Call Javascript alert from c# code?

I have seen several examples of how to call a JavaScript alert from a web button.  I would like to call it from the following though:
if (dtEntry.Rows.Count == 0)
            {
               //Display JavaScript Alert("No data to display.");
            }
            else
            {
                GridViewEntry.DataSource = dtEntry;
                GridViewEntry.DataBind();
            }
Suggestions?

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