When writing custom validation using the OnValidate method of a partial class an unhandled exception is thrown in the page:

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

This seems to be linked to the fact that the table has foreign key relationships (it works fine for tables without foreign keys or if I remove the foreign key relationship from the table).

Can anyone help on this issue?

Trace output below:

[NullReferenceException: Object reference not set to an instance of an object.]
   System.Data.Linq.ChangeProcessor.SendOnValidate(MetaType type, TrackedObject item, ChangeAction changeAction) +197
   System.Data.Linq.ChangeProcessor.ValidateAll(IEnumerable`1 list) +130
   System.Data.Linq.ChangeProcessor.SubmitChanges(ConflictMode failureMode) +76
   System.Data.Linq.DataContext.SubmitChanges(ConflictMode failureMode) +331
   System.Data.Linq.DataContext.SubmitChanges() +19
   System.Web.UI.WebControls.LinqToSqlWrapper.SubmitChanges(DataContext dataContext) +9
   System.Web.UI.WebControls.LinqDataSourceView.InsertDataObject(Object dataContext, Object table, Object newDataObject) +83
   System.Web.UI.WebControls.LinqDataSourceView.ExecuteInsert(IDictionary values) +448
   System.Web.UI.DataSourceView.Insert(IDictionary values, DataSourceViewOperationCallback callback) +86
   System.Web.UI.WebControls.DetailsView.HandleInsert(String commandArg, Boolean causesValidation) +274
   System.Web.UI.WebControls.DetailsView.HandleEvent(EventArgs e, Boolean causesValidation, String validationGroup) +676
   System.Web.UI.WebControls.DetailsView.OnBubbleEvent(Object source, EventArgs e) +95
   System.Web.UI.Control.RaiseBubbleEvent(Object source, EventArgs args) +37
   System.Web.UI.WebControls.DetailsViewRow.OnBubbleEvent(Object source, EventArgs e) +113
   System.Web.UI.Control.RaiseBubbleEvent(Object source, EventArgs args) +37
   System.Web.UI.WebControls.LinkButton.OnCommand(CommandEventArgs e) +118
   System.Web.UI.WebControls.LinkButton.RaisePostBackEvent(String eventArgument) +135
   System.Web.UI.WebControls.LinkButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +10
   System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +13
   System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +175
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1565

Can you post the OnValidate method so I can duplicate with Northwind please.

Steve,

The previous message was the wrong error, I have investigated some more and I have currently identified the issue as follows:

I have a table with a status field, The allowable values are contained in another table, and this is the foreign key for the main table status value. The Dynamic Data framework helpfully creats a dropdownlist for the field with the values from the status table, as you would expect and want.

But if you have a validation routine for that field then you will get an javascript error on the page

Private Sub OnEditStatusChanging(ByVal value As Integer)
Throw New ValidationException("Status changing")
End Sub

if you remove the foreign key constraint then the exception is handled on the page correctly by the validator display control.

But the larger issue is that it is not clear to me when the database is accessed and changed. I really want to compare the changed status with the original status, as some  status transitions are not allowed. How do I reference the original value to compare it with the new one?

Thanks

Hi Zorgit, are you using L2S or EF?

Steve,

L2S

Basically what I want to know is, when the user clicks update to change some values, what is the best way to access the current values (i.e. the ones that were there before the user changed them).

It is not at all obvious - are the original values stored in the page somewhere, in which case where? or do I need to read the database again to test the changed values?

OK, I think the issue here is that there is no validator in the FK Edit you should add a DynamicValidator to the FK Edit FieldTemplate that may do the trick

Yep tested it that does the trick I added in c# to a test table:

[MetadataType(typeof(TestMD))]
public partial class Test
{
    partial void OnLongChanging(string value)
    {
        throw new ValidationException("Test Eception");
    }

    public partial class TestMD
    {
        public object ID { get; set; }

        public object Short { get; set; }

        public object Long { get; set; }
    }
}

And I got the exeption showing on the page  

Steve,

Thanks that's very helpful I will make the change to the FK_Edit template.

And for the second part of the question - how do I see the values that were in a field BEFORE the user changed it so I can make a comparison for validation purposes?

Thanks

the String value has the value for the field but not the other properties :(

Steve,

The answer seems to be to create a member variable in the Partial Class, and in the fields OnFieldChanging event you can store the current value in the member variable.

Then in OnValidate you can compare the new value (which has been written in the original member variable) to the value stored previously.

e.g. assume a table Articles with a field EditStatus (which is a Foreign Key)

Partial Public Class Article

REM needs to be private or protected not public or you will get a LINC error
Protected OldStatus As Integer

That's a good solution if there are more than one properties previous value you want to check. What would be ideal is if the OnXXXUpdate partial class had OldEntity and NewEntity availble in it.