The core for asynchronous programming are the objects Task and Task<T>. Both of them are compatible with the keywords async and await.

First of all we need to identify if the code’s I/O-bound or CPU-bound.

• the code’s limited for external operations and waits for something a lot of time. Examples of this are DDBB calls, or a server’s response. In this case we have to use async/await to free the thread while we wait
• the code does a CPU-intensive operation. Then we move the work to another thread using Task.Run() so we don’t block the main thread.

async code vs parallel code

(!) Asynchronous code is not the same as parallel code (!)

• In async code you are trying to make your threads do as little work as possible. This will keep your app responsibe, capable to serve many requests at once and scale well.
• In parallel code you do the opposite. You use and keep a hold on a thread to do CPU-intensive calculations

async code

The importante of async programming is that you choose when to wait on a task. This way, you can start other tasks concurrently

In async code, one single thread can start the next task concurrently before the previous one completes.
(!) async code doesn’t cause additional threads to be created because an async method doesn’t run on its own thread. (!) It runs on the current synchronization context and uses time on the thread only when the method is active.

parallel code

For parallelism you need multiple threads where each thread executes a task, and all of those tasks are executed at the same time

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En el Program tenemos código que depende del nugget y la implementación de la base de datos que vamos a integrar.
Este usa una cadena del appsettings.json para conectar a la BBDD

"server=localhost;database=postgres;uid=postgres;password=thisisSomepassword"

Para crear una nueva entidad en la base de datos:

• Creamos el Model pertinente (por ej.) Coche
• Lo añadimos al ApplicationDbContext como un DbSet<Coche>
• Añadimos una migration add-migration addedCoche
• Ejecutamos update-database

Migration

Mecanismo para aplicar cambios en el modelo de datos de la aplicacion a la BBDD de manera incremental y versionable. Cada migracion contiene los pasos necesarios para llevar a cabo las modificaciones en la BBDD como agregar nuevas tablas, cambiar columnas existentes o eliminar indices.

Esto permite tener:

• Despliegues consistentes: aplica mismos cambios en diferentes entornos.
• Control de versiones
• Evolucionar la BBDD

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Go to Tools > Nuget Package Manager > Package Manager Console

Adapt and paste the following code. This includes:

• db’s connection string
• name for the context it’s going to create
• Where it pastes the data classes to

Scaffold-DBContext "Host=host_here;Database=database_name;Username=username_here;Password=pwd_here" Npgsql.EntityFrameworkCore.PostgreSQL -DataAnnotations -Context ContextNameHereDbContext -ContextDir Data -o Models/DB -force -NoOnConfiguring -verbose

This creates:

• DbContext.cs class
• all Model/Data classes

Remember to modify Startup.cs to set the service classes as AddTransient for the services which use this DbContext

services.AddTransient<SomethingService, SomethingService>(); 

Add into the service, the context you’ve just created and set it at the constructor.

private readonly ApplicationDbContext _context;

Cuando se van a realizar consultas de solo lectura, las cuales NO se van a usar para actualizar en la base de datos, se puede llamar al método .AsNoTracking() para mejorar la performance.
Es recomendable utilizarla para manejar grandes volumenes de entidades.

Las entidades de las queries donde se use .AsNoTracking() no se podrán actualizar, por lo que no es recomendable para ADD, UPDATE o DELETE

// operacion de solo lectura
var libros = context.Libros
	.AsNoTracking()
	.Where(l => l.Autor == "Something")
	.ToList();

You can easily mark fields for a model which are primary key or are required.

public class User
{
	[Key]
	public int Id { get; set; }
	
	[Required(ErrorMessage = "Name is mandatory")]
	public string Name { get; set; }
	
	public string Telephone { get; set; }
	
	public string Mobile { get; set; }
	
	[Required(ErrorMessage = "Email is mandatory")]
	public string Email { get; set; }
}

Tipos Nullable (?)

El operador ? se usa para convertir un tipo de valor en un tipo de valor nullable, el cual puede contener un valor nulo.

int? num = null;
if(num is int numValue)
{
	Console.WriteLine(numValue); 
}

Acceso condicional nulo a miembros (?. o ?[])

(!) (revisar abajo como usarlo mejor junto al operador ??) (!)

Este operador permite acceder a un miembro de un objeto solo si el objeto no es nulo, evitando asi una NullReferenceException. Si no se cumple, devuelve null.

• si a es nulo, el resultado de a?.x o a?[x] es null
• si a no es nulo, el resultado de a?.x o a?[x] es el mismo que a.x o a[x]

string[] nombres = null;
int? longitud = nombres?.Length; // asigna null en lugar de lanzar una excepcion

Coalescencia de Nulos (??)

Se usa para proporcionar un valor por defecto en caso de que una expresion nullable contenga un null.

sin este operador

// DON'T DO THIS
string name = GetName();
if(name == null)
{
	name = "unknown";
}

se reemplaza por esto usando el operador

// do this instead
string name = GetName() ?? "unknown";

Se recomienda usar ?? en vez de expresiones ternarias para comprobar un null

// DON'T DO THIS
var v = x == null ? x : y;

// do this instead
var v = x ?? y;

uso de operador .? junto a ??

