If you want to show msg if you are posting the page to server through update panal (asynchronous postback) then you have to use this function.
//During Asynchronous Post Back in Update
Panal Use ScriptManager (Inside Update Panal)
{
ScriptManager.RegisterStartupScript(Page,
this.GetType(), "Key", string.Format("alert('{0}');",
Message), true);
}
If you want to show message in browser and want
to show it through javascript (alert function)
then use this function.
// During synchronous normal Post
Back Use ClientScript
public static void
ShowMessage(string MessageText, Page MyPage)
{
MyPage.ClientScript.RegisterStartupScript(MyPage.GetType(),
"MessageBox", "alert('" + MessageText.Replace("'", "\'") + "');", true);
"MessageBox", "alert('" + MessageText.Replace("'", "\'") + "');", true);
}