ASP.NET MVC 在进行聚合时抛出错误 - 任何人都可以帮助我吗？

Question

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I have a scoreboard. In this table, when users make a transaction, the score is recorded with that user's id. But someone who is a new member of the site gives the above error because they do not have any data in this table. It can add users with data in the table, but it gives an error when a new user becomes a member. How can I solve this? (I have attached the link of the error image. You can open and see)

 using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using ProjectTiklaKazan.Models.Sınıf;
namespace ProjectTiklaKazan.Controllers
{
    [Authorize]
    public class AnaMenuController : Controller
    {
        Context c = new Context();
        // GET: AnaMenu
        
        public ActionResult Index()
        {
            Users u = Session["UserID"] as Users;
            var toptohum = c.PuanBilgis.Where(x => x.UserID == u.UserID).Sum(x => x.PuanMiktar).ToString();
            if (toptohum !=null)
            {
                TempData["toptohum"] = toptohum;
            }
            else
            {
                TempData["toptohum"] = 0;
            }
            return View();
        }
    }
}

Answer 1

You're hitting this issue because IEnumerable<T> only knows that a collection has no items in it by iterating over the collection.

In your case, c.PuanBilgis.Where(x => x.UserID == u.UserID) yields an IEnumerable<> that has no contents, but it doesn't know that it has no contents. Then the Sum(...) LINQ extension is called, and tries to iterate over the IEnumerable<> , but hits an exception when it tries to read from the database and there is no data to be read.

However, IList<T> has a Count property on it that keeps track of the number of elements in the collection. So if you can convert your IEnumerable<T> into an IList<T> , then some of the LINQ extensions will be able to take advantage of the Count property and save itself from the cost of enumerating the collection.

This does have an up-front perf cost of enumerating the data entirely instead of on-demand, but it's not much, especially if the collection is empty.

In your case, you could do something like this:

c.PuanBilgis.Where(x => x.UserID == u.UserID).ToList().Sum(x => x.PuanMiktar).ToString()