如何在对异步方法的调用上追加对扩展方法的调用

Question

I have a custom extension method "ToSelectList" and I am currently using it like so:

        var myList = await _myService.GetItems();
        var mySelectList = myList.ToSelectList(nameof(MyEntity.Id));

That's fine and works great, however, I tried to do this as one line:

        var myList = await _myService.GetItems().ToSelectList(nameof(MyEntity.Id));

But I get:

'Task<List<MyEntity>>' does not contain a definition for 'ToSelectList'
and no extension method 'ToSelectList' accepting a first argument of type
'Task<List<MyEntity>>' could be found (are you missing a using directive 
or an assembly reference?)

My ToSelectList method is as follows:

using System.Collections.Generic;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc.Rendering;

namespace MyProject.Extensions
{
    public static class ListExtensions
    {
        public static List<SelectListItem> ToSelectList<T>(this List<T> list, string id, string text = "Name")
        {
            var selectListItems = new List<SelectListItem>();

            list.ForEach(item =>
            {
                selectListItems.Add(new SelectListItem
                {
                    Text = item.GetType().GetProperty(text).GetValue(item).ToString(),
                    Value = item.GetType().GetProperty(id).GetValue(item).ToString()
                });
            });

            return selectListItems;
        }
    }

I tried changing method signature to public async static Task<List<SelectListItem>> but no joy.

Answer 1

The problem is that await applies to the entire line so you either need to wrap it in parentheses:

var myList = (await _myService.GetItems())
    .ToSelectList(nameof(MyEntity.Id));

Or split it onto 2 lines:

var items = await _myService.GetItems();
var myList = items.ToSelectList(nameof(MyEntity.Id));

Answer 2

While @DavidG 's answer is correct, I would suggest you to simplify your logic. You don't need any of that reflection code if you use the built-in Tuple<T1, T2> :

// let's assume this is in your service class
// you most likely want a better name
public Task<List<Tuple<int, string>>> GetItemsAsEnumAsync()
{
    return _context.SomeTable
        .Select(x => Tuple.Create(x.Id, x.Name))
        .ToListAsync();
}

Your ToSelectList would then just be:

public static List<SelectListItem> ToSelectList(this List<Tuple<int, string>> list)
{
    return list
        .Select(x => new SelectListItem
        {
            Value = x.Item1.ToString(),
            Text = x.Item2
        })
        .ToList();
}

This assumes that your service is not in the same project as the ASP.NET Core application and hence cannot return the List<SelectListItem> itself. You could make ToSelectList more generic by using ToSelectList<T1, T2>(this List<KeyValuePair<T1, T2>> list) if you find it useful.

Answer 3

You can either go for:

var myList = _myService.GetItems().Result.ToSelectList(nameof(MyEntity.Id));

but this will just block the thread and you will execute code non async

or change result of _myService.GetItems() to List<SelectListItem> .

Anyway what is the point of this async if you immediately need the result?