How to create a Message Box in ASP.NET using C#

public void MsgBox1(string msg, Page refP)
   {
       Label lbl = new Label();
       string lb = "window.alert('" + msg + "')";
       ScriptManager.RegisterClientScriptBlock(refP, this.GetType(), "UniqueKey", lb, true);
       refP.Controls.Add(lbl);
   }

MsgBox1("Your Message",this);

Page.ClientScript.RegisterStartupScript(this.GetType(),"Hello","javascript:alert("Give Message Here");",true);

  public void fnAlert(string str)
  {
    string alertScript = "<script language="JavaScript">";
    alertScript += "alert('" + str + "');";
    alertScript += ";        
    if (!ClientScript.IsClientScriptBlockRegistered("alert"))
         ClientScript.RegisterStartupScript(typeof(string),"alert", alertScript);
  }

</script>

try
{
throw new Exception("<script language="javascript">\n" + "alert('abc');" + "</script>");
}

catch(Exception ex)
{
   label1.text = ex.message.text;
}