沒有類型爲「IEnumerable <SelectListItem」的ViewData項目

Question

我有關於下拉的問題。從我的代碼中顯示數據庫的下拉值。但是，當我沒有填寫任何值，然後按輸入錯誤沒有ViewData項目類型'IEnumerable顯示。

以下是我的代碼：

Category.cs

using System; 
using System.Collections.Generic; 
using System.Linq; 
using System.Text; 
using System.ComponentModel.DataAnnotations; 
using System.Web.Mvc; 

namespace BusinessLayer 
{ 

    public interface ICategory 
    { 
     int CategoryId { get; set; } 
     string CategoryDescription { get; set; } 

    } 
    public class Category:ICategory 
    { 
     [Required] 
     public int CategoryId { get; set; } 
     [Required] 
     public string CategoryName { get; set; } 
     [Required] 
     public string CategoryDescription { get; set; } 
     [Required] 
     public SelectList CategoryList { get; set; } 
    } 
} 

CategoryBusinessLayer.cs

using System; 
using System.Collections.Generic; 
using System.Linq; 
using System.Text; 
using System.Configuration; 
using System.Data; 
using System.Data.SqlClient; 

namespace BusinessLayer 
{ 
    public class CategoryBusinessLayer 
    { 
     public IEnumerable<Category> Categories 
     { 
      get 
      { 
       string connectionString = 
        ConfigurationManager.ConnectionStrings["DBCSS"].ConnectionString; 

       List<Category> categoryList = new List<Category>(); 

       using (SqlConnection con = new SqlConnection(connectionString)) 
       { 
        SqlCommand cmd = new SqlCommand("spGetAllCategory", con); 
        cmd.CommandType = CommandType.StoredProcedure; 
        con.Open(); 
        SqlDataReader rdr = cmd.ExecuteReader(); 
        while (rdr.Read()) 
        { 
         Category categories = new Category(); 
         categories.CategoryId = Convert.ToInt32(rdr["CategoryId"]); 
         categories.CategoryName = rdr["CategoryName"].ToString(); 
         categories.CategoryDescription = rdr["CategoryDescription"].ToString(); 
         categoryList.Add(categories); 
        } 
       } 

       return categoryList; 
      } 
     } 
} 

ProductController.cs

[HttpGet] 
     [ActionName("Create")] 
     public ActionResult Create_Get() 
     { 
      Category category = new Category(); 
      CategoryBusinessLayer cat = new CategoryBusinessLayer(); 
      category.CategoryList = new SelectList(cat.Categories, "CategoryId", "CategoryName"); 
      ViewBag.category = category.CategoryList; 
      return View(); 
     } 

Create.cshtml

@Html.DropDownList("CategoryId", (IEnumerable<SelectListItem>)ViewBag.category as SelectList, "Select Category") 

謝謝。

Answer 1

這通常發生在您提交表單時，並且您的HttpPost操作方法返回相同的視圖而無需重新加載構建SELECT元素所需的ViewData項目。

在您的GET和POST操作中，您可以簡單地將一列SelectListItem設置爲ViewBag/ViewData。此外，您目前的代碼不必要地執行了兩次投射！在此視圖下使用此表達式

(IEnumerable<SelectListItem>)ViewBag.category as SelectList 

請確保在返回相同視圖之前重新加載此數據。

在你看來

[HttpPost] 
public ActionResult Create(SomeViewModel model) 
{ 

    // your existing code to get category 
    ViewBag.category = category.CategoryList 
           .Select(x=> new SelectListItem { Value=x.Id, 
                Text= x.CategoryName }).ToList(); 
    return View(model); 
} 

現在，

@Html.DropDownList("CategoryId", ViewBag.category as List<SelectListItem>, 
                     "Select Category")