Reading the Data and Schema

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ADO.NET Programming in Visual Basic .NET
By Steve  Holzner, Bob  Howell

Table of Contents
Chapter 11.   Creating Your Own Data Control


Before the control does anything, it has to be able to read the schema from the database based on the Select statement. Let's create a public subprocedure and call it ReadData. We want the procedure to behave a little differently when the control is displayed in a form designer than when it is running in an application. When the control is in a designer, we don't want to read data from the database. What we really want to do is to read the schema. The DataAdapter has a method just for this purpose called FillSchema. The question is, how do we detect if the control is in a designer or in design mode?

If you never developed controls in VB 6, you might be wondering what the heck I'm talking about. This can get a little confusing so please be patient. Controls execute in two modes, User and Design. When a control is in Design mode, it is sitting on a form that is being designed or open in the form designer. The control is actually executing. The form is being designed. The control is executing in Design mode.

When a control is in User mode it is running in the actual application. The control is executing in User (or application) mode. Both modes refer to a compiled control executing. One is executing in a designer, the other is executing in an application. Got It? Good, you're half way there!

In VB 6, you could access a property of the ambient object called User-Mode. This returned a value that indicated how the control was running. VB .NET controls no longer need the ambient object, so they have a Direct property called DesignMode which returns True if the control is running in a designer. So here is our ReadData procedure. We also will add a few more support procedures:

 Public Sub ReadData()      Try          If mAdapter.SelectCommand.CommandText = "" Then Exit Sub          If mDataSet.Tables.Count > 0 Then              mDataSet.Tables.RemoveAt(mintBoundTable)          End If          If Me.DesignMode Then              mAdapter.FillSchema(mDataSet, SchemaType.Source, _                                  mintBoundTable)          Else              mAdapter.Fill(mDataSet, mintBoundTable)              RaiseEvent DataSetFilled(mDataSet)              UpdatePosition()          End If      Catch errobj As Exception          Throw errobj      End Try  End Sub  Private Sub UpdatePosition()      If Not Me.ParentForm Is Nothing Then          If mblnAutoText Then              lblText.Text = "Record: " & _                     Me.ParentForm.BindingContext(Me).Position + 1 & _                       " of " & _                       mDataSet.Tables(mintBoundTable).Rows.Count          End If      End If  End Sub  Public Property AutoText() As Boolean      Get          AutoText = mblnAutoText      End Get      Set(ByVal Value As Boolean)          mblnAutoText = Value          Me.Text = "AutoText"      End Set  End Property  Public Property SelectCommandText() As String      Get          SelectCommandText = mAdapter.SelectCommand.CommandText      End Get      Set(ByVal Value As String)          mAdapter.SelectCommand.CommandText = Value          ReadData()      End Set  End Property 

It's pretty straightforward. If there is no Select statement assigned, the procedure exits. If the DataSet already has any table objects, then they are removed. We could also have used mDataset.Clear. This would clear all of the DataTables from the DataSet. If the DataSet were created outside the control and then passed in via the DataSet property, this is not what we would want to happen. We really want to remove only the bound table, or the table we will use to bind controls to. We want to leave any other tables alone.

Next we check to see if we are in Design mode. If we are, we want to get the schema from the database, but not the actual data. If we are not, we grab the whole kit and caboodle. We then fire an event that notifies the host that we have replaced the data in the DataSet. We then call a procedure that updates the Text property with Record: n of x if we have the autoText property turned on. The autoText property is a Boolean value that indicates if we want the Text property automatically updated when the DataTable's current row changes. If any errors occur they are thrown back to the calling procedure.


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ADO. NET Programming in Visual Basic. NET
ADO.NET Programming in Visual Basic .NET (2nd Edition)
ISBN: 0131018817
EAN: 2147483647
Year: 2005
Pages: 123

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