issue in calling a modal dialog box

I have an issue in a asp.net web page 

 I have a page page1, on which if a button is clicked , i need to perform some validations (like whether necessary data in supplied) and if any data is missing i need to show a message box asking for data
If all data is there then i need to store this data and then call another web page page2.aspx like a modal dialog box.

I put the below code in the page_load event for page1, so the page is redirected to page2 without doing any validation

Button1.Attributes.Add("onclick", "window.showModalDialog('page2.aspx', null,'status:no;dialogWidth:370px;dialogHeight:220px;dialogHide:true;help:no;scroll:no');");

Please help me
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