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How to merge two lists of objects based on a property?

I have the two lists below where one comes from the database and the other from a JSON. The one from the database has it's Id associated.
List from database:
EmployeeId FirstName LastName EmployeeNumber
1234 Tom Cruise 98372829
5555 James Bond 93932228
The employeeId is a GUID Saved in the database.
Now I retrieve a list of Employees again to detect changes - James Bond Lastname changed. And I used automapper to map in the same format as my database entity.
List from JSON:
EmployeeId FirstName LastName EmployeeNumber
000-0000... Tom Cruise 98372829
000-0000... James Carter 93932228
Now I want to update the first list with the FirstName and LastName based on the EmployeeNumber.
// Employees retrieved in JSON
var retrievedEmployees = JsonSerializer.Deserialize<List<EmployeeDto>>(methodToRetrieveEmployees()))!.ToList();
var mappedEmployees = _mapper.Map<IEnumerable<Employee>>(retrievedEmployees);
var existingEmployeeFromDatabase = await GetExistingEmployees();
var employeesWithLatestUpdates = mappedEmployees
.Where(y => existingEmployeeFromDatabase.Any(z => z.Number == y.Number)).ToList();
So What I need to do is to update employeeswithLatestChanges (Id,FirstName and LastName) with the values from existingEmployees from the database. Since they don't have Id, this should be mapped by the EmployeeNumber.
I have tried to use Union/joins but no luck.

Updating by linq in c# 6
var updatedEmployee = employeeswithLatestChanges.Select(x => new Employee
{
FirstName = existingEmployees.FirstOrDefault(y => y.EmployeeId == x.EmployeeNumber)?.FirstName?? x.FirstName,
LastName = existingEmployees.FirstOrDefault(y => y.LastName == x.code)?.LastName ?? x.LastName ,
});
Can Use Loop also
foreach (var dbEmp in existingEmployees)
{
foreach(var emp in (employeeswithLatestChanges.Where(t => t.EmployeeNumber == dbEmp.EmployeeId)))
{
emp.FirstName= dbEmp.FirstName;
emp.LastName= dbEmp.LastName;
}
}

To fix the idea we can assume the following class to represent an employee:
public sealed class Employee
{
public string FirstName { get; set; }
public string LastName { get; set; }
public Guid Id { get; set; }
public int Number { get; set; }
}
Suppose you have a collection of employees named employees that you want to update by using another collection of employees named updatedEmployees:
IEnumerable<Employee> employees = ....;
IEnumerable<Employee> updatedEmployees = ....;
The simplest way I can think of to solve your problem is the following:
public static void Main(string[] args)
{
// previous code omitted for brevity
Dictionary<int, Employee> employeeNumberToUpdatedEmployee = updatedEmployees.ToDictionary(x => x.Number);
foreach (var employee in employees)
{
if (employeeNumberToUpdatedEmployee.TryGetValue(employee.Number, out var updatedEmployee)
{
employee.FirstName = updatedEmployee.FirstName;
employee.LastName = updatedEmployee.LastName;
}
}
// subsequent code omitted for brevity
}
An alternative way to solve this problem is to perform a join operation by using LINQ to objects, as in the following code:
var employees = new List<Employee>
{
new Employee{ Id = Guid.NewGuid(), Number = 11, FirstName = "Bob", LastName = "Red" },
new Employee{ Id = Guid.NewGuid(), Number = 13, FirstName = "Alice", LastName = "Smith" },
new Employee{ Id = Guid.NewGuid(), Number = 5, FirstName = "Max", LastName = "Brown" },
};
var updatedEmployees = new List<Employee>
{
new Employee{ Id = Guid.NewGuid(), Number = 11, FirstName = "Bob", LastName = "Verdi" },
new Employee{ Id = Guid.NewGuid(), Number = 13, FirstName = "Alice", LastName = "Rossi" },
new Employee{ Id = Guid.NewGuid(), Number = 78, FirstName = "Sam", LastName = "Smith" },
};
// here we are using the fact that we can have, at most, one match
var query = from employee in employees
join updatedEmployee in updatedEmployees on employee.Number equals updatedEmployee.Number into matches
from match in matches.DefaultIfEmpty()
select new Employee
{
Id = employee.Id,
Number = employee.Number,
FirstName = match?.FirstName ?? employee.FirstName,
LastName = match?.LastName ?? employee.LastName,
};
foreach (var item in query)
{
Console.WriteLine($"{item.FirstName} {item.LastName}");
}

Is there an easy way to flatten an array of objects while making an array of the one column that would be different?

