medium-2016-09-22c.cs

using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Threading.Tasks;
using AutoMapper;
using MediatR;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.EntityFrameworkCore;

public class TasksEditViewModel
{
    public int Id { get; set; }
    
    public bool IsAdding => Id == 0;
    
    [Required(ErrorMessage = "Please provide a description")]
    [StringLength(500, ErrorMessage = "The description cannot be 
        longer than 500 characters")]
    public string Description { get; set; }
    
    [Required]
    [Display(Name = "Category")]
    public int CategoryId { get; set; }
    
    public SelectList CategoryOptions { get; set; }
}

public class TasksEditViewModelQuery : 
    IAsyncRequest<TasksEditViewModel>
{
    public int Id { get; set; }
}

public class TasksEditViewModelQueryHandler : 
    QueryHandlerBase, 
    IAsyncRequestHandler<TasksEditViewModelQuery,TasksEditViewModel>
{
    public TasksEditViewModelQueryHandler(ToDoContext context, 
            IMapper mapper)
        : base(context, mapper)
    {
    }

    public async Task<TasksEditViewModel> Handle(
        TasksEditViewModelQuery query)
    {
        var model = new TasksEditViewModel();
        if (query.Id > 0)
        {
            var task = await GetTask(query.Id);
            Mapper.Map(task, model);
        }
        
        model.CategoryOptions = new SelectList(
            await Context.Categories
                .OrderBy(x => x.Name)
                .ToListAsync(), "Id", "Name", model.CategoryId);
        return model;
    }
    
    private async Task<Models.Task> GetTask(int id)
    {
        var task = await Context.Tasks
            .SingleOrDefaultAsync(x => x.Id == id);
        if (task == null)
        {
            throw new NullReferenceException($"Task with id: {id} not found.");
        }
        
        return task;
    }       
}