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Composition vs. Rendering

.NET Framework 1.1

While it is relatively easy to author a composite control, composition has a performance overhead because it has to create child controls. If you want optimal performance for your control, you can implement the rendering logic yourself by overriding the Render method. Additionally, you must implement any postback data processing and postback event handling needed by your control.

The following sample shows the control Rendered, which renders a user interface that is equivalent to that provided in the Composite Server Control Sample. Rendered does not use composition and hence is more complex to author.

The following members in Rendered are identical to those in the Composite Server Control Sample.

  • The public Text property. (However, in this case the property is not delegated to a child control.)
  • The public Number property.
  • The private Sum property.
  • The custom Check event.

Because Rendered does not have child controls, it has to do the following additional work.

  • Provide its own implementation for the text boxes.
  • Provide its own implementation for a submit button.
  • Implement IPostBackEventHandler.
  • Implement IPostBackDataHandler.
  • Override the Render method.

Note that Rendered does not use ViewState to persist properties across round trips because state restoration is not needed in this scenario. Note also that in this sample the Render method writes raw HTML to the Write method of HtmlTextWriter. For an example of using the utility methods of HtmlTextWriter for rendering, see Rendering Server Controls Samples.

To build the sample, see the instructions in Server Control Samples.

using System;
using System.Web;
using System.Web.UI;
using System.Collections.Specialized;

namespace CustomControls
{
   
   public class Rendered : Control, IPostBackDataHandler, IPostBackEventHandler
   {
   
      private String text1;
      private String text2;
      private String text = "Press the button to see if you won.";
      private int number = 100;
      

      private int Sum
      {
         get 
         {        
            return Int32.Parse(text1) + 
               Int32.Parse(text2);
         }  
      }
      
      public int Number
      {
         get
         {
            return number;
         }
         set
         {
            number = value;
         }
      }
      
      
      public String Text {
         get {
            return text;
         }
         set { 
            text = value;
         }        
      }
      
      public event CheckEventHandler Check;
      
      protected virtual void OnCheck(CheckEventArgs ce)
      {
         if (Check != null)
         {
            Check(this,ce);
         }
      }
      
      public virtual bool LoadPostData(string postDataKey, 
         NameValueCollection values) {
         text1 = values[UniqueID + "t1"];
         text2 = values[UniqueID+ "t2"];
         Page.RegisterRequiresRaiseEvent(this);
         return false;
      }
      
      public virtual void RaisePostDataChangedEvent() {
      }
      
      public void RaisePostBackEvent(string eventArgument){
         
         OnCheck(new CheckEventArgs(Sum - Number));
      }
      
      
      protected override void Render(HtmlTextWriter output) {
         output.Write ("<h3>Enter a number : ");
            output.Write("<INPUT type= text name = " + this.UniqueID + "t1" + 
            " value = '0' >");
            output.Write("</h3><br>");   
            output.Write ("<br><h3>Enter another number : ");
         output.Write("<INPUT type= text name = " + this.UniqueID + "t2" + 
            " value =  '0' >");
         output.Write("</h3><br>");   
         output.Write("<br><br><input type= submit name = " + 
            this.UniqueID + " value= 'Submit'>");
         output.Write("<br><br><span style='height:50px;width:500px;'>"
            + Text + "</span>");
      }
   }     

} 

// CheckEvent.cs.
// Contains the code for the custom event data class CheckEventArgs.
// Also defines the event handler for the Check event.
using System;

namespace CustomControls
{
      public class CheckEventArgs : EventArgs
      {
            private bool match = false;
            
            public CheckEventArgs (int difference)
            {
                  if (difference == 0)
                  {
                        match = true;
                  }
            }
            public bool Match
            {
                  get
                  {
                        return match;
                  }
            }
      }
      
      public delegate void CheckEventHandler(object sender, CheckEventArgs ce);
}

[Visual Basic]
Option Explicit
Option Strict

Imports System
Imports System.Web
Imports System.Web.UI
Imports System.Collections.Specialized

