ASP.NET Web Application Message Box

You want to use an Alert. Unfortunately it's not as nice as with windows forms.

ClientScript.RegisterStartupScript(this.GetType(), "myalert", "alert('" + myStringVariable + "');", true);

Similar to this question here: http://forums.asp.net/t/1461308.aspx/1

Or create a method like this in your solution:

public static class MessageBox {
    public static void Show(this Page Page, String Message) {
       Page.ClientScript.RegisterStartupScript(
          Page.GetType(),
          "MessageBox",
          "<script language='javascript'>alert('" + Message + "');</script>"
       );
    }
}

Then you can use it like:

MessageBox.Show("Here is my message");