Chapter 14 errors for Inserting and Deleting Data with the ListView Control

winkimjr2 · May 14th, 2013, 09:47 AM

Can someone help me with this error from Try It Out Inserting and Deleting Data with the ListView Control?

Error
Server Error in '/Site' Application.

Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

Line 6: Protected Sub EntityDataSource1_Inserted(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.EntityDataSourceChangedE ventArgs) Handles EntityDataSource1.Inserted
Line 7: Dim myPhotoAlbum As PhotoAlbum = CType(e.Entity, PhotoAlbum)
Line 8: Response.Redirect(String.Format("ManagePhotoAlbum. aspx?PhotoAlbumId={0}", New String(myPhotoAlbum.Id.ToString())))
Line 9: End Sub
Line 10: End Class

Source File: N:\NewASP\Site\NewPhotoAlbum.aspx.vb Line: 8

Here is my code for aspx
<%@ Page Title="Create New Photo Album" Language="VB" MasterPageFile="~/MasterPages/Frontend.master" AutoEventWireup="false" CodeFile="NewPhotoAlbum.aspx.vb" Inherits="_NewPhotoAlbum" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="cpMainContent" runat="Server">
<asp:DetailsView ID="DetailsView1" runat="server" AutoGenerateRows="False" DataKeyNames="Id" DataSourceID="EntityDataSource1" DefaultMode="Insert" Height="50px" Width="125px">
<Fields>
<asp:BoundField DataField="Id" HeaderText="Id" SortExpression="Id" InsertVisible="False"/>
<asp:BoundField DataField="Name" HeaderText="Name" SortExpression="Name" />
<asp:CommandField ShowInsertButton="true" />
</Fields>
</asp:DetailsView>
<asp:EntityDataSource ID="EntityDataSource1" runat="server"
ConnectionString="name=TesterEntities" DefaultContainerName="PlanetWroxEntities" EnableFlattening="False"
EnableInsert="True" EntitySetName="PhotoAlbums" OnInserted="EntityDataSource1_Inserted">
</asp:EntityDataSource>
</asp:Content>

Code behind
Imports TesterModel

Partial Class _NewPhotoAlbum
Inherits BasePage

Protected Sub EntityDataSource1_Inserted(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.EntityDataSourceChangedE ventArgs) Handles EntityDataSource1.Inserted
Dim myPhotoAlbum As PhotoAlbum = CType(e.Entity, PhotoAlbum)
Response.Redirect(String.Format("ManagePhotoAlbum. aspx?PhotoAlbumId={0}", New String(myPhotoAlbum.Id.ToString())))
End Sub
End Class

Imar · May 14th, 2013, 10:08 AM

Hi there,

Can you comment out the line with the redirect statement? EF may be throwing an error that you don't see because of the cast to null.

Also, you don't need the New String part. This should work:

Code:

Dim myPhotoAlbum As PhotoAlbum = CType(e.Entity, PhotoAlbum)
Response.Redirect(String.Format("ManagePhotoAlbum.aspx?PhotoAlbumId={0}", myPhotoAlbum.Id.ToString()))

Finally, check your database. Are you sure that both PhotoAlbum and Picture have a column called Id that is of type int and set to an Identity? Also, did you link Picture.PhotoAlbumId to PhotoAlbum.Id? If you have to (or already have) make changes to the database, don't forget to refresh or recreate the EF model before you try again.

Cheers,

Imar

winkimjr2 · May 14th, 2013, 10:43 AM

Hi Imar and thanks for your quick response.
However I did not understand what you meant by
"Can you comment out the line with the redirect statement? EF may be throwing an error that you don't see because of the cast to null."
Where is that line?
1. The tables Picture and PhotoAlbum both have Id set as primary key. After I removed the part you asked me to remove, I still got the following error.

2. You asked if I "did you link Picture.PhotoAlbumId to PhotoAlbum.Id?" I dont know the answer to this. How do I check to make sure I link them?

Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

Line 6: Protected Sub EntityDataSource1_Inserted(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.EntityDataSourceChangedE ventArgs) Handles EntityDataSource1.Inserted
Line 7: Dim myPhotoAlbum As PhotoAlbum = CType(e.Entity, PhotoAlbum)
Line 8: Response.Redirect(String.Format("ManagePhotoAlbum. aspx?PhotoAlbumId={0}", myPhotoAlbum.Id.ToString()))
Line 9: End Sub
Line 10: End Class

Source File: N:\NewASP\Site\NewPhotoAlbum.aspx.vb Line: 8

Imar · May 14th, 2013, 10:53 AM

Just comment out this line:

Code:

Response.Redirect(String.Format("ManagePhotoAlbum.  aspx?PhotoAlbumId={0}", myPhotoAlbum.Id.ToString()))

E.g.:

Quote:

' Response.Redirect(String.Format("ManagePhotoAlbum. aspx?PhotoAlbumId={0}", myPhotoAlbum.Id.ToString()))

To check the relationships, check out the database diagram you created earlier. Then check the relationship between these two tables (right-click the line between them and choose Properties). Then make sure they are linked on the tables and columns I mentioned earlier.

Note: doing this from memory so you may see other stuff than I am describing here. In that case, let me know and I'll look up the page numbers that show how to do this.

Cheers,

Imar

winkimjr2 · May 14th, 2013, 11:04 AM

Yes the tow tables are linked with Picture Id and PhotoAlbumId. I did recreate the link and refreshed the database.

I did comment on the line of code you asked me to:
'Response.Redirect(String.Format("ManagePhotoAlbum .aspx?PhotoAlbumId={0}", myPhotoAlbum.Id.ToString()))

And now I have the following error.

Violation of PRIMARY KEY constraint 'PK_PhotoAlbum'. Cannot insert duplicate key in object 'dbo.PhotoAlbum'. The duplicate key value is (0).
The statement has been terminated.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Data.SqlClient.SqlException: Violation of PRIMARY KEY constraint 'PK_PhotoAlbum'. Cannot insert duplicate key in object 'dbo.PhotoAlbum'. The duplicate key value is (0).
The statement has been terminated.

Imar · May 14th, 2013, 11:14 AM

Quote:

And now I have the following error.

Violation of PRIMARY KEY constraint 'PK_PhotoAlbum'. Cannot insert duplicate key in object 'dbo.PhotoAlbum'. The duplicate key value is (0).

Ah, there we have it. This seems to suggest that the Id column of the PhotoAlbum table is set to be the key, but not an identity. An identity auto numbers each row in the table, generating unique IDs. Without it, the Id will be zero (the default for an int in .NET) the first time you insert an album. The second time, the code will crash as you try to insert another row with the same ID of zero.

Check the section of the book that shows you how to create the PhotoAlbum and Picture tables. Make sure you set Is Identity for both Id columns to Yes. Then save the changes and refresh or recreate the EF model.

Hope this helps,

Imar

winkimjr2 · May 14th, 2013, 05:15 PM

When I click insert on the ManagePhotoAlbum, I am getting the following error. I am not sure how to fix it.

The INSERT statement conflicted with the FOREIGN KEY constraint "FK_Picture_PhotoAlbum". The conflict occurred in database "Tester", table "dbo.PhotoAlbum", column 'Id'.
The statement has been terminated.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Data.SqlClient.SqlException: The INSERT statement conflicted with the FOREIGN KEY constraint "FK_Picture_PhotoAlbum". The conflict occurred in database "Tester", table "dbo.PhotoAlbum", column 'Id'.
The statement has been terminated.

Imar · May 15th, 2013, 02:28 PM

This error usually happens for one of the following two reasons:

1. You're not passing the correct ID of the album to the ManagePicture.aspx page. This could be the case if you pass something like PhotoAlbumId in the query string, but try to retrieve it as PhotoAlbum.

2. The code that associates the picture with the album is not correct or does not execute.

For both cases, posting the full code (mark up and code behind) would help in figuring this out.

Cheers,

Imar