medium-2016-09-22d.cs

using System.Threading.Tasks;
using MediatR;

public class TaskCompleteCommand : IAsyncRequest<CommandResult>
{
    public int Id { get; set; }
}

public class TaskCompleteCommandHandler : CommandHandlerBase, 
    IAsyncRequestHandler<TaskCompleteCommand, CommandResult>
{
    public TaskCompleteCommandHandler(ToDoContext context)
        : base(context)
    {
    }
    
    public async Task<CommandResult> Handle(
        TaskCompleteCommand command)
    {
        var task = await GetTask(command.Id);
        task.MarkComplete();
        await Context.SaveChangesAsync();
        return CommandResult.SuccessResult();
    }
    
    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;
    }       
}