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In the default.aspx sample page, I have taken txtemail text box and an asp.net button called btnValidate. If user has not specified the emailed in the txtemail text box, show error to the user and don’t post back the page to the server. If user has entered something in the txtemail, process the event and go to the default2.aspx page.

To avoid the post back handle the OnClientClick event call the javascript function which retuns true on successful validation otherwise false. If OnClientClick gets true from the script function then it would raise the OnClick event, otherwise it would not.

Button in aspx page would look like:

javascript method would look like:

function ValidateEmail()
{
var x = document.getElementById('txtemail');
var val = x.value;

if(val == "")
{
alert("Please enter your email id");
x.focus();
return false;
}
return true;
}

Summary: In this article we have seen that without using any hidden button, how we can control the post back on the asp.net button click.

The general rule for how events are raised is :

1. The initialization events are raised from the innermost control to the outermost one
2. All other events are raised from the outermost control to the innermost one

Web server controls such as the Repeater, DataList, and DataGrid controls can contain button controls that themselves raise events. For example, each row in a DataGrid control can contain one or more buttons created dynamically by templates.

Rather than each button raising an event individually, events from the nested controls are bubbled — that is, they’re sent to the container. The container in turn raises a generic event called ItemCommand with parameters that allow you to discover which individual control raised the original event. By responding to this single event, you can avoid having to write individual event handlers for child controls. The ItemCommand event includes the two standard event arguments, an object referencing the source of the event and an event object containing event-specific information.

Note The DataGrid and DataList Web server controls support additional events, such as EditCommand, DeleteCommand, and UpdateCommand, which are special cases of bubbled events.
With buttons, you can use the CommandArgument property to pass a user-specified string to the event handler to help you identify what button raised the event. For example, in a DataList control, buttons raise the ItemCommand event. You can set the CommandArgument property of each button to a different value — perhaps one button’s value is “ShowDetails” and another button’s value is “AddToShoppingCart” and then capture those values in the event handler later.

1. Compiled at runtime
2. HTML design (Visual design possible)
3. ASP.Net page model with code behind
4. No design time interface (Only a box representing the user control is available on an .aspx page)
6. Cannot be added to the ToolBox. Can only be used in the current web application (although there are ways to share it).

Custom Server Control

1. Precompiled
2. No visual design. HTML needs to be declared programmatically
3. Component model
4. Can be used in .aspx pages, user controls or other custom server controls.
5. Has design-time and run-time interface
6. Can be added to the ToolBox (using drag and drop)

After investigating and reading about all of them in details, I finally decided to go with WMD since it exactly matches with what I needed and was very easy to integrate (works on all browser, light weight, minimum configuration easily possible, support markdown, live preview etc). So far I haven’t seen any major issues with it and this seems to be a good choice.

Let me try to make this clear by simple single line definitions and then point to some very useful posts on the same topics.

For Value Types (Integer, Double, Structures, Boolean etc)

By Value – A copy of the value is passed (new storage location is created) on to the function/sub routine. Any changes made will not be reflected whatsoever.

By Ref – Same memory storage location is used, therefore any changes made to them will be reflected.

For Reference Types (Class , interface , delegate and array)

By Value – A copy of the object reference is passed. Change in data values being referenced is reflected but change in the reference itself will not be reflected back.

By Reference – Reference itself is passed, changes in the data values being referenced and change in the reference will be reflected.

Here are some must read posts which I find very useful to clarify this difference -

Jon Skeet article on passing values by reference.

Jon Skeet article on how value and reference types are saved.