DataBinding Complex Type properties

Question

 

I've been playing with Complex Types in databinding scenarios.

 

Let's use this example :

 

Contact

  firstname

  lastname

  Address (complex type)

      street

      city

      state

 

When I create an EntityDataSource that points to an entity with a complex type, it surfaces the complex type's properties. E.g. if I bind the EDS to a datagrid, I can see as my grid columns:

 

contactID

firstname

lastname

Address.street

Address.city

Address.state

 

However if I try to do databinding without the eds I'm hitting a wall.

 

Example (which is not real world, just something to test out the functionality)

 

dim mycontacts=context.Contacts.ToList

 

with dropdownList1

 .DataTextField="Address.street"

.DataValueField="contactID"

.datasource=mycontacts

.databind

end with

 

I have paid attention to case in the strings for binding.

 

When I hit databind, I get an exception telling me that Contact does not contain a property with the name Address.street.

 

In debug I can definitely get a value back if I request myContacts(0).Address.street.

 

When trying this with a Windows Forms ComboBox using "Address.street" as the DisplayMember, rather than an exception , it displays the value member (contactid). If I use "firstname" as the DisplayMember, I get firstname.

 

So what's the problem? Me again?

 

thanks

 

julie

 

Answer 1

Hello Julie,

I don't think you are the problem this time

There are different ways of specifying which property a databound control binds to in ASP.NET, like Eval() Bind(), BoundField.DataField, ListControl.DataTextField, etc. And in general, they behave differently.

The flattening we did on EntityDataSource is an attempt to make the properties that are exposed by EDM entities available for 2-way databinding in most cases.

For instance, in your example, we provide a property descriptor for customer.Address, and also for customer.Address.Street.

At runtime, in the case of a control binding to Eval(“Address.Street”) from a customer, Eval will use the property descriptor corresponding to Address, and it will drill down on it to extract the value of the Street property on it.

A grid column of a BoundField derived type with DataField = “Address.Street” will work differently: it will just look for a property descriptor in the data item with a name as “Address.Street”. In fact, EntityDataSource is the first DataSource control that I know off that will provide such a thing.

Bind(“Address.Street”) will work in a similar fashion to Eval() when reading the properties into the data bound control, but will act a little bit more like BoundField when sending back changes to the DataSource.

There are a few cases in which the behavior is not any of the above and hence you end up with a control that cannot have access to a complex type’s properties. You can expect us to work closely with the ASP.NET team in making the experience smoother in future versions.

Also, in this case, I think it is worthy of mentioning, you have to consider that flattening of properties in the EntityDataSource only happen under certain conditions. You can take a second look at the rules of wrapping (which is the mechanism we use to obtain flattened properties) in this blog post.

Besides, we worked very closely with the ASP.NET Dynamic Data in this release, to enable their technology to work EDM through the EntityDataSource. I think it is very worthy of trying.

Regarding what you see in the Windows Forms ComboBox, we don't do any flattening there, and so there is no property descriptor for "Address.Street". The way to get it is to do an explicit projection of "ContactId, Address.Street AS Street", and then bind the control to that. This will actually also work for you in any ASP.NET ListControl.

Hope this helps,
Diego