Question

I have a control if a review already exist, if it exist I want to warn the visitor that if the person click OK/yes in the alert the review will be overwritten, if the person press no/cancel the review will not be updated. But it doesn’t work, in the debugging the alert line just passes by and updates without any alert.

if (ReviewExist(StoreID, UserID) != 0)
{
    ScriptManager.RegisterStartupScript(this, this.GetType(), "Message", "confirm('Are you sure?');", true);
    UpdateStoreReview(Description);
    Response.Redirect("Default");
}
else      
{
    AddStoreReview(Description);
}

Answer 1

It is because your Response.Redirect redirects the control to another page which will ignore your RegisterScript.

What you need to do is, if you are trying to acheive this in the button click (update button), then in the Page_Load try

if(!IsPostBack)    
{
 btnUpdate.Attributes.Add("OnClick","confirm('Are you sure?');");
}

Then the above code can be changed to

if (ReviewExist(StoreID, UserID) != 0)
{
    UpdateStoreReview(Description);
    Response.Redirect("Default");
}
else      
{
    AddStoreReview(Description);
}

Note
I think you need to redirect to Default.aspx; but you are missing .aspx in the response.redirect.