calling alert from code-behind

i have a dropdownlist and a listbox both asp.net controls

i am trying to prevent the user add duplciate items to listbox control

i able to block it but 开发者_如何学Ci want to display DIV or Alert box saying,"duplciate names are not allowed"

protected void btn_AddRecipientAction_OnClick(object sender, EventArgs e)
        {
            if (Convert.ToInt32(this.ddlRecipient.SelectedValue) > 0)
            {
                if (ddlRecipient.Text.Length > 0)
                {
                    //var items = new System.Collections.ArrayList(this.lstRecipient.Items);  
                    for(var i = lstRecipient.Items.Count - 1; i >= 0; --i) 
                    {                      
                        if (lstRecipient.Items[i].Text == ddlRecipient.SelectedItem.Text)
                        {
                            lstRecipient.Items.RemoveAt(i);
                            **//alert("duplicate entry not allowed")
                            //div display the message and disappears after few seconds?**
                        } 
                    } 
                    ListItem newList = new ListItem();
                    newList.Text = ddlRecipient.SelectedItem.Text;
                    newList.Value = ddlRecipient.SelectedValue;
                    this.lstRecipient.Items.Add(newList);
                }


            } 
        }

alert way:

You could use this line assuming you have a ScriptManager

ScriptManager.RegisterClientScriptBlock(this,this.GetType(),"alert","alert('duplicate entry not allowed');",true);

This, however still does a postback since the script is run when the page is loaded again after the click. A better solution is to validate in client using javascript before submitting the page.

What you want is actually two separate things.

You should be validating on in the code behind, checking for duplicates on the post back. Then, use some javascript to do the same check on the client.

You MUST check for duplicates on the server since the user may not have javascript turned on.

Wow! Please don't inject js in the page to alert the user. You should instead have a notification control that receive a dataset of messages like an array then display the messages to the user. You want to separate your concerns.

You can achieve that in js. At the server you can set the array in json in a hidden field and then at the document ready event in js read that json data, parse it and loop on the array and display you messages. If you must you can use alert to display them but you should avoid it since it's so 1990's.

But I would go beyond that. I you do all the processing and validation in javascript before it gets to the server. So you don't rely on a post back to execute your validation. So as soon as the user adds the item it's told that it's a duplicate. Then, once the list is filled by the user he could save with a ajax call or post the page and at the server you parse the list, validate it and save it. If you have to compare the list to one already persisted at the server you can do that there. SOme thing goes wrong? you add the message to the notification control.

Please think about it. Try using a framework like MVC to separate you concerns. I makes the hole thing much faster to develop and so easier to maintain.

To call some JS from the code behind you can use Page.ClientScript property and call the RegisterStartupScript() method

http://msdn.microsoft.com/en-us/library/asz8zsxy.aspx