Alternative for ShowDialog in ASP .net

Am coming from C#.net, VB.net back ground to ASP.net. I used them for long time; but the current project is in ASP.Net.

I want alternative for ShowDialog. One important feature of showdialog (which I want) was the code following it is not executed until after the dialog box is closed.

Some jquery alternatives for showdialog I have found but they don’t stop execution of code following it (until form or dialog is closed).

Any suggestions