Async in ASP.NET Core MVC - asp.net-core

Question

I am new to async programming. FirstOrDefaultAsync is throwing an error
List does not contain a definition for FirstOrDefaultAsync()
Can someone explain me what I am doing wrong?
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.EntityFrameworkCore;
namespace PatientManagement.Models
{
public class PatientService : IPatientService
{
private List<Patient> patients;
public PatientService()
{
patients = new List<Patient>()
{
new Patient(){ ID = 1, Name = "Akash", Email = "aakash1027#gmail.com" },
new Patient(){ ID = 2, Name = "John", Email = "John#gmail.com" },
new Patient(){ ID = 3, Name = "Mike", Email = "Mike#gmail.com" },
};
}
public async Task<Patient> GetPatient(int id)
{
return await patients.FirstOrDefaultAsync(x => x.ID == id);
}
}
}

Answer 1

The problem is there really is no FirstOrDefaultAsync method on a List!
Async code is appropriate for cases where you, for instance, cross a boundary and have to wait for a database or some other process to do work. Your case is different: your process actually has to do the work of finding this record in the list. So you should just use the normal synchronous FirstOrDefault:
public Patient GetPatient(int id)
{
return patients.FirstOrDefault(x => x.ID == id);
}