Entity Framework: Winforms Combobox Databinding is just weird...

I wanted to use Databinding on a Combobox in Winforms, like I did with Objects or a Dataset on Master-Detail relations.
Databinding with the Designer is just so easy :-)

Figure: Designer properties on the combobox

I have 2 Bindingsources, the Master-Bindingsource and the the Detail-Bindingsource.
I choose the Combobox and set the different properties (DataSource, ValueMember, DisplayMember)

  • I set the ValueMember to the primary key of the Detail-Bindingsource and
  • I set the SelectedValue (Databindings) to the Master-Bindingsource.Doctor (= Entity class)

Figure: Combobox properties in Designer

Problem on runtime

I get the following error

Invalid cast from System.Int32 to ClientName.Business.Entities.Doctors

Reason: The Primary key (Int32) doesn’t fit into the Entity property of the Master-Table (entity)

The problem is: the Entity Framework hides the foreign-key from me (DoctorsId).
I cannot bind to it in the Designer!

How to solve that?

Don’t use Databinding.
There is no easy way of converting a primary key (Int32) to the actual Entity, with that Integer.

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
            //HACK: AARRRGHHH: Why I have todo this manually??
            // With other Objects or Datasets we can do this in Designer
            Doctors preferredDoctor = comboBox1.SelectedValue as Doctors;
            if (preferredDoctor != null)
                if (CurrentPatientFromBindingSource != null)
                    CurrentPatientFromBindingSource.Doctor = preferredDoctor;

private void LoadDoctors()
            doctorsBindingSource.DataSource = patientsBL.GetDoctors();

            //HACK: Arrggghhh: Why do I have todo this manually?
            comboBox1.SelectedItem = CurrentPatientFromBindingSource.Doctors;

Todo: Investigate if Linq to Entites provides a flag to configure: Show Foreign Key property for entity


Anonymous said...

You need to set SelectedItem, not SelectedValue :)

Peter Gfader said...

I am pretty sure, that I tried that. But I will give it another try and let you know.
THX Anonymous

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