Question

c#报错 Object reference not set to an instance of an object.

string strCateGory = "CateGoryID=1";
string strPrice = "UnitPrice>0";
private void FillDropDownList()
{
SqlConnection Conn = new SqlConnection("Data Source=.\\sqlexpress;Integrated Security=SSPI;Initial Catalog=Northwind");
string strSqlCategories = "select CategoryName,CategoryID from Categories";
SqlCommand cmd = new SqlCommand(strSqlCategories,Conn);
Conn.Open();
ddlCategoryFilter.DataSource = cmd.ExecuteReader();
ddlCategoryFilter.DataTextField = "CategoryName";
ddlCategoryFilter.DataValueField= "CategoryID";
ddlCategoryFilter.DataBind();
Conn.Close();
}
private void FilterByPrice(string strChoice)
{
switch (strChoice)
{
case "Any Price":
strPrice = "UnitPrice>0";
break;
case "Cheap":
strPrice = "UnitPrice<20";
break;
case "Moderate":
strPrice = "UnitPrice>=20 AND UnitPrice<50";
break;
case "Expensive":
strPrice = "UnitPrice>=50";
break;
case "Absurdly Expensive":
strPrice="UnitPrice>100";
break;
}
}
private void FilterByCategory(string strChoice)
{
strCateGory="CategoryID="+strChoice;
}
private void DataFiller()
{
SqlConnection Conn = new SqlConnection("Data Source=.\\sqlexpress;Integrated Security=SSPI;Initial Catalog=Northwind");
string strSql = "select ProductID,ProductName,CategoryID,UnitPrice from Products";
SqlDataAdapter Adapter = new SqlDataAdapter(strSql,Conn);
Conn.Open();
DataSet ds = new DataSet();
Adapter.Fill(ds,"dtProducts");
DataView dvUK = new DataView(ds.Tables["dtProducts"]);
dvUK.RowFilter = strCateGory + "AND(" + strPrice + ")";
dgProducts.DataSource = dvUK;
dgProducts.DataBind();
Conn.Close();
}
protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
{
FillDropDownList();
DataFiller();
}
}

protected void FilterChange(object sender, EventArgs e)
{
FilterByCategory(ddlCategoryFilter.SelectedItem.Value.ToString());
FilterByPrice(rdbtPrice.SelectedItem.Text.ToString());
DataFiller();
}
}
Source Error:

Line 78: {
Line 79: FilterByCategory(ddlCategoryFilter.SelectedItem.Value.ToString());
Line 80: FilterByPrice(rdbtPrice.SelectedItem.Text.ToString());
Line 81: DataFiller();
Line 82: }

Source File: Line: 80
Stack Trace:

[NullReferenceException: Object reference not set to an instance of an object.]
_3._22.WebForm8_61.FilterChange(Object sender, EventArgs e) in D:\2010\3.22\3.22\WebForm8_61.aspx.cs:80
System.Web.UI.WebControls.ListControl.OnSelectedIndexChanged(EventArgs e) +118
System.Web.UI.WebControls.DropDownList.RaisePostDataChangedEvent() +143
System.Web.UI.WebControls.DropDownList.System.Web.UI.IPostBackDataHandler.RaisePostDataChangedEvent() +10
System.Web.UI.Page.RaiseChangedEvents() +134
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +5201
有内容的，断点设在哪里

Answer 1

rdbtPrice.SelectedItem有内容吗？

就设在你出错的那一行。

FilterByPrice(rdbtPrice.SelectedItem.Text.ToString());
看看 rdbtPrice.SelectedItem是不是null的。

Answer 2

FilterByPrice(rdbtPrice.SelectedItem.Text.ToString());
这个代码你看你的 rdbtPrice.SelectedItem.Text 这个值获取到了吗
如果是空值，肯定是不能tostring()的

Answer 3

难道你的下拉控件没有 绑上值？

设断点一步步看 ，学会用断点，按F10 跟着走

Answer 4

在那里引用对象的时候没有实例化，或者使用了null变量。只能单步跟踪了，看看问题在那里！