我使用的 reader 是错误的还是我的 reader 可能有空值？

Question

我正在从事一个电子商务项目，但在使用 reader 时遇到了一些麻烦。这是我的 ShoppingCartController 代码，它不起作用

public List<PRODUCT> GetCartItems()
    {
        SqlConnection sqlConnection1 = new SqlConnection("MyConnection");
        SqlCommand cmd = new SqlCommand("uspGetCart", sqlConnection1);
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.Parameters.Add("@Username", SqlDbType.VarChar).Value = User.Identity.GetUserName();
        sqlConnection1.Open();
        SqlDataReader reader = cmd.ExecuteReader();
        List<PRODUCT> cartList = new List<PRODUCT>();
        PRODUCT product;

        while (reader.Read())
        {
            product = new PRODUCT();
            product.MANUFACTURER.ManufacturerName = reader["ManufacturerName"].ToString();
            product.Name = reader["Name"].ToString();
            product.Cost = decimal.Parse(reader["Cost"].ToString());
            cartList.Add(product);
        }
        sqlConnection1.Close();
        return cartList;
    }

我得到的错误是对象引用未设置到对象的实例。

堆栈跟踪是

    [NullReferenceException: Object reference not set to an instance of an object.]
    SeeSharpBeans.Controllers.ShoppingCartController.GetCartItems() in c:\Users\Andrew\Documents\Visual Studio 2013\Projects\SeeSharpBeans\SeeSharpBeans\Controllers\ShoppingCartController.cs:54
    SeeSharpBeans.Controllers.ShoppingCartController.Index() in c:\Users\Andrew\Documents\Visual Studio 2013\Projects\SeeSharpBeans\SeeSharpBeans\Controllers\ShoppingCartController.cs:22
    lambda_method(Closure , ControllerBase , Object[] ) +101
    System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase controller, Object[] parameters) +59
    System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) +435
    System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +60
    System.Web.Mvc.Async.ActionInvocation.InvokeSynchronousActionMethod() +76
    System.Web.Mvc.Async.AsyncControllerActionInvoker.<BeginInvokeSynchronousActionMethod>b__39(IAsyncResult asyncResult, ActionInvocation innerInvokeState) +36
    System.Web.Mvc.Async.WrappedAsyncResult`2.CallEndDelegate(IAsyncResult asyncResult) +73
    System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +136
    System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +102



System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult) +49
   System.Web.Mvc.Async.AsyncInvocationWithFilters.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3d() +117
   System.Web.Mvc.Async.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f() +323
   System.Web.Mvc.Async.<>c__DisplayClass33.<BeginInvokeActionMethodWithFilters>b__32(IAsyncResult asyncResult) +44
   System.Web.Mvc.Async.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult) +47
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +136
   System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +102
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethodWithFilters(IAsyncResult asyncResult) +50
   System.Web.Mvc.Async.<>c__DisplayClass2b.<BeginInvokeAction>b__1c() +72
   System.Web.Mvc.Async.<>c__DisplayClass21.<BeginInvokeAction>b__1e(IAsyncResult asyncResult) +185
   System.Web.Mvc.Async.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult) +42
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +133
   System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +56
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +40
   System.Web.Mvc.Controller.<BeginExecuteCore>b__1d(IAsyncResult asyncResult, ExecuteCoreState innerState) +34
   System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +70
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +133
   System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +56
   System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +37
   System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +44
   System.Web.Mvc.Controller.<BeginExecute>b__15(IAsyncResult asyncResult, Controller controller) +39
   System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +62
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +133
   System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +56
   System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +37
   System.Web.Mvc.Controller.EndExecute(IAsyncResult asyncResult) +39
   System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.EndExecute(IAsyncResult asyncResult) +39

我的产品模型是这样的

public partial class PRODUCT
{
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
    public PRODUCT()
    {
        this.PURCHASES = new HashSet<Purchase>();
        this.TRANSACTIONS = new HashSet<TRANSACTION>();
    }

    public int ProductID { get; set; }
    public int ManufacturerID { get; set; }
    public string Name { get; set; }
    public int Quantity { get; set; }
    public decimal Cost { get; set; }

    public virtual MANUFACTURER MANUFACTURER { get; set; }
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<Purchase> PURCHASES { get; set; }
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<TRANSACTION> TRANSACTIONS { get; set; }
}

Answer 1

如果您的数据不包含空值，那么可能的问题是您没有将 MANUFACTURER 属性初始化为 PRODUCT 构造函数中的值，这意味着它是当您尝试将 product.MANUFACTURER.ManufacturerName 设置为一个值时为 null。

另请注意，命名类和全部大写的成员通常是错误的形式 - 考虑将它们更改为 Product 和 Manufacturer。

Answer 2

我认为问题可能出在行

product.MANUFACTURER.ManufacturerName = reader["ManufacturerName"].ToString();

MANUFACTURER 是否在产品 class 中初始化？

Answer 3

尝试将集合项定义移动到 while 循环的范围内：

while (reader.Read()) 
{ 
PRODUCT product = new PRODUCT(); 
product.MANUFACTURER.ManufacturerName = reader["ManufacturerName"].ToString(); 

product.Name = reader["Name"].ToString();

product.Cost = decimal.Parse(reader["Cost"].ToString()); 
cartList.Add(product); }

Answer 4

This line is the issue
product.MANUFACTURER.ManufacturerName = reader["ManufacturerName"].ToString();

Use
 product.MANUFACTURER.ManufacturerName = Convert.ToString(reader["ManufacturerName"])  

 product.Cost = decimal.Parse(Convert.ToString(reader["Cost"]));
instead of .ToString();

Answer 5

需要使用它们的构造函数来初始化 MANUFACTURER，例如：-

while (reader.Read())
            {
                product = new PRODUCT();
                product.MANUFACTURER=new MANUFACTURER();
                product.MANUFACTURER.ManufacturerName = reader["ManufacturerName"].ToString();
                product.Name = reader["Name"].ToString();
                product.Cost = decimal.Parse(reader["Cost"].ToString());
                cartList.Add(product);
            }

我使用的 reader 是错误的还是我的 reader 可能有空值？

Am I using reader wrong or is it possible my reader has null values?

.net

c#

reader