Window alerts in C#

the javascript way

if(error) //your logic here
{
  ClientScript.RegisterStartupScript(typeof(Page), "test", "<script>alert('Hello');return false;</script>");
}

to a button

btn.Attributes.Add("onClick", "javascript:alert('Message Here');");

using messagebox class (using System.Windows.Forms;)

if (MessageBox.Show("Do you want to continue?", "Question", MessageBoxButtons.YesNo) == DialogResult.Yes)
        {
            //if user clicks yes, do stuff
        }

or

System.Windows.Forms.MessageBox.Show("This is a message");

If errors should occur, go to project -> add reference,
then scroll down to System.Windows.Forms and select it.

This will add to web.config:

<add assembly="System.Windows.Forms, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/></assemblies>

Ajax Modal Popup

http://www.asp.net/AJAX/AjaxControlToolkit/Samples/ModalPopup/ModalPopup.aspx

Using Response on .Net Apps

Response.Write("<script type='text/javascript'>alert('This is my Message')</script>");

Using render objects

(HtmlWriter writer)

writer.Write("<script type='text/javascript'>alert('Hello');return false;</script>");