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IPostBackDataHandler Interface

Defines methods that ASP.NET server controls must implement to automatically load post back data.

For a list of all members of this type, see IPostBackDataHandler Members.

[Visual Basic]
Public Interface IPostBackDataHandler
[C#]
public interface IPostBackDataHandler
[C++]
public __gc __interface IPostBackDataHandler
[JScript]
public interface IPostBackDataHandler

Classes that Implement IPostBackDataHandler

Class Description
CheckBox Displays a check box that allows the user to select a true or false condition.
CheckBoxList Creates a multi selection check box group that can be dynamically created by binding the control to a data source.
DropDownList Represents a control that allows the user to select a single item from a drop-down list.
HtmlInputCheckBox Allows programmatic access to the HTML <input type= checkbox> element on the server.
HtmlInputFile Allows programmatic access to the HTML <input type= file> element on the server.
HtmlInputHidden Allows programmatic access to the HTML <input type=hidden> element on the server.
HtmlInputImage Allows programmatic access to the HTML <input type= image> element on the server.
HtmlInputRadioButton Allows programmatic access to the HTML <input type= radio> element on the server.
HtmlInputText Allows programmatic access to the HTML <input type= text> and <input type= password> elements on the server.
HtmlSelect Allows programmatic access to the HTML <select> element on the server.
HtmlTextArea Allows programmatic access to the <textarea> HTML element on the server.
ImageButton A control that displays an image and responds to mouse clicks on the image.
ListBox Represents a list box control that allows single or multiple item selection.
RadioButtonList Represents a list control that encapsulates a group of radio button controls.
TextBox Displays a text box control for user input.

Remarks

If you want a server control you design to examine form data that is posted back to the server by the client, you must implement the IPostBackDataHandler interface. The contract that this interface defines allows a server control to determine whether its state should be altered as a result of the post back, and to raise the appropriate events. For more information, see Processing Postback Data.

Example

[Visual Basic, C#, C++] The following example demonstrates a custom text box server control that implements the IPostBackDataHandler interface. The Text property is changed as a result of the post back and the server control raises a text changed event after post-back data has been loaded.

[Visual Basic] 
Imports System
Imports System.Web
Imports System.Web.UI
Imports System.Collections
Imports System.Collections.Specialized

Namespace CustomWebFormsControls
    
    <System.Security.Permissions.PermissionSetAttribute(System.Security.Permissions.SecurityAction.Demand, Name:="FullTrust")> Public Class MyTextBox
        Inherits Control
        Implements IPostBackDataHandler
        
        
        Public Property Text() As String
            Get
                Return CType(Me.ViewState("Text"), String)
            End Get
            
            Set
                Me.ViewState("Text") = value
            End Set
        End Property
        
        
        Public Event TextChanged As EventHandler
        
        
        Public Overridable Shadows Function LoadPostData( _
        postDataKey As String, _
        postCollection As System.Collections.Specialized.NameValueCollection) _
        As Boolean Implements IPostBackDataHandler.LoadPostData
            
            Dim presentValue As String = Text
            Dim postedValue As String = postCollection(postDataKey)
            
            If presentValue Is Nothing Or Not presentValue.Equals(postedValue) Then
                Text = postedValue
                Return True
            End If
            
            Return False
        End Function
        
        
        Public Overridable Shadows Sub RaisePostDataChangedEvent() _
        Implements IPostBackDataHandler.RaisePostDataChangedEvent
        
            OnTextChanged(EventArgs.Empty)
        End Sub
        
        
        Protected Overridable Sub OnTextChanged(e As EventArgs)
            RaiseEvent TextChanged(Me, e)
        End Sub
        
        
        Protected Overrides Sub Render(output As HtmlTextWriter)
            output.Write("<INPUT type= text name = " & Me.UniqueID & _
                " value = " & Me.Text & " >")
        End Sub
        
    End Class
    
End Namespace


[C#] 
using System;
using System.Web;
using System.Web.UI;
using System.Collections;
using System.Collections.Specialized;


namespace CustomWebFormsControls {

   [System.Security.Permissions.PermissionSet(System.Security.Permissions.SecurityAction.Demand, Name="FullTrust")] 
   public class MyTextBox: Control, IPostBackDataHandler {
    

      public String Text {
         get {
            return (String) ViewState["Text"];
         }

         set {
            ViewState["Text"] = value;
         }
      }      
      

      public event EventHandler TextChanged;
      

      public virtual bool LoadPostData(string postDataKey, 
         NameValueCollection postCollection) {

         String presentValue = Text;
         String postedValue = postCollection[postDataKey];

         if (presentValue == null || !presentValue.Equals(postedValue)) {
            Text = postedValue;
            return true;
         }

         return false;
      }

      
      public virtual void RaisePostDataChangedEvent() {
         OnTextChanged(EventArgs.Empty);
      }
      

      protected virtual void OnTextChanged(EventArgs e) {
         if (TextChanged != null)
            TextChanged(this,e);
      }
      

      protected override void Render(HtmlTextWriter output) {
         output.Write("<INPUT type= text name = "+this.UniqueID
            + " value = " + this.Text + " >");
      }
   }   
}
   

[C++] 
#using <mscorlib.dll>
#using <System.dll>
#using <System.Web.dll>
using namespace System;
using namespace System::Web;
using namespace System::Web::UI;
using namespace System::Collections;
using namespace System::Collections::Specialized;

[System::Security::Permissions::PermissionSet(System::Security::Permissions::SecurityAction::Demand, Name=S"FullTrust")] 
public __gc class MyTextBox: public Control, public IPostBackDataHandler {

public:
   __property String* get_Text() {
      return (String*) ViewState->Item[S"Text"];
   }

   __property void set_Text( String* value ) {
      ViewState->Item[S"Text"] = value;
   }

   __event EventHandler* TextChanged;

   virtual bool LoadPostData(String* postDataKey, 
      NameValueCollection* postCollection) {

         String* presentValue = Text;
         String* postedValue = postCollection->Item[postDataKey];

         if (presentValue == 0 || !presentValue->Equals(postedValue)) {
            Text = postedValue;
            return true;
         }

         return false;
   }

   virtual void RaisePostDataChangedEvent() {
      OnTextChanged(EventArgs::Empty);
   }

protected:
   virtual void OnTextChanged(EventArgs* e) {
      if (TextChanged != 0)
         TextChanged(this,e);
   }

   void Render(HtmlTextWriter* output) {
      output->Write(S"<INPUT type= text name = {0} value = {1} >", this->UniqueID, this->Text);
   }
};   

[JScript] No example is available for JScript. To view a Visual Basic, C#, or C++ example, click the Language Filter button Language Filter in the upper-left corner of the page.

Requirements

Namespace: System.Web.UI

Platforms: Windows 2000, Windows XP Professional, Windows Server 2003 family

Assembly: System.Web (in System.Web.dll)

See Also

IPostBackDataHandler Members | System.Web.UI Namespace | Postback Data Processing Sample | Processing Postback Data

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