Namespace CustomControls
   Public Class Rendered
      Inherits Control
      Implements IPostBackDataHandler, IPostBackEventHandler
      
      Private text1 As String
      Private text2 As String
      Private _text As String = "Press enter to see if you won."
      Private _number As Integer = 100
      
      Private ReadOnly Property Sum() As Integer
         Get
            Return Int32.Parse(text1) + Int32.Parse(text2)
         End Get
      End Property
      
      Public Property Number() As Integer
         Get
            Return _number
         End Get
         Set
            _number = value
         End Set
      End Property
      
      Public Property Text() As String
         Get
            Return _text
         End Get
         Set
            _text = value
         End Set
      End Property
      
      Public Event Check As CheckEventHandler
      
      Protected Overridable Sub OnCheck(ce As CheckEventArgs)
         RaiseEvent Check(Me, ce)
      End Sub
      
      Public Overridable Function LoadPostData(postDataKey As String, values As NameValueCollection) As Boolean Implements IPostBackDataHandler.LoadPostData
         text1 = values(UniqueID & "t1")
         text2 = values(UniqueID & "t2")
         Page.RegisterRequiresRaiseEvent(Me)
         Return False
      End Function
      
      Public Overridable Sub RaisePostDataChangedEvent() Implements IPostBackDataHandler.RaisePostDataChangedEvent
      End Sub
      
      Public Sub RaisePostBackEvent(eventArgument As String) Implements IPostBackEventHandler.RaisePostBackEvent
         OnCheck(New CheckEventArgs(Sum - Number))
      End Sub
      
      Protected Overrides Sub Render(output As HtmlTextWriter)
         output.Write("<h3>Enter a number : ")
         output.Write("<INPUT type= text name = " & _
                        Me.UniqueID & "t1" & _
                        " value = '0' >")
         output.Write("</h3><br>")
         output.Write("<br><h3>Enter another number : ")
         output.Write("<INPUT type= text name = " & _
                        Me.UniqueID & "t2" & _
                        " value =  '0' >")
         output.Write("</h3><br>")
         output.Write("<br><br><input type= submit name = " & _
                        Me.UniqueID & _
                        " value= 'Submit'>")
         output.Write("<br><br><span style='height:50px;width:500px;'>" & _
                        Text & "</span>")
      End Sub
   End Class
End Namespace

' CheckEvent.vb
' Contains the code for the custom event data class CheckEventArgs.
' Also defines the event handler for the Check event.
Imports System
Namespace CustomControls
   Public Class CheckEventArgs
      Inherits EventArgs
      Private _match As Boolean = False
      
      Public Sub New(difference As Integer)
         If difference = 0 Then
            _match = True
         End If
      End Sub
      
      Public ReadOnly Property Match() As Boolean
         Get
            Return _match
         End Get
      End Property
   End Class
   
   Public Delegate Sub CheckEventHandler(sender As Object, ce As CheckEventArgs)
End Namespace

Using the Rendered Control on a Page

The following page uses the custom control Rendered, defined in the preceding sample. Note that this page is identical to the page in Composite Server Control Sample, which uses a composite control instead of a rendered control.

<%@ Register TagPrefix="Custom" Namespace="CustomControls" Assembly = "CustomControls" %>

<script language="VB" runat=server>
   Private Sub Sum_Checked(sender As Object, e As CheckEventArgs)
      If e.Match = True Then
         Rendered.Text = "<h2> You won a million dollars!!!! </h2>"
      Else
         Rendered.Text = "Sorry, try again. The numbers you entered don't add up to" & _
                  " the hidden number."
      End If
   End Sub
</script>

<html>            
<body>
<h1> The Mystery Sum Game </h1><br>             
<form runat=server>                             
<Custom:Rendered id = "Rendered" OnCheck = "Sum_Checked" Number= "10" runat = server/>                                                                          
</form>                                               
</body>                                         
</html>
                                  

See Also

Developing a Composite Control

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