I have an array of objects similar to this:
class StateVisitor
{
string FirstName { get; set; }
string LastName { get; set; }
string StateViseted { get; set; }
}
StateVisitor[] StateVisitors = {
new() { FirstName = "Bob", LastName = "Smith", StateViseted = "AL" },
new() { FirstName = "Bob", LastName = "Smith", StateViseted = "AK" },
new() { FirstName = "Bob", LastName = "Jones", StateViseted = "AL" },
new() { FirstName = "Sam", LastName = "Smith", StateViseted = "UT" }
}
And I want to do something like this:
class VisitorsWithCombinedStates {
string FirstName { get; set; }
string LastName { get; set; }
string[] StatesVisetedArray { get; set; }
}
VisitorsWithCombinedStates[] visitorsWithCombinedStates = StateVisitors... /* Linq magic? */
visitorsWithCombinedStates.ForEach(v
=> Console.WriteLine($"{v.FirstName} {v.LastName} visited {string.Join(", ",v.StatesVisitedArray)}"));
// "Bob Smith visited AL, AK"
// "Bob Jones visited AL"
// "Sam Smith visited UT"
Is there an easy way, in C# (probably with LINQ) to flatten that first array into the second array, where it makes an array of the states visited?

I think you are looking for GroupBy then projecting the groups into your new class:
var visitorsWithCombinedStates = StateVisitors.GroupBy(sv => new { sv.FirstName, sv.LastName }, sv => sv.StateVisited)
.Select(svg => new VisitorsWithCombinedStates {
FirstName = svg.Key.FirstName,
LastName = svg.Key.LastName,
StatesVisitedArray = svg.ToArray()
})
.ToArray();
NOTE: Corrected spelling of visited

You want to group your items by a combination of first and last name and project the result of the grouping as just the state visited
var groups = stateVisitors.GroupBy(sv => new
{
sv.FirstName,
sv.LastName,
},
sv => sv.StateVisited );
Output:
foreach(var g in groups)
{
// g.Key.FirstName
// g.Key.LastName
// g is IEnumerable<string> of visited states
}

An example, to fill your VisitorsWithCombinedStates class:
VisitorsWithCombinedStates[] visitorsWithCombinedStates = StateVisitors.GroupBy(x => new { x.FirstName, x.LastName},x => x.StateViseted)
.Select(x => new VisitorsWithCombinedStates { FirstName = x.Key.FirstName, LastName = x.Key.LastName, StatesVisetedArray = x.ToArray() }).ToArray();

How do I initialize this anonymous function?

var groups = new List<Group>
{
new Group{
Name = "Train",
Members = new List<Colleague>{
{FirstName = "Thomas", LastName = "Tank"},
{FirstName = "Honey", LastName = "Booboo"}
}
},
new Group{Name = "Bus"}
};
I am getting a red underline under 'FirstName' and 'LastName'... meaning that I've initialised it wrong...
How can I initialise that (Colleague) List? I think I am incorrectly initialising that list (Members)...
Edit: To make things clearer,
public class Group
{
public string Name { get; set; }
public List<Colleague> Members { get; set; }
}
Edit: Following up discussion with Kirk Woll:
This is what I did with your advice. Members is null though...
var groups = new List<Group>
{
new Group
{
Name = "Train",
Members = new List<Colleague>{
new Colleague { FirstName = "Thomas", LastName = "Tank" },
new Colleague { FirstName = "Jet", LastName = "Starr" }
}
},
new Group{Name = "Bus"}
};
To summarise, there's a breakpoint after the initialisation of groups. Each Group (x2) has Name defined, but Members = null in the first Group ('Train'). Members must be initialised! Cheers.

You want:
Members = new List<Colleague>
{
new Colleague { FirstName = "Thomas", LastName = "Tank" },
new Colleague { FirstName = "Honey", LastName = "Booboo" }
}
Since it's a list of Colleague to which you're trying to add it.

