Sv: System.NullReferenceException och dropdownlist

System.NullReferenceException och dropdownlist

Postades av 2003-10-09 12:34:04 - Martin Söderlund, i forum asp.net generellt, Tråden har 2 Kommentarer och lästs av 879 personer

Hej!
Jag får ett felmeddelande hela tiden när jag ska redigera en länk i mitt länkscript. Jag använder mig av en dropdownlista för att visa kategorier, och binder data till den.

På default.aspx så har jag en codebehind-sida som heter links.vb, och klassen edit_link som ser ut så här:

<code>Public Class edit_link : Inherits UserControl

protected withevents choose_cat as dropdownlist
protected Withevents tbl_links_edit as repeater

Function fix(str)
str = Replace(str,"'","")
fix = str
End Function

Sub Page_Load(sender as object, e as eventargs)
choose_category()
edit()
End Sub

'lista kategorier
Sub choose_category()

'plocka ut unika kategorier ut länktabellen
Dim SQL As string = "SELECT DISTINCT category FROM tbl_links"

Dim Cn As OleDbConnection = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;data source=" & Server.MapPath("db_links.mdb"))
Dim Cm As OleDbCommand = new OleDbCommand(SQL,Cn)

Cn.Open()

'skapa ett OleDbDataReader-objekt
Dim objDataReader As OleDbDataReader = Cm.ExecuteReader()

'skapa en ny samling av listposter (option-val i dropdownlistan i default.aspx)
Dim ddlBox As new ListItemCollection

ddlBox.Add(new ListItem("Välj"))
ddlBox.Add(new ListItem())

while(objDataReader.Read())
'lägg till val i kategoridropdownlistan
ddlBox.Add(new ListItem(objDataReader("category").toString()))
end while

'bind data till dropdownlistan
choose_cat.DataSource = ddlBox
choose_cat.DataBind()

'stäng objekt
objDataReader.Close()
Cn.Close()

End Sub

'redigera länk
Sub edit()

Dim Cn As New OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;data source=" & Server.MapPath("db_links.mdb"))
Dim SQL As string = "SELECT url,sitedescription FROM tbl_links WHERE Id=" & fix(Request.Querystring("id"))

Dim Cm As New OleDbCommand(SQL,Cn)
Dim dbRead As OleDbDataReader

Dim Container As object
Cn.Open()
dbRead = Cm.ExecuteReader()

tbl_links_edit.DataSource = dbRead
tbl_links_edit.DataBind()

dbRead.Close()
Cn.Close()
End Sub

'utför redigering av länk
Sub performedit(sender as object,e as eventargs)

Dim eUrl As string = fix(Request.Form("url"))
Dim eDescription As string = fix(Request.Form("description"))
Dim eCategory As string = choose_cat.SelectedItem.toString

If eCategory = "" Then eCategory = fix(Request.Form("alt_category"))

Dim SQL As string = "UPDATE tbl_links SET URL='" & eURL & "',SiteDescription='" & eDescription & "' WHERE Id=" & fix(Request.Querystring("id"))
Dim Cn As New OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;data source=" & Server.MapPath("db_links.mdb"))

Dim Cm As New OleDbCommand(SQL,Cn)
Cm.Connection.Open()
Cm.ExecuteNonQuery()

Cn.Close
Response.Redirect("default.aspx?action=edit&id=" & Request.Querystring("id") & "&cat=" & Request.Querystring("cat"))

End Sub

End Class</code>

i default.aspx så laddar jag användarkontrollen edit.ascx, och koden i användarkontrollen ser ut så här:

<code><%@ Control Language="VB" Inherits="edit_link" src="links.vb" %>

<asp:repeater id="tbl_links_edit" runat="server">

<HeaderTemplate>
<span class="head">Redigera lÃ¤nk</span>
</HeaderTemplate>

<ItemTemplate>
<p>
URL:
<br />
<input type="text" size="40" value='<%# Container.DataItem("url") %>' name="url" />
<br />
<br />
Namn:
<br />
<input type="text" size="40" value='<%# Container.DataItem("SiteDescription") %>' name="description" />
<br />
<br />
Kategori:
<br />
<asp:DropDownList id="choose_cat" runat="server"></asp:DropDownList>
<input type="text" size="30" name="alt_category" />
</p>
<asp:button onClick="performedit" text="Uppdatera" runat="server" />

</ItemTemplate>
</asp:repeater>
</code>

Felmeddelandet jag får kan ses här: http://www.dileno.hel.nu/l3/default.aspx?action=edit&id=89&cat=Litteratur

Jag har tre andra klasser till i links.vb och exakt samma kod eller likadan kod där fungerar problemfritt.

