Rendering a Server Control Samples

  • Article

This topic provides two samples of rendering a server control that illustrate the following concepts.

  • Overriding the rendering method of WebControl.
  • Using the utility methods of HtmlTextWriter.

Overriding the Rendering Methods of WebControl

The control (MyLinkButton2) in the following sample is similar to the link button described in the Postback Using Client-Side Script Sample, with the following differences.

  • MyLinkButton2 derives from WebControl and thus inherits style properties whose rendering is implemented by WebControl.
  • MyLinkButton2 does not pass raw HTML to the Write method of the HtmlTextWriter instance; instead, it overrides the AddAttributesToRender method to pass the script function to the client and overrides the RenderContents method to pass the text for the hyperlink.
using System;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Collections;

namespace CustomControls {
   
   public class MyLinkButton2: WebControl, IPostBackEventHandler{
      
      public MyLinkButton2() : base(HtmlTextWriterTag.A) {
      }
      
      public virtual string Text {
                  get {
                        object o = ViewState["Text"];
                        return((o == null) ? String.Empty : (string)o);
                  }
                  set {
                        ViewState["Text"] = value;
                  }
      }
      // Defines the Click event.
      //
      public event EventHandler Click;
      
      // Invokes delegates registered with the Click event.
      //
      protected virtual void OnClick(EventArgs e) {
         
         if (Click != null) {
            Click(this, e);
         } 
      }
      
      
      // Method of IPostBackEventHandler that raises change events.
      //
      public void RaisePostBackEvent(string eventArgument){
         
         OnClick(new EventArgs());
      }

      protected override void AddAttributesToRender(HtmlTextWriter writer) {
                  
                  base.AddAttributesToRender(writer);
                  writer.AddAttribute(HtmlTextWriterAttribute.Href, "javascript:" + Page.GetPostBackEventReference(this));
                        
            } 
    protected override void RenderContents(HtmlTextWriter writer) {
                        writer.Write(Text);
            }
 
   }    
}
[Visual Basic]
Option Explicit
Option Strict

Imports System
Imports System.Web.UI
Imports System.Web.UI.WebControls
Imports System.Collections

Namespace CustomControls
   Public Class MyLinkButton2
      Inherits WebControl
      Implements IPostBackEventHandler
      
      Public Sub New()
         MyBase.New(HtmlTextWriterTag.A)
      End Sub
      
      
      Public Overridable Property Text() As String
         Get
            Dim o As Object = ViewState("Text")
            If o Is Nothing Then
               Return String.Empty
            Else
               Return CStr(o)
            End If
         End Get
         Set
            ViewState("Text") = value
         End Set
      End Property
      
      ' Defines the Click event.
      '
      Public Event Click As EventHandler
      
      ' Invokes delegates registered with the Click event.
      '
      Protected Overridable Sub OnClick(e As EventArgs)
         RaiseEvent Click(Me, e)
      End Sub
      
      ' Method of IPostBackEventHandler that raises change events.
      '
      Public Sub RaisePostBackEvent(eventArgument As String) Implements IPostBackEventHandler.RaisePostBackEvent
         OnClick(New EventArgs())
      End Sub
      
      Protected Overrides Sub AddAttributesToRender(writer As HtmlTextWriter)
         MyBase.AddAttributesToRender(writer)
         writer.AddAttribute(HtmlTextWriterAttribute.Href, "javascript:" + Page.GetPostBackEventReference(Me))
      End Sub
       
      Protected Overrides Sub RenderContents(writer As HtmlTextWriter)
         writer.Write(Text)
      End Sub
   End Class 
End Namespace

Using the Utility Methods of HtmlTextWriter

The following sample control (Rendered2) is identical to the control described in Composition vs. Rendering, with the difference that Rendered2 uses the utility methods of HtmlTextWriter instead of passing raw HTML to the Write method of the HtmlTextWriter instance. This sample illustrates use of the utility methods of HtmlTextWriter and also shows how to emit multiple HTML elements from a single control. The sample uses the following methods of HTMLTextWriter.

  • AddAttribute
  • AddStyleAttribute
  • RenderBeginTag
  • RenderEndTag
  • Write

Additionally, the sample uses the HtmlTextWriterAttribute, HtmlTextWriterStyle, and HtmlTextWriterTag enumerations.

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 Rendered2 : Control, IPostBackDataHandler, IPostBackEventHandler
   {
   
      private String text1;
      private String text2;
      private String text = "Press 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 writer) {
         writer.RenderBeginTag(HtmlTextWriterTag.H3); 
         writer.Write("Enter a number:"); 
      writer.RenderEndTag(); 
         
      writer.AddAttribute(HtmlTextWriterAttribute.Type,"Text");
      writer.AddAttribute(HtmlTextWriterAttribute.Name,this.UniqueID + "t1");
      writer.AddAttribute(HtmlTextWriterAttribute.Value,"0");
      writer.RenderBeginTag(HtmlTextWriterTag.Input);       
         writer.RenderEndTag(); 

         writer.RenderBeginTag(HtmlTextWriterTag.H3);  
         writer.Write("Enter another number:"); 
      writer.RenderEndTag(); 

         writer.AddAttribute(HtmlTextWriterAttribute.Type,"Text");
      writer.AddAttribute(HtmlTextWriterAttribute.Name,this.UniqueID + "t2");
      writer.AddAttribute(HtmlTextWriterAttribute.Value,"0");
      writer.RenderBeginTag(HtmlTextWriterTag.Input);       
      writer.RenderEndTag(); 
      
         writer.RenderBeginTag(HtmlTextWriterTag.Br);
      writer.RenderEndTag(); 

      writer.AddAttribute(HtmlTextWriterAttribute.Type,"Submit");
      writer.AddAttribute(HtmlTextWriterAttribute.Name,this.UniqueID);
      writer.AddAttribute(HtmlTextWriterAttribute.Value,"Submit");
      writer.AddStyleAttribute(HtmlTextWriterStyle.Height,"25 px");
      writer.AddStyleAttribute(HtmlTextWriterStyle.Width,"100 px");
      writer.RenderBeginTag(HtmlTextWriterTag.Input);    
         writer.RenderEndTag(); 
         
         writer.RenderBeginTag(HtmlTextWriterTag.Br);
         writer.RenderEndTag(); 
         
         writer.RenderBeginTag(HtmlTextWriterTag.Span); 
         writer.Write(this.Text);
         writer.RenderEndTag(); 
      }
   }     
}

// 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 Rendered2
      Inherits Control
      Implements IPostBackDataHandler, IPostBackEventHandler
      
      Private text1 As String
      Private text2 As String
      Private _text As String = "Press the button 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(writer As HtmlTextWriter)
         writer.RenderBeginTag(HtmlTextWriterTag.H3)
         writer.Write("Enter a number:")
         writer.RenderEndTag()
         
         writer.AddAttribute(HtmlTextWriterAttribute.Type, "Text")
         writer.AddAttribute(HtmlTextWriterAttribute.Name, Me.UniqueID + "t1")
         writer.AddAttribute(HtmlTextWriterAttribute.Value, "0")
         writer.RenderBeginTag(HtmlTextWriterTag.Input)
         writer.RenderEndTag()
         
         writer.RenderBeginTag(HtmlTextWriterTag.H3)
         writer.Write("Enter another number:")
         writer.RenderEndTag()
         
         writer.AddAttribute(HtmlTextWriterAttribute.Type, "Text")
         writer.AddAttribute(HtmlTextWriterAttribute.Name, Me.UniqueID + "t2")
         writer.AddAttribute(HtmlTextWriterAttribute.Value, "0")
         writer.RenderBeginTag(HtmlTextWriterTag.Input)
         writer.RenderEndTag()
         
         writer.RenderBeginTag(HtmlTextWriterTag.Br)
         writer.RenderEndTag()
         
         writer.AddAttribute(HtmlTextWriterAttribute.Type, "Submit")
         writer.AddAttribute(HtmlTextWriterAttribute.Name, Me.UniqueID)
         writer.AddAttribute(HtmlTextWriterAttribute.Value, "Submit")
         writer.AddStyleAttribute(HtmlTextWriterStyle.Height, "25 px")
         writer.AddStyleAttribute(HtmlTextWriterStyle.Width, "100 px")
         writer.RenderBeginTag(HtmlTextWriterTag.Input)
         writer.RenderEndTag()
         
         writer.RenderBeginTag(HtmlTextWriterTag.Br)
         writer.RenderEndTag()
         
         writer.RenderBeginTag(HtmlTextWriterTag.Span)
         writer.Write(Me.Text)
         writer.RenderEndTag()
      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

See Also

Rendering an ASP.NET Server Control