Pulling objects from a collection that satisfies a condition

I was wondering whether you know using LINQ in C# to pull a list of objects from a collection that satisfies a condition?
I am trying to pull a list of Person objects from a list whose IDs match in an integer list. Please see the code below (I am trying to pull all person objects whose IDs are in the integer list).
public class Person
{
public int Id { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
}
...
var intList = new List<int>() { 1, 2, 3, 4 };
var perList = new List<Person>
{
new Person {Id = 1, FirstName = "Thomas", LastName = "Joseph"},
new Person {Id = 2, FirstName = "Joseph", LastName = "Austin"},
new Person {Id = 3, FirstName = "Lee", LastName = "Hentry"},
new Person {Id = 4, FirstName = "Abraham", LastName = "Tony"}
};

You can use the Where and Contains methods:
var people = perList
.Where(person => intList.Contains(person.Id))
.ToList();

You can use the Where and Any methods:
var filteredPeople = perList.Where(person => intList.Any(person.Id)).ToList();

How to Display the Members of a Class

I'm trying to create a wrapper for selecting multiple items from a single array. I get the result at the end of the code below. Not sure what I'm doing wrong.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Tester.cs
{
class Program
{
static void Main(string[] args)
{
var customers = new[]
{
new { CustomerID = 1, FirstName = "Orlando", LastName = "Gee",
CompanyName = "A Bike Store" },
new { CustomerID = 2, FirstName = "Keith", LastName = "Harris",
CompanyName = "Bike World" },
new { CustomerID = 3, FirstName = "Donna", LastName = "Carreras",
CompanyName = "A Bike Store" },
new { CustomerID = 4, FirstName = "Janet", LastName = "Gates",
CompanyName = "Fitness Hotel" },
new { CustomerID = 5, FirstName = "Lucy", LastName = "Harrington",
CompanyName = "Grand Industries" },
new { CustomerID = 6, FirstName = "David", LastName = "Liu",
CompanyName = "Bike World" },
new { CustomerID = 7, FirstName = "Donald", LastName = "Blanton",
CompanyName = "Grand Industries" },
new { CustomerID = 8, FirstName = "Jackie", LastName = "Blackwell",
CompanyName = "Fitness Hotel" },
new { CustomerID = 9, FirstName = "Elsa", LastName = "Leavitt",
CompanyName = "Grand Industries" },
new { CustomerID = 10, FirstName = "Eric", LastName = "Lang",
CompanyName = "Distant Inn" }
};
var addresses = new[] {
new { CompanyName = "A Bike Store", City = "New York", Country = "United States"},
new { CompanyName = "Bike World", City = "Chicago", Country = "United States"},
new { CompanyName = "Fitness Hotel", City = "Ottawa", Country = "Canada"},
new { CompanyName = "Grand Industries", City = "London", Country = "United Kingdom"},
new { CompanyName = "Distant Inn", City = "Tetbury", Country = "United Kingdom"}
};
IEnumerable<Names> customerfullName = customers.Select(data => new Names {
FirstName = data.FirstName,
LastName = data.LastName});
foreach (Names entry in customerfullName)
{
Console.WriteLine(entry);
}
}
}
class Names
{
public string FirstName { get; set; }
public string LastName { get; set; }
}
}
Tester.cs.Names is what i get repeated when I run the program.

Console.WriteLine uses the ToString method of the object class. By default, that displays the name of the class.
This method is overridden by classes derived from object to display whatever they want. You have not overridden it, so you get the default.
You can reproduce your problem, without LINQ, as follows:
class Names
{
public string FirstName { get; set; }
public string LastName { get; set; }
}
var name = new Names {FirstName = "John", LastName = "Saunders"};
Console.WriteLine(name); // Will display "Tester.cs.Names"

default the ToString will be used, use:
class Names
{
public string FirstName { get; set; }
public string LastName { get; set; }
public override string ToString()
{
return string.Format("{0} {1}", FirstName, LastName);
}
}
It's also possible to create an extra property for the fullname
class Names
{
public string FirstName { get; set; }
public string LastName { get; set; }
public string FullName
{
get
{
return string.Format("{0} {1}", FirstName, LastName);
}
}
}
usage:
foreach (Names entry in customerfullName)
{
Console.WriteLine(entry.FullName);
}

Your Names class has not overridden the ToString method, so it is using the default implementation from object and printing out it's type name. You either need to override ToString in Names to print out the underlying strings, or you need to print out the individual string properties in your foreach loop.