Deleting An item from a List Box from a Database

Question

Hi Im trying to delete an item from the listbox which the user selects here you can find my code and a screenshot tnx.

Code:

   public void deleteEvent(ListBox list_event)
        {
            int i = list_event.SelectedIndex;

            ds.Tables["tblEvents"].Rows[i].Delete();
            da.Update(ds.Tables["tblEvents"]);
        }

Im getting Object reference not set to an instance of an object Exception.

ScreenShot: http://i53.tinypic.com/ad1209.jpg

Vulpes · Answer

Well according to the link you posted earlier ( http://i53.tinypic.com/n5kb9v.jpg ) there is a column called Member_ID in tblEvents in the database. So are you saying that you didn't cater for it at all when you created the tblEvents DataTable in your DataSet? I was hoping that you had allowed for it but had called it something else.

Mark Fenech · Answer

the thing is that i dont have any relation between tblEvents and tblMembers

Vulpes · Answer

Here's an example of using a TableMapping. Suppose in the 'tblEvents' table in the database you have a column called 'Member_ID'. However in the 'tblEvents' DataTable in the DataSet you've called this column 'ID' instead. When you create your DataAdapter add these lines: ITableMapping mapping = da.TableMappings.Add(Table tblEvents); mapping.ColumnMappings.Add(Member_ID ID); Now when you update the database it should look for the Table mapping by default and (hopefully) all will be well.

Vulpes · Answer

Are you using any column names in that particular DataTable which are different from the table in the underlying database? I'm thinking in particular of Member_ID which is the one that was showing up before when you received the InvalidOperationException. If you are then either: 1. We'll need to create TableMappings for those columns so that the Update() method can find them. 2. You'll need to change the names of those columns in the DataTable so that they're the same as the ones in the database.

Mark Fenech · Answer

i cant because of this exception : Update unable to find TableMapping['Table'] or DataTable 'Table'.

Vulpes · Answer

In that last line I'd update the whole DataSet rather than the individual table: da.Update(ds);

Mark Fenech · Answer

Vulpes it worked like that correctly the delete but when i refresh or reload form the items i deleted will pop up again i dont think i deleted them from the db this is my code : public void deleteEvent(ListBox list_event) { int i = list_event.SelectedIndex; ds.Tables[tblEvents].Rows[i].Delete(); ds.AcceptChanges(); da.Update(ds.Tables[tblEvents]); }

Vulpes · Answer

Well the error message is suggesting that the DataTable is corrupt though I wouldn't accept that at face value until you've tried a few other things. One thing I've found with ADO.NET is that it's usually best to update the whole DataSet rather than an individual DataTable in case the updates have affected relations between the tables. So I'd try replacing this line: da.Update(ds.Tables[tblEvents]); with these two lines: ds.AcceptChanges(); da.Update(ds);

Mark Fenech · Answer

im getting this exception now :/ http://i53.tinypic.com/n5kb9v.jpg Help pls

Ankit Nandekar · Answer

check ur table name or check when u r filling Dataset using DataAdapter u r passing Table name or notin Fill Method.

Vulpes · Answer

A couple of possibilities: 1. Are you sure that there is a selected item in the listbox? If there isn't then SelectedIndex would be -1 and Rows[-1] wouldn't exist. 2. Are you sure that the TableName property of the table is tblEvents and this isn't just the name of a variable which you used elsewhere in the code to refer to the DataTable object?