视图中抛出Null异常

Question

Using ASP.NET MVC, .NET Framework 4.5.2, Entity Data Model for SQL DB, Visual Studio 2017.

I have a class generated from the ADO.NET(EF Designer from Database) :

BookInfo.cs

 namespace LibraryMS
{
 using System;
 using System.Collections.Generic;

public partial class BookInfo
{
    public string BookID { get; set; }
    public string Title { get; set; }
    public string Author { get; set; }
    public string Publisher { get; set; }
    public string PublishDate { get; set; }
    public string Edition { get; set; }

    public virtual Inventory Inventory { get; set; }
}
}

The database is designed where the "BookID" in the BookInfo table has a foreign key "BookID" in the Inventory table. In a view to update an inventory's properties referenced by "BookID", I then proceed to query the list and update the correct instance.

Screenshot of update inventory page:

When landing on page to enter info the [HttpGet] UpdateInventory() is called, when clicking "Create" button as seen above, the [HttpPost] UpdateInventory(...) is called.

Logic/Code in Controller:

    [HttpGet]
    public  ActionResult UpdateInventory()
    {
        return View();
    }


    [HttpPost]
    public async Task<ActionResult> UpdateInventory(string bookID, string ttlIn, string lowin, string outnow)
    {
        var bf = await SqlRestApiHelper.searchFromBooks(bookID);

        bf.Inventory.TotalIn = Convert.ToInt16(ttlIn);
        bf.Inventory.LowIn = Convert.ToInt16(lowin);
        bf.Inventory.Out = Convert.ToInt16(outnow);
        await SqlRestApiHelper.UpdateBookInfoInventory(bf.Inventory);

        await SqlRestApiHelper.SaveChanges();
        return View("All");            
    }

 [HttpGet]
    public async Task<ActionResult> All()
    {
        return View(await SqlRestApiHelper.getAllBooksInfo(0, 10));
    }

SqlRestApiHelper.cs

namespace LibraryMS
{
public static class SqlRestApiHelper
{
   private static libraryDBEntities entities = new libraryDBEntities();


    public static async Task<LibraryMS.BookInfo> searchFromBooks(string id)
    {
       return entities.BookInfoes.ToList().Find(book => book.BookID == id);
    }

    public static async Task UpdateBookInfoInventory(LibraryMS.Inventory inv)
    {
        var newInv = inv;

        var el = entities.BookInfoes.ToList().Find(x => x.Inventory.BookID == newInv.BookID);
        if (el != null) 
        {
            el.Inventory.TotalIn = newInv.TotalIn;
            el.Inventory.LowIn = newInv.LowIn;
            el.Inventory.Out = newInv.Out;
            // the above updates the list item referenced
        }

    }

    public static async Task SaveChanges()
    {
       await entities.SaveChangesAsync();
    }

 public static async Task<IPagedList<BookInfo>> getAllBooksInfo(int page, int itemsPerPage)
    {
        List<BookInfo> bookinfo = new List<BookInfo>();

            bookinfo = (from o in entities.BookInfoes
                      orderby o.Title descending //use orderby, otherwise Skip will throw an error
                      select o)
                      .Skip(itemsPerPage * page).Take(itemsPerPage)
                      .ToList();
        int totalCount = bookinfo.Count();//return the number of pages
        IPagedList<BookInfo> pagebooks = new StaticPagedList<BookInfo>(bookinfo, page + 1,10,totalCount);
        return pagebooks;//the query is now already executed, it is a subset of all the orders.
    }

The Null Exception Thrown:

Code for all.cshtml view page:

 @model PagedList.IPagedList<LibraryMS.BookInfo>
 @using PagedList.Mvc;
 @{
ViewBag.Title = "All";
 }

 <h2>all</h2>

 <table class="table">

 @foreach (var item in Model) {
 <tr>
    <td>
        @Html.DisplayFor(modelItem => item.Title)
    </td>
    <td>
        @Html.DisplayFor(modelItem => item.Author)
    </td>
    <td>
        @Html.DisplayFor(modelItem => item.Publisher)
    </td>
    <td>
        @Html.DisplayFor(modelItem => item.PublishDate)
    </td>
    <td>
        @Html.DisplayFor(modelItem => item.Edition)
    </td>

    <td>
        @Html.ActionLink("Details","Details",new { item.BookID})
    </td>
</tr>
 }

 </table>

 @Html.PagedListPager(Model, page => Url.Action("All","BookInfoController", new { page }))

Answer 1

your view is throwing error because you are returning the view without passing the model, you are using return View("All") without passing the model the right way is by passing the model with the view, you can do it this way View("ViewName", ModelData);

in your case

return View("All", await SqlRestApiHelper.getAllBooksInfo(0, 10));

for the saving part I am not sure why, but i can see few errors, first what if book does not have inventory info ? it will throw null error, so first check if null, create new inventory, if not update accordingly here is how i would do it

 public static async Task UpdateBookInfoInventory(Inventory inv)
 {
    var newInv = inv;
     // get book info
    var el = entities.BookInfoes.FirstOrDefault(x => x.BookID == inv.BookID);
    if (el != null) 
    {
         if(el.Inventory != null)
       {
           // update accordingly
        el.Inventory.TotalIn = newInv.TotalIn;
        el.Inventory.LowIn = newInv.LowIn;
        el.Inventory.Out = newInv.Out;
        // the above updates the list item referenced
    }
    else
      { 
        /// add new if null 
         el.inventory = newInv;  
     }
   await SqlRestApiHelper.SaveChanges();

}

Answer 2

As a first step , put the breakpoint in the getAllBooksInfo method and see whether the list count is coming or not in visual studio. This will help you a lot.

And As an alternative step, you can also solve this error by using the ToPagedList(pageIndex, pageSize); method, i have used it personally and it was worked well

ToPagedList example as per your code:

     **public static async Task<IPagedList<BookInfo>> getAllBooksInfo(int page, int itemsPerPage)
        {
            List<BookInfo> bookinfo = new List<BookInfo>();

                bookinfo = (from o in entities.BookInfoes
                          orderby o.Title descending //use orderby, otherwise Skip will throw an error
                          select o)
                          .Skip(itemsPerPage * page).Take(itemsPerPage)
                          .ToList();
            int totalCount = bookinfo.Count();//return the number of pages

//changes made to the below line

            IPagedList<BookInfo> pagebooks = bookinfo.ToPagedList(1, 10);


            return pagebooks;//the query is now already executed, it is a subset of all the orders.
        }**

official source: https://github.com/troygoode/PagedList

Note: Even with this approach, kindly check whether you are getting the data or not from the database by using the breakpoint in the changed line.

Hope this will surely solve your problem kindly let me know thoughts or feedbacks

Thanks

karthik