window.showModalDialog

In Asp.NET , when we want to create a modal dialog box we can use the javascript like

vReturnValue = window.showModalDialog(sURL [, vArguments] [, sFeatures])
Detail in MSDN.

But there is a problem,if we have to post to server in modal dialog box , you can find
another window will be opened and an error box will be shown.
To resolve it we can use FRAMESET , that's to say we create a aspx file include FRAMESET
in which link to the page we want to show in modal dialog box.