Klassen links ser ut så här:

<code>Public Class links : Inherits Page

protected withevents categories As dropdownlist
protected withevents tbl_links As repeater
protected withevents iu_holder as placeholder

Dim CnStr As string = "Provider=Microsoft.Jet.OLEDB.4.0;data source=" & Server.MapPath("db_links.mdb")

Function fix(str)
str = Replace(str,"'","")
fix = str
End Function

Sub Page_Load()

If Not Session("axxezz")=True Then Response.Redirect("login.aspx")

'visa kategorier
show_categories()

Dim Action As string
Action = fix(Request.Querystring("action"))

Select Case Action

Case ""
'visa länkar

Dim SQL As string

Select Case fix(Request.Querystring("cat"))
Case ""
SQL = "SELECT Id,URL,SiteDescription,dt,Clicks FROM tbl_links WHERE Id IN" & _
" (SELECT TOP 20 SiteId FROM tbl_stats ORDER BY ID DESC) ORDER BY Clicks DESC"

Case Else
SQL = "SELECT Id,URL,SiteDescription,dt,Clicks FROM tbl_links WHERE category = '" & fix(Request.Querystring("cat")) & "' ORDER BY Clicks DESC"

End Select

Dim Cn As new OleDbConnection(CnStr)
Dim Cm As new OleDbCommand(SQL,Cn)
Dim dbRead As OleDbDataReader

Dim Container As object
Cn.Open()
dbRead = Cm.ExecuteReader()

tbl_links.DataSource = dbRead
tbl_links.DataBind()

dbRead.Close()
Cn.Close()

Case "click"
click()

Case "logout"
Session.Contents.Remove("axxezz")
Response.Redirect("login.aspx")

Case "delete"
delete()

Case "edit"
iu_holder.Controls.Add(Page.LoadControl("edit.ascx"))

Case "new"
iu_holder.Controls.Add(Page.LoadControl("insert.ascx"))

Case "cat"
iu_holder.Controls.Add(Page.LoadControl("cat.ascx"))

End Select

End Sub

'kategorier i dropdownlistan
Sub show_categories()

'plocka ut unika kategorier ut länktabellen
Dim SQL As string = "SELECT DISTINCT category FROM tbl_links"

Dim Cn As OleDbConnection = new OleDbConnection(CnStr)
Dim Cm As OleDbCommand = new OleDbCommand(SQL,Cn)

Cn.Open()

'skapa ett OleDbDataReader-objekt
Dim objDataReader As OleDbDataReader = Cm.ExecuteReader()

'skapa en ny samling av listposter (option-val i dropdownlistan i default.aspx)
Dim ddlBox As new ListItemCollection

ddlBox.Add(new ListItem("Välj"))
ddlBox.Add(new ListItem())

while(objDataReader.Read())
'lägg till val i kategoridropdownlistan
ddlBox.Add(new ListItem(objDataReader("category").toString()))
end while

'bind data till dropdownlistan
categories.DataSource = ddlBox
categories.DataBind()

'stäng objekt
objDataReader.Close()
Cn.Close()

End Sub

'klick på länk
Sub Click()

'SQL-satser för uppdatering & ny info som sätts in databasen
Dim uSQL As string = "UPDATE tbl_links SET Clicks = Clicks --1 WHERE Id=" & fix(Request.Querystring("dilenoid"))
Dim iSQL As string = "INSERT INTO tbl_stats(dt,SiteId) VALUES('" & Now.ToString("d") & "'," & fix(Request.Querystring("dilenoid")) & ")"

Dim Cn As new OleDbConnection(CnStr)
Cn.Open()

'uppdatera antal klicks i databasen
Dim uCm As new OleDbCommand(uSQL,Cn)
uCm.ExecuteNonQuery()

'sätt in Id & dagens datum för gällande url i statistiktabellen
Dim iCm As new OleDbCommand(iSQL,Cn)
iCm.ExecuteNonQuery()

Cn.Close()
'skicka vidare till egentliga url:en
Response.Redirect(fix(Request.Querystring("url")))

End Sub

'radera länk
Sub delete()

Dim lSQL As string = "DELETE * FROM tbl_links WHERE Id=" & fix(Request.Querystring("id"))
Dim sSQL As string = "DELETE * FROM tbl_stats WHERE SiteId=" & fix(Request.Querystring("id"))

Dim Cn As New OleDbConnection(CnStr)
Cn.Open()

Dim lCm As New OleDbCommand(lSQL,Cn)
lCm.ExecuteNonQuery()

Dim sCm As New OleDbCommand(sSQL,Cn)
sCm.ExecuteNonQuery()

Cn.Close

Response.Redirect("default.aspx?cat=" & Request.Querystring("cat"))

End Sub

End Class
</code>

Proceduren show_categories i klassen links är mycket snarlik proceduren choose_category i klassen edit_link, men skillnaden är den att här fungerar proceduren för att visa en dropdownlista, medan den inte fungerar i show_categories.

Jag är mycket tacksam för hjälp. :)

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