How to use alert message from base class

In your base class for example,
Utils.cs write the following method

public static void ShowAlert(Page currentPage, string message)
      {
          StringBuilder sb = new StringBuilder();
          sb.Append("alert('");
          sb.Append(message);
          sb.Append("');");
          currentPage.ClientScript.RegisterStartupScript(typeof(Utils), "showalert", sb.ToString(), true);
      }

Now where ever you want to show your alert message just call it.

for example

In Hello.aspx.cs

protected void Page_Load(object sender, EventArgs e)
{
       Utils.ShowAlert(this, "Hello");
}