ASP.NET Design Time Databinding Custom Controls

Introduction

This article describes how to provide design-time databinding functionality to a custom web control when it is used in a data bound composite control's template. An example of this would be if you added your custom web control to an asp:Repeater's ItemTemplate.

Background

When a custom web control is used inside of a databound composite control's template, the custom web control's designer is not used at design-time and databound properties are not set even though the Databind event is called. In order to set the custom web controls databound properties to correctly display the control at design time, a DataBindingHandler must be used.

Using the code

Shown below is sample code for a generic DataBinderHandler. This handler should correctly display any Bind("PropertName"), Eval("PropertyName"), or DataBinder.Eval(Container,"PropertyName") expression at design time.

In order to tell the designer to use a DataBinderHandler, you must add the DataBindingHandler attribute to the custom web control.

<Designer(GetType(InvoiceControlDesigner)), _
DataBindingHandler(GetType(DEDataBindingHandler)), _
ToolboxData("<{0}:InvoiceControl runat="server"></{0}:InvoiceControl>")> _
Public Class InvoiceControl
    Inherits System.Web.UI.WebControls.CompositeControl

.....

End Class

Public Overrides Sub DataBindControl(ByVal designerHost As _
       System.ComponentModel.Design.IDesignerHost, _ 
       ByVal control As System.Web.UI.Control)

  Dim myIDataBindingAccesor As IDataBindingsAccessor = control
  If myIDataBindingAccesor.HasDataBindings Then
  For Each mydatabinding As DataBinding In _
           myIDataBindingAccesor.DataBindings
    Dim pinfo As System.Reflection.PropertyInfo
    pinfo = control.GetType.GetProperty(mydatabinding.PropertyName, _
            mydatabinding.PropertyType, System.Type.EmptyTypes)
    If pinfo IsNot Nothing Then
        Dim MyValue As Object = Nothing
        Select Case mydatabinding.PropertyType.FullName
          Case "System.String"
            MyValue = "ABC"
          Case "System.Date"
            MyValue = #1/1/1970#
          Case "System.Integer", "System.Long"
            MyValue = 123
          Case "System.Single", "System.Double"
            MyValue = 12.34
          Case Else
            MyValue = "DataBound"
        End Select
        Try
          If control.BindingContainer IsNot Nothing AndAlso _ 
            TypeOf control.BindingContainer Is IDataItemContainer Then
             If mydatabinding.Expression.ToLower.StartsWith("bind(""") Or _
              mydatabinding.Expression.ToLower.StartsWith("eval(""") Or _
              mydatabinding.Expression.ToLower.StartsWith("databinder.eval(container") Then
               Dim StartPosition As Integer = mydatabinding.Expression.IndexOf("(")
               Dim FormatPosition As Integer
               Dim ExPrefix As String = ""
               If Not mydatabinding.Expression.ToLower.StartsWith(_
                          "databinder.eval(container") Then
                 ExPrefix = "DataItem."
               Else
                 StartPosition = mydatabinding.Expression.IndexOf(",", StartPosition) + 2
               End If
               FormatPosition = mydatabinding.Expression.IndexOf(",", StartPosition)
               Dim mydatabindingex As String = ""
               If FormatPosition > 4 Then
                 mydatabindingex = mydatabinding.Expression.Substring(StartPosition, _
                                   FormatPosition - StartPosition + 1)
                 Dim FormatString As String
                 FormatString = mydatabinding.Expression.Substring(FormatPosition + 2, _
                                mydatabinding.Expression.Length - FormatPosition - 4)
                 MyValue = DataBinder.Eval(control.BindingContainer, _
                           ExPrefix & mydatabindingex, FormatString)
               Else
                 mydatabindingex = mydatabinding.Expression.Substring(StartPosition, _
                                   mydatabinding.Expression.Length - StartPosition - 2)
                 MyValue = DataBinder.Eval(control.BindingContainer, _
                           ExPrefix & mydatabindingex)
               End If
             End If
          End If
        Catch ex As Exception
          Debug.WriteLine("DataBinderHandler Failed " & ex.ToString)
        End Try
        If System.Type.Equals(MyValue.GetType, mydatabinding.PropertyType) Then
          pinfo.SetValue(control, MyValue, Nothing)
        End If
      End If
    Next
  End If
End Sub
End Class

The ASP.NET code:

<asp:Repeater ID="Repeater1" runat="server" 
  DataMember="InvoiceList" DataSourceID="DEDataSource1">

<ItemTemplate>

<asp:Label ID="Label1" runat="server" 
  Text='<%# Bind("InvoiceID") %>'></asp:Label>

<DE:InvoiceControl ID="InvoiceControl1" runat="server" 
  Invoice='<%# Databinder.Eval(Container,"DataItem") %>' 
  LogoImagePath="http://www.digitalexample.com/images/delogo.gif" 
  InvoiceControlMode="ReadOnly" InvoiceControlStyle="Default" >

<InvoiceTableStyle CellPadding="3" 
  CellSpacing="0" Height="100%" Width="100%" />

</DE:InvoiceControl>

Points of interest

It took a lot of digging before I was able to figure out why my custom control would not work once placed inside of a Repeater. Debugging revealed that the databound properties were not getting set but the databinding event was getting called. I also noted that at design time, the designer did not call the PreRender event. I am not sure why Microsoft did not include a generic DataBindingHandler to be used when there was not a DataBindingHandler attribute set.

History

• Originally posted on 08/21/2007 by Andrew DeVries.