关于.NET 未将对象引用设置到对象的实例

Question

public partial class Admin_AddSortOrCategory : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        BinData2();
    }
///绑定DropDownList（CategoryList）
  public void BinData2()
    {
        CategoryManager categoryManager = new CategoryManager();
        List<Category> List_Category = new List<Category>();
        List_Category = (List<Category>)categoryManager.GetAllCategories();        this.CategoryList.DataSource = List_Category;
        this.CategoryList.DataTextField = "Name";
        this.CategoryList.DataValueField = "Id";
        this.CategoryList.DataBind();    }///单击事件
  protected void SortButton_Click(object sender, EventArgs e)
    {
        SortManager sortManager = new SortManager();
        Sort sort = new Sort();
        sort.Name = SortText.Text.ToString();
        sort.Category.Id = Convert.ToInt32(CategoryList.SelectedItem.Value);
        sortManager.AddSort(sort);        Response.Redirect("AddSortOrCategory.aspx");
    }
为什么 【sort.Category.Id = Convert.ToInt32(CategoryList.SelectedItem.Value);】报错：未将对象引用设置到对象的实例
///AddSort()方法  
public int AddSort(Sort sort)
        {
            DBHelper dbHelper = new DBHelper();
            SqlParameter[] prams = new SqlParameter[]
            {
                dbHelper.CreateInParam("@Name",SqlDbType.NVarChar,20,sort.Name),
                dbHelper.CreateInParam("@CategoryId",SqlDbType.Int,4,sort.Category.Id)            };
            try
            {
                return (dbHelper.RunProc("Add_Sort", prams));
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
                return 0;
            }
        }

Answer 1

CategoryList.SelectedItem 是 null，没有 Item 是出于被选择状态。应该先判断一下，SelectedItem 是否是 null。

Answer 2

CategoryList.SelectedItem可能是null

Answer 3

CategoryList.SelectedValue
是否选择值。判断if(CategoryList.SelectedValue!=null)

Answer 4

sort.Category.Id = Convert.ToInt32(CategoryList.SelectedValue==null?"0":CategoryList.SelectedValue); 
sort.Category是否为null

Answer 5

CategoryList已经绑定了 为什么CategoryList.SelectedValue是Null呢

Answer 6

设置CategoryList.SelectedValue默认值
判断sort.Category看看

Answer 7

还是有问题  我水平不行--!
又遇到昨天那问题了
public partial class Admin_SortBook : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        BinCategoryData();
        
    }    public void BinSortData()//绑定小类别
    {
        
        int categoryId = Convert.ToInt32(this.CategoryList.SelectedItem.Value);
        SortManager sortManager = new SortManager();        this.SortList.DataSource = sortManager.GetSortByCategoryId(categoryId);
        this.SortList.DataTextField = "Name";
        this.SortList.DataValueField = "Id";
        this.SortList.DataBind();
       
    }    public void BinCategoryData()//绑定大类别
    {
        CategoryManager categoryManager = new CategoryManager();        this.CategoryList.DataSource = categoryManager.GetAllCategories();
        this.CategoryList.DataTextField = "Name";
        this.CategoryList.DataValueField = "Id";
        this.CategoryList.DataBind();
        BinSortData();
    
    }
根据大类的Id查出小类  但只能查出第一个