show alert message from server side

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)

public void alertBox(string Message)

{

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);

}