DropDownList and BindingManger

[sebastianslutzky]

Hi,
I have a web page (extending SPring.Web.Page) that displays a drop down list with the following code:

...

protected override void OnInitializeControls(EventArgs e)
{

if (!IsPostBack)
BindLocations();
}


protected override void InitializeDataBindings()
{
base.InitializeDataBindings();
BindingManager.AddBinding("ddlLocations.SelectedVa lue", "SearchCriteria.LocationCode");
}

private void BindLocations()
{
ddlLocations.DataSource = PageController.GetAllLocations();
ddlLocations.DataTextField = "Name";
ddlLocations.DataValueField = "Identity";
ddlLocations.DataBind();
ddlLocations.Items.Insert(0, DropDownUtils.UnselectedItem);
}
....
The data source is kept in the controller, which is kept in the session.

This works fine the first time I select a value and navigate somewhere else.

However, the second time I select a value and press ''Go', I get the following error :

"Cannot have multiple items selected in a DropDownList."


It seems that the BIndingManager should call 'ClearSelection' on the drop down list before binding from the model to the ddl.


Am I using the wrong syntax for this? Is there a workaround I could use?

Thanks very much
Sebastian

[Erich Eichinger]

Hi,

I'm afraid in this case it's your job to call ClearSelection() on the ddl. DataBindingManager is not aware of the ddl. It just copies property values from source to target and vice versa.

Try overriding OnPreRender() like this:

void OnPreRender( EventArgs e )
{
  ddl.ClearSelection();
  base.OnPreRender(); // DataBinding is done here!
}

cheers,
Erich

[Bruno Baia]

Hi,

another tricky solution when you know all the possibilities of the expression framework is something like this :

protected override void InitializeDataBindings()
{
   base.InitializeDataBindings();
   BindingManager.AddBinding("ddlLocations.SelectedValue", "SearchCriteria.LocationCode", BindingDirection.SourceToTarget);
   BindingManager.AddBinding("{ddlLocations.ClearSelection();ddlLocations}.SelectedValue", "SearchCriteria.LocationCode", BindingDirection.TargetToSource);
}

The first binds from the ddl.SelectedValue to the model.
The second call ddl.ClearSelection() before to bind from the model to the ddl.SelectedValue.

-Bruno

[Erich Eichinger]

Nice idea - I didn't think of this ;-)

cheers,
Erich

[sebastianslutzky]

Hi,
Thanks a lot to both for your time and dedication!
I've tried both approaches, but in the two cases the same error is still happening.

I've stepped into Spring code and was able to see the 'SelectedValue' property of the ddl being correctly set at 'SetPropertyOrFieldValueInternal' method.

Also, I confirmed that at this point the ddl had no item selected (inspected each of them and saw the 'Selected' property beinf 'false') after the property was set , that is, after the line with the code...

pi.SetValue(context, newValue, null);


at the PropertyOrFIeldNode class, the ddl had the right item selected.

However, the same httpexception is being thrown by the web server at the point of render.

It seems I'll need to bind the ddl manually.

Thank you again,

Sebastian

[Erich Eichinger]

Hi Sebastian,

could you please post your aspx + codebhind (as attachment)? I would like to investigate this - if it's a spring-problem, we need to fix it.

at the moment I have no idea for this behaviour.

tia,
Erich

[sebastianslutzky]

Hi Erich,

I just found it was a problem with my code.

Basically, I added a 'dummy' entry to all ddls in the page ('--SELECT--' item)

I didn't realise this item was shared rather than being a diff instance per ddl.

Thereforem if it was selected in one ddl, it was selected in all of them, resulting in multiple selections when a ddl had a real selection.

I changed this and everything works fine. There is no need for calling 'ClearSelection' at all: using the default Bidirectional Binding works fine for all dlls.

Sorry about that.

Sebastian