Se puede usar el operador ?? junto a .? para asignar un valor por defecto, en caso de que alguna propiedad accedida mediante .? sea nula en algún punto

ejemplo 1

string[] nombres = // something 
string nombre = nombres?[index] ?? "NotFound"; // array is null or name not found

ejemplo 2

// if the value someList from request is null, this if will check as false
if ((request?.someList?.Count ?? 0) <= 0)
{
	// ... do something
}

También se puede usar junto a un throw para hacer más concisa la asignación de valores.

public string Name
{
	get => name;
	set => name = value ?? throw new ArgumentNullException(nameof(value), "Name cannot be null");
}

Asignacion de coalescencia de nulos (??=)

reemplaza este formato de código

if (variable is null) 
{
	variable = "something";
}

por esto. lo hace más conciso

variable ??= "something";

Reference(s)

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/null-coalescing-operator
https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/style-rules/ide0029-ide0030-ide0270
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/member-access-operators#null-conditional-operators–and-

Retrieve a simple subset of properties for all items in list

Usually, anon data types are used in the select clause to return a specific subset of properties for each object in the collection.

we have the employee class

public class Employee
{
	public int ID { get; set; }
	public string Name { get; set; }
	public int Age { get; set; }
	public string Address { get; set; }
}

List<Employee> employees = // ...

// we convert an Employee into an anon object type with a subset of properties
var employeeDetails = from emp in employees
	select new { Id = emp.ID, Name = emp.Name };

How to map List of classes

public List<MappedUser> MapListOfUsers(List<User> users)
{
	// method 1
	List<MappedUser> mappedUsers = users.ConvertAll(user => MapSingleUser(user));
	
	// method 2
	List<MappedUser> mappedUsers2 = 
		(from user in users select MapSingleUser(user)).ToList();
}

method to encapsulate mapping itself

private MappedUser MapSingleUser(User user)
{
 var mapped = new MappedUser
 {
	 Id = user.Id,
	 Name = user.Name,
	 Email = user.Email
 };
 return mapped;
}

This provides easier and more legible than doing a foreach to iterate everything.

How to filter list per properties (Where)

var adminUserTask = users
	.Where(user => "admin".Equals(user.type.ToLower()))
	.Select(async user => { return await ProcessAdmin(user);});
List<UserResults> results = (await Task.WhenAll(adminUserTask)).ToList();

filter by properties in a nullable list and return true if there’s any row that match. If the list is null, it returns false.

return response.results?.rows
	.Where(row => row.id == requestId && (row.owner == requestOwner || row.responsible == requestResponsible))
	.Any() ?? false;

another example

var task = response.results.rows
	.AsParallel()
	.Where(row => "specifictype".Equals(row.type.ToLower()))
	.Select(async row => {
		if(row.type.Equals("specificType"))
		{
			return await Something(row, id, log);
		} else 
		{
			return row;
		}
	});

How to select records based on another list (ids)

This is how to select a list of items, selecting them by id, based on another list of items

List<string> idList = // ...
var profiles = _context.UserProfiles
		.Where(userProf => idList.Contains(userProf.Id));

How to order based on a dynamic parameter

// Direction is an Enum w. values ASC or DESC
private List<Person> SortPersons(Direction direction, List<Person> persons, Func<Person, string> sortBy)
{
	if(direction.DESC)
	{
		return persons.OrderByDescending(sortBy.Invoke).ToList();
	} else 
	{
		return persons.OrderBy(sortBy.Invoke).ToList();
	}
}

How to use it

var sortedPersons = SortPersons(direction, persons, person => person.Name);

Single param lambda

remember that for lambdas with a single param where it comes from the same query, you don’t need to explicitely set it

this transforms

public List<int> FilterList(List<int> listToFilter, List<int> filterintList)
{
	return listToFilter.Where(number => filteringList.Contains(number)).ToList();
}

Methods worth a mention

Remove duplicates Distinct()

var task = response.results.rows
	.AsParallel()
	.Where(row => "specificType".Equals(row.type.ToLower()))
	.Select(async row => { return await Something(row, id, log);});

Get the difference of two lists Except()

var list = listToFilter
	.Except(filteringList)
	.ToList();

First() vs Single()

// gets the first element that matches and stops there
string list = listToFilter
	.First(s => s.StartsWith("ohno"));

// gets the matching element, or throws an exception if there's more than 1
string list = listToFilter
	.Single(s => s.StartsWith("ohno"));

C# tiene los DataAnnotation

[Required] hace que sea un campo obligatorio

[Required(ErrorMessage = "Nombre es obligatorio")]
public string NombreCategoria { get; set; }

[Required(ErrorMessage = "Orden es obligatorio")]
[Range(1, int.MaxValue, ErrorMessage = "El orden debe de ser mayor a cero")]
public int Orden {get; set; }

Se controla mediante el siguiente codigo en el controller. El código de ModelState es código base de un Controller.

if(ModelState.IsValid) 
{
	// code if everything's valid
}

Interfaces must start by capital I

public interface IDataService 
{ 
	public Task SendData(DataModel model);
}

We use PascalCase for:

• classes’ name
• methods’ name
• public variables

We use CamelCase for:

• private or internal field names (they must include the prefix _)
• methods’ paramters

public class DataService
{
	const int TAX = 7;
	
	public bool IsValid { get; private set; };
	private IWorkerQueue _workerQueue;
	
	public async Task SendData(DataModel model)
	{
		string someValue = "";
		// ... whatever
	}
}

async methods must end by Async

public async Task<string> GetUrlAsync()
{
	// ... whatever
}

Reference(s)

https://learn.microsoft.com/es-es/dotnet/csharp/fundamentals/coding-style/identifier-names

(related tool on how to upgrade a project to .NET 8)

Setup a project for testing

Agregar > nuevo proyecto

Proyecto de prueba NUnit / xunit

Ponerle como nombre `.Tests

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CLR (Common language runtime)

Entorno de ejecucion para .NET. En tiempo de ejecucion el compilador de CLR convierte el codigo CIL en codigo nativo para el SO. Facilita la integración entre lenguajes.

CLR es la MV en la que se ejecutan nuestras apps. CLR se hizo para tener una capa de abstraccion entre las propias apps y el SO donde se ejecutaban.

El CLI se puede ejecutar en otros SO. El CLR se ejecuta solo en Windows.

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