Message Box in Asp.Net

public void ShowMessageBox(string message)
{
Page currentPage = (Page)HttpContext.Current.Handler;
if (currentPage != null)
{
ScriptManager manager = ScriptManager.GetCurrent(currentPage);
string _msg = "alert('" + message + "');";
if (manager != null && manager.IsInAsyncPostBack)
{
ScriptManager.RegisterClientScriptBlock(currentPage, currentPage.GetType(), "msg", _msg, true);
}
else
{
currentPage.ClientScript.RegisterStartupScript(currentPage.GetType(), "msg", _msg, true);
}
}
}