Working with Data Sources in Code

Up to this point, we have been setting up data sources directly on the designer surface with very little code. ComponentOne Studio for Entity Framework (SEF) has made it very easy, but sometimes you want or need to do everything in code. SEF makes this possible as well. Everything we did previously can be done at run time in code.

An obvious way to go about this would be to use the ClientViewSource object that we have, in effect, been setting up in the designer as elements of the ViewSourceCollection of a C1DataSource, given that it can be created on its own without a C1DataSource. We could, however, take a step further back and use a lower level class ClientView<T>. This would provide full control over loading data from the server and, since it is derived from C1.LiveLinq.LiveViews.View<T>, we can apply any LiveLinq operators to it. The ability to bind this to any GUI control whose datasource can be set to a View<T> also means that we’ll end up with a fully editable view of our data.

Server-side filtering is, perhaps, the most common operation, because no one usually wants entire database tables brought to the client unrestricted. Earlier we saw how SEF made it simple to perform without code, but now we’ll try it in run-time code.

To begin using SEF in run-time code without a C1DataSource, add a few lines to our project's main class to create a global client-side data cache. When we used C1DataSource, it was created for us behind the scenes. Now we can create it explicitly using the following code:

Imports C1.Data.Entities

Public Class Program
    Public Shared ClientCache As EntityClientCache
    Public Shared ObjectContext As NORTHWNDEntities

    <STAThread()> _
    Shared Sub Main()

        ObjectContext = New NORTHWNDEntities
        ClientCache = New EntityClientCache(ObjectContext)

        Application.EnableVisualStyles()
        Application.SetCompatibleTextRenderingDefault(False)
        Application.Run(New MainForm())
    End Sub
End Class

using C1.Data.Entities;

static class Program
{
    public static EntityClientCache ClientCache;
    public static NORTHWNDEntities ObjectContext;

    [STAThread]
    static void Main()
    {
        ObjectContext = new NORTHWNDEntities();
        ClientCache = new EntityClientCache(ObjectContext);

        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);
        Application.Run(new MainForm());
}

This code creates a single application-wide (static) ObjectContext and associates it with EntityClientCache. As noted previously in The Power of Client Data Cache topic, the ability to have a single context (and cache) for the entire application is a great simplification made possible by SEF.

2. Add a grid (dataGridView1), a combo box (comboBox1), and a button (btnSaveChanges) to the form.

Imports C1.Data.Entities
Imports C1.Data

Public Class DataSourcesInCode

    Private _scope As EntityClientScope

    Public Sub New()

        InitializeComponent()

        _scope = Program.ClientCache.CreateScope()

        Dim viewCategories As ClientView(Of Category) = _scope.GetItems(Of Category)()

        comboBox1.DisplayMember = "CategoryName"
        comboBox1.ValueMember = "CategoryID"
        comboBox1.DataSource = viewCategories

        BindGrid(viewCategories.First().CategoryID)
    End Sub

    Private Sub BindGrid(categoryID As Integer)
        dataGridView1.DataSource =
                (From p In _scope.GetItems(Of Product)().AsFiltered(Function(p As Product) p.CategoryID.Value = categoryID)
                 Select New With
                 {
                     p.ProductID,
                     p.ProductName,
                     p.CategoryID,
                     p.Category.CategoryName,
                     p.SupplierID,
                     .Supplier = p.Supplier.CompanyName,
                     p.UnitPrice,
                     p.QuantityPerUnit,
                     p.UnitsInStock,
                     p.UnitsOnOrder
                 }).AsDynamic()
    End Sub

    Private Sub btnSaveChanges_Click(sender As System.Object, e As System.EventArgs) Handles btnSaveChanges.Click
        Program.ClientCache.SaveChanges()
    End Sub

    Private Sub comboBox1_SelectedIndexChanged(sender As System.Object, e As System.EventArgs) Handles comboBox1.SelectedIndexChanged
        If comboBox1.SelectedValue IsNot Nothing Then
            BindGrid(CType(comboBox1.SelectedValue, Integer))
        End If
    End Sub
End Class

namespace TutorialsWinForms
{
    using C1.Data.Entities;
    using C1.Data;

    public partial classDataSourcesInCode : Form
    {
        private EntityClientScope _scope;

        public DataSourcesInCode()
        {
            InitializeComponent();

            _scope = Program.ClientCache.CreateScope();

            ClientView<Category> viewCategories =_scope.GetItems<Category>();

            comboBox1.DisplayMember = "CategoryName";
            comboBox1.ValueMember = "CategoryID";
            comboBox1.DataSource = viewCategories;

            BindGrid(viewCategories.First().CategoryID);
        }

        private void comboBox1_SelectedValueChanged(object sender, EventArgs e)
        {
            if (comboBox1.SelectedValue != null)
                BindGrid((int)comboBox1.SelectedValue);
        }

        private void BindGrid(int categoryID)
        {
            dataGridView1.DataSource =
                (from p in _scope.GetItems<Product>().AsFiltered(
                     p => p.CategoryID == categoryID)
                 select new
               {
                     p.ProductID,
                     p.ProductName,
                     p.CategoryID,
                     CategoryName = p.Category.CategoryName,
                     p.SupplierID,
                     Supplier = p.Supplier.CompanyName,
                     p.UnitPrice,
                     p.QuantityPerUnit,
                     p.UnitsInStock,
                     p.UnitsOnOrder
                 }).AsDynamic();
        }

        private void btnSaveChanges_Click(object sender, EventArgs e)
        {
            Program.ClientCache.SaveChanges();
        }

    }
}

4. Save, build and run your application. You should see similar results to those you saw in the Server-Side Filtering example, the only difference being that this time we’ve implemented it all in code.

The private field _scope is the form’s gateway to the global data cache. It is a pattern we recommend you follow in all the forms where you do not employ a C1DataSource component directly, as that does this for you automatically. It ensures that the entities the form needs stay in the cache while the form is alive, and that they are automatically released when all forms (scopes) holding them are released.

To create the view to which the grid is bound that only provides those products associated with the chosen category in the combo box required one additional operator; AsFiltered(<predicate>).

Note that when this query is executed, the result does not necessarily require a round trip to the server to retrieve the products requested. The cache is examined first to see if it already contains the requested data, either because the required data has already been requested once before within this form or from another form in the application. Or, possibly a completely separate query run elsewhere in the application had requested that all products be returned, so the cache would already have all the product data. Again, this is a fundamental strength of SEF. By providing your application with a global cache of data, its performance is continually improved throughout its lifetime.

Here we chose to create a new view, and bind the grid to it, every time the user selects a new category in the combo box (see the combo box’s SelectedValueChanged event). However, we could have avoided the need to create new views all the time and instead created one single view using a special BindFilterKey, which we'll learn more about in the Simplifying MVVM topic.

So, in summary, we replicated in code what we did on the design surface with C1DataSource in Server-Side Filtering . We have even thrown in a little extra; we customized the fields shown in the grid columns as we did in Customizing View by adding a Select to our LiveLinq statement:

select new
{
    p.ProductID,
    p.ProductName,
    p.CategoryID,
    CategoryName = p.Category.CategoryName,
    p.SupplierID,

Supplier = p.Supplier.CompanyName,

    p.UnitPrice,
    p.QuantityPerUnit,
    p.UnitsInStock,
    p.UnitsOnOrder
};

Had we just wanted the raw product data returned from the table without any special formatting, we could have simply said;