I have a submit button on a form that I want to first do the validation and if valid call a client side javascript function and then run through its click event handler 
currently I have this set up by adding this line in the Page_Load
mybutton.Attributes.Add("onclick", "javascript:ShowRunningDialog(this);");
But if the page is not valid I don't want to call this client side function. As it is now the 'ShowRunningDialog' is obviously going to be called no matter what
 
 

 

 if (Page.IsValid) mybutton.Attributes.Add("onclick", "javascript:ShowRunningDialog(this);");
Was that what you meant? 
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"If we learn from our mistakes, I should be brilliant by now."

 Duh. Perfect. Thanks.

You still need to post back which is unnecessary.  This can be done using all JavaScript see the following:
 
function ShowValid()
{
     ValidatePage();
    if(Page_IsValid)
        //Do Something
    else
        return false;

}