.Data.DataSetExtensions;
public class Program
{
 private static readonly Func<DataRow, User> fnUserInfo = s => 
 new User
 {
 Id = s.Field<int>("Id"),
 Username = s.Field<string>("Username"),
 Name = s.Field<string>("Name"),
 Email = s.Field<string>("Email"),
 CreatedDate = s.Field<DateTime>("CreatedDate"),
 Phone = s.Field<string>("Phone"),
 City = s.Field<string>("City"),
 State = s.Field<string>("State"),
 Country = s.Field<string>("Country")
 };
 public static void Main()
 {
 Console.WriteLine("Hello World");
 var user = GetUserById(1);
 Console.WriteLine(user.Name);
 var user1 = GetUserByUsername("user3");
 Console.WriteLine(user1.Name);
 var user2 = GetUserByEmail("[email protected]");
 Console.WriteLine(user2.Name);
 }
 public static User GetUserById(int id)
 {
 var users = GetUsers().AsEnumerable().Where(row => row.Field<int>("Id") == id);
 var userInfo = users.Select(fnUserInfo).FirstOrDefault();
 return userInfo;
 }
 public static User GetUserByUsername(string username)
 {
 var users = GetUsers().AsEnumerable().Where(row => row.Field<string>("Username") == username);
 var userInfo = users.Select(fnUserInfo).FirstOrDefault();
 return userInfo;
 }
 public static User GetUserByEmail(string email)
 {
 var users = GetUsers().AsEnumerable().Where(row => row.Field<string>("Email") == email);
 var userInfo = users.Select(fnUserInfo).FirstOrDefault();
 return userInfo;
 }
 static DataTable GetUsers()
 {
 // Consider these are the data from the SQL table.
 DataTable table = new DataTable();
 table.Columns.Add("Id", typeof(int));
 table.Columns.Add("Username", typeof(string));
 table.Columns.Add("Name", typeof(string));
 table.Columns.Add("Email", typeof(string));
 table.Columns.Add("CreatedDate", typeof(DateTime));
 table.Columns.Add("Phone", typeof(string));
 table.Columns.Add("City", typeof(string));
 table.Columns.Add("State", typeof(string));
 table.Columns.Add("Country", typeof(string));
 // Here we add five DataRows.
 table.Rows.Add(1, "user1", "David", "[email protected]", DateTime.Now, "9999999999", "City 1", "State 1", "India");
 table.Rows.Add(2, "user2", "Sam", "[email protected]", DateTime.Now, "8888888888", "City 2", "State 2", "USA");
 table.Rows.Add(3, "user3", "Christoff", "[email protected]", DateTime.Now, "7777777777", "City 3", "State 3", "UK");
 table.Rows.Add(4, "user4", "Janet", "[email protected]", DateTime.Now, "6666666666", "City 4", "State 4", "Germany");
 table.Rows.Add(5, "user5", "Melanie", "[email protected]", DateTime.Now, "5555555555", "City 5", "State 5", "France");
 return table;
 }
}
public class User
{
 public int Id { get; set; }
 public string Username { get; set; }
 public string Name { get; set; }
 public string Email { get; set; }
 public DateTime CreatedDate { get; set; }
 public string Phone { get; set; }
 public string City { get; set; }
 public string State { get; set; }
 public string Country { get; set; }
}

DotNetFiddle: https://dotnetfiddle.net/FhDRR6

• Ramesh Kannan: Is there any other way or better approach to achieve this?
• Pieter Witvoet: Any method with a DataRow parameter and a return type of User can be used in your Select. In this case, a static method will do fine, there's no need to explicitly declare it as a Func.