Question

我遇到了nullpointer的问题。我似乎无法理解为什么我的代码是错误的。任何帮助将不胜感激。这是我的代码：

     protected void Page_Load(object sender, EventArgs e)
   {

    Product aProd = new Product(); 
     // Get Product ID from querystring 
     string prodID = Request.QueryString["ProdID"].ToString(); (null pointer occurs at this line. )

    prod = aProd.getProduct(prodID); 
      //Display product details on web form 

     lbl_ProdName.Text = prod.Product_Name;    
    lbl_ProdDesc.Text = prod.Product_Desc;    
    lbl_Price.Text = prod.Unit_Price.ToString("c");
    img_Product.ImageUrl = "~\\Images\\" + prod.Product_Image;    

    //Store the value in invisible fields 
    lbl_Price.Text = prod.Unit_Price.ToString();      
    lbl_ProdID.Text = prodID.ToString(); 
}

这些是Product.cs的代码。我仍然不知道我的代码出错了

      public class Product
   {

string _connStr = ConfigurationManager.ConnectionStrings["HealthDBContext"].ConnectionString;
private string _prodID = null;
private string _prodName = string.Empty;
private string _prodDesc = ""; // this is another way to specify empty string
private decimal _unitPrice = 0;
private string _prodImage = "";
private int _stockLevel = 0;

// Default constructor
public Product()
{
}

// Constructor that take in all data required to build a Product object
public Product(string prodID, string prodName, string prodDesc,
               decimal unitPrice, string prodImage, int stockLevel)
{
    _prodID = prodID;
    _prodName = prodName;
    _prodDesc = prodDesc;
    _unitPrice = unitPrice;
    _prodImage = prodImage;
    _stockLevel = stockLevel;
}

// Constructor that take in all except product ID
public Product(string prodName, string prodDesc,
       decimal unitPrice, string prodImage, int stockLevel)
    : this(null, prodName, prodDesc, unitPrice, prodImage, stockLevel)
{
}

// Constructor that take in only Product ID. The other attributes will be set to 0 or empty.
public Product(string prodID)
    : this(prodID, "", "", 0, "", 0)
{
}

// Get/Set the attributes of the Product object.
// Note the attribute name (e.g. Product_ID) is same as the actual database field name.
// This is for ease of referencing.
public string Product_ID
{
    get { return _prodID; }
    set { _prodID = value; }
}
public string Product_Name
{
    get { return _prodName; }
    set { _prodName = value; }
}
public string Product_Desc
{
    get { return _prodDesc; }
    set { _prodDesc = value; }
}
public decimal Unit_Price
{
    get { return _unitPrice; }
    set { _unitPrice = value; }
}
public string Product_Image
{
    get { return _prodImage; }
    set { _prodImage = value; }
}
public int Stock_Level
{
    get { return _stockLevel; }
    set { _stockLevel = value; }
}

Answer 1

Request.QueryString["ProdID"]返回null。因此，ToString()来电会导致System.NullPointerException

解决方案：确保您的Request.QueryString["ProdID"]来电回复正确的对象，或自行测试结果：

var obj = Request.QueryString["ProdID"];
string prodID = obj == null ? String.Empty : obj.ToString();

遇到Nullpointer错误

1 个答案: