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First of all I will give a brief overview of Postback and Callback. When ever you register a control’s server side event, ASP.Net register a JavaScript which will submit the form’s details and on the server it fires the event which is registered for the control.

ASP.net register __doPostBack javascript function. Syntax for the script is __doPostBack(‘Event Target’,'Event Argument’); Here Event Target can be a control or it can be a function created by developer. You can also generate a postback script reference for a control by using GetPostBackScript function. Now ASP.Net page always handle the PostBack event by it self.

For Callback you have to impliment a ICallBackEventHandler interface. After implimenting ICallBackEventHandler a page must contain two events, RaiseCallBackEvent with return type void and GetCallBackResult with return type string. Your code logic will be contained in the RaiseCallBackEvent function code block and the result of the Callback will be contained returned by GetCallBackResult. Function GetCallBackResult will call the client side callback result function. I will explain CallBack in detail in my future post.

Many of ASP.Net users are facing a problem with invalid Postback or Callback argument error. Invalid PostBack or CallBack argument error is basically raise because of Event Validation feature. The EventValidation feature is a new feature in ASP.NET 2.0, and provides an additional level of checks to verify that a postback from a control on the client is really from that control and not from someone malicious using something like a cross-site script injection to try and manipulate things. It is part of our overall strategy of increasingly adding security in depth levels to the programming model — so that developers can be secure by default even if they forget to add security checks of their own.

Now, Invalid PostBack or CallBack argument error may occur when you are firing click event and the object is rebinding or its properties are changed in Page_Load event or someone is trying to hack into your system with cross site scripting. Each time .Net Framework render a page then it associate a unique Guid for all the controls. When binding a gridview or repeater, on each databind framework will associate a new guid for the contorl. So every time when you are firing event make sure Page_Load event does not change the control, because if the control changed the it will have a different Guid which have acutally fired the event for postback. Here are some scenario with this error.

1) Invalid Postback or Callback argument in GridView Problem may be: You are binding data in Page_Load event with either Object Data Source or Manual Binding with function call. This will make your GridView bind data on every event fire of any control. When you are firing any GridView command with OnRowCommand, before RowCommand fire your GridView will rebind and all control within it will be assigned to new id. So RowCommand could not get the item which have fired the event.  for Invalid Postback or Callback argument in GridView: You can bind your data within this if condition

if (!IsPostBack)
{
     //Your code for Bind data
}

This code will definitely give you solution if this not work then check whether any other control is not giving error.

2) Invalid Postback or Callback argument while submitting form. Problem may be: You have many textboxes and textareas and when user is entering “<” or “>” char then it is giving error. This is because of .Net framework is giving facility to validate request. This function will allow user not to submit script or html code directly so it is blocked. Solution for Invalid Postback or Callback argument while submitting form: You can replace this char with javascript before submitting the form replace “<” with “<” and “>” with “>” the javascript code is here

function ReplaceChar(obj)
{
     //Here obj is your textbox object
     var Textvalue = obj.value;
     Textvalue = Textvalue.replace("<","<");
     Textvalue = Textvalue.replace(">",">");
     obj.value = Textvalue;
}

The other solution for both of this issue is to set enableEventValidation=false You can set this option declaration <@ Page > or even you can put this code in your web. config file in block
//set it true if you want to validate your each request. If you do not validate the request then your security of data will be decrease so this is not the perfect solution If both of this solution not work then you can contact me on my email and if you find any new solution please post it comment.

This function creates a thumbnail that is exactly as big as the size you give it. The image is resized to best fit the size of the thumbnail. If it does not fit exactly in both directions, it’s centered in the thumnail.

The Function:

     function thumbnail_box($img, $box_w, $box_h) {
          //create the image, of the required size
          $new = imagecreatetruecolor($box_w, $box_h);
          if($new === false) {
               //creation failed -- probably not enough memory
               return null;
          }

          //Fill the image with a light grey color
          //(this will be visible in the padding around the image,
          //if the aspect ratios of the image and the thumbnail do not match)
          //Replace this with any color you want, or comment it out for black.
          //I used grey for testing =)
          $fill = imagecolorallocate($new, 200, 200, 205);
          imagefill($new, 0, 0, $fill);

          //compute resize ratio
          $hratio = $box_h / imagesy($img);
          $wratio = $box_w / imagesx($img);
          $ratio = min($hratio, $wratio);

          //if the source is smaller than the thumbnail size,
          //don't resize -- add a margin instead
          //(that is, dont magnify images)
          if($ratio > 1.0)
               $ratio = 1.0;

          //compute sizes
          $sy = floor(imagesy($img) * $ratio);
          $sx = floor(imagesx($img) * $ratio);

          //compute margins
          //Using these margins centers the image in the thumbnail.
          //If you always want the image to the top left,
          //set both of these to 0
          $m_y = floor(($box_h - $sy) / 2);
          $m_x = floor(($box_w - $sx) / 2);

          //Copy the image data, and resample
          //If you want a fast and ugly thumbnail,
          //replace imagecopyresampled with imagecopyresized
          if(!imagecopyresampled($new, $img, $m_x, $m_y, 0, 0, $sx, $sy, imagesx($img), imagesy($img))) {
               //copy failed
               imagedestroy($new);
               return null;
          }

          //copy successful
          return $new;
     }

Example Usage:

$i = imagecreatefromjpeg("img.jpg");
$thumb = thumbnail_box($i, 210, 150);
imagedestroy($i);

if(is_null($thumb)) {
    /* image creation or copying failed */
    header('HTTP/1.1 500 Internal Server Error');
    exit();
}

header('Content-Type: image/jpeg');
imagejpeg($thumb);

Recently I had to work on customizing an ASP.NET Calendar control by adding text to the day cells for my team’s Logon Message project. This proved to be not as straight-forward as I thought so here is what I did in case someone else might want to do the same.

The best place to modify the text in a cell seems to be the DayRender event handler. So the first thing I tried was to just modify the e.Cell.Text property like this:

void Calendar1_DayRender(object sender, DayRenderEventArgs e)
{
     e.Cell.Text += “My text”;
}

The main problem was that after adding text to a day cell in the control I wasn’t able to select that day by clicking on the day number. Here is what you can do if you still want the select day functionality:

void Calendar1_DayRender(object sender, DayRenderEventArgs e)
{
     AddTextToDayCell(e, Datetime.Today, “MyText“);
}

void AddTextToDayCell(DayRenderEventArgs e, Datetime d, string text)
{
     if(e.Day.Date == d.Date)
     {
          string ID = ((System.TimeSpan)(e.Day.Date - new DateTime(2000, 1, 1))).Days.ToString();

          e.Cell.Text = "<a href=\"javascript:__doPostBack('Calendar1','" + ID + "')\" style=\"color:#663399\">" + e.Day.DayNumberText; //assuming the name of the calendar control is Calendar1.

          e.Cell.Text +=text;
     }
}

If you want your new text to act as a link to some other URL, you could modify the AddTextToCell function as follows:

private void Calendar1_DayRender(object sender, System.Web.UI.WebControls.DayRenderEventArgs e)
{
     AddTextToDayCell(e, DateTime.Today, "MyText", "<a href="http://fathir.com/">http://fathir.com</a>"); //this will add the MyText link to <a href="http://fathir.com/">http://fathir.com</a> to the current day's cell
}

void AddTextToDayCell(DayRenderEventArgs e, DateTime d, string text, string URL)
{
     if(e.Day.Date == d.Date)
     {
          string ID = ((System.TimeSpan)(e.Day.Date - new DateTime(2000, 1, 1))).Days.ToString();
          e.Cell.Text = "<a href=\"javascript:__doPostBack('Calendar1','" + ID + "')\" style=\"color:#663399\">" + e.Day.DayNumberText;
          e.Cell.Text += "<br /> <a href=\""+ URL + "\">" + text;
     }
}

Hope you’ll find this helpful.

Send e-mail without SMTP

public void sendSmtpDotnet()
{
System.Web.Mail.MailMessage m = new System.Web.Mail.MailMessage();
m.From = "someone@domain.com";
m.To = "to@domain.com";
m.Subject = "Test";
m.Body = "Body Test";

System.Web.Mail.SmtpMail.SmtpServer = "smtp.domain.com";
System.Web.Mail.SmtpMail.Send(m);
}

Send e-mail with SMTP

public void sendSmtpDotnet()
{
System.Web.Mail.MailMessage m = new System.Web.Mail.MailMessage();
m.From = "someone@domain.com";
m.To = "to@domain.com";
m.Subject = "Test";
m.Body = "Body Test";

m.Fields["http://schemas.microsoft.com/cdo/configuration/smtpauthenticate"] = 1;
//1 --> Authenticate=true
m.Fields["http://schemas.microsoft.com/cdo/configuration/smtpserverport"] = 2525;
m.Fields["http://schemas.microsoft.com/cdo/configuration/sendusing"] = 2;
//2 = Send the message using the network (SMTP over the network).

m.Fields["http://schemas.microsoft.com/cdo/configuration/sendusername"] = @"DOMAINNAME\USERNAME";
m.Fields["http://schemas.microsoft.com/cdo/configuration/sendpassword"] = "PASSWORD";

System.Web.Mail.SmtpMail.SmtpServer = "smtp.domain.org";
System.Web.Mail.SmtpMail.Send(m);
}

If you make unfounded assumptions about the type, length, format, or range of input, your application
is unlikely to be robust. Input validation can become a security issue if an attacker discovers that you
have made unfounded assumptions. The attacker can then supply carefully crafted input that
compromises your application by attempting SQL injection, cross-site scripting, and other injection
attacks. To avoid such vulnerability, you should validate text fields (such as names, addresses, tax
identification numbers, and so on) and use regular expressions to do the following:

• Constrain the acceptable range of input characters.
• Apply formatting rules. For example, pattern-based fields, such as tax identification numbers, ZIP Codes, or postal codes, require specific patterns of input characters.
• Check lengths.

Regular expression support is available to ASP.NET applications through the
RegularExpressionValidator control and the Regex class in the System.Text.RegularExpressions
namespace.

File Download : How To: Use Regular Expressions to Constrain Input in ASP.NET

Source : Microsoft patterns & practices Developer Center