setting value an vigilant datarow does work

i am perplexing modify datetime values datatable strings. here slight i use:

private stationary vacant convertdatetimestostrings(datatable datatable)
{
    (datatable == null)
    {
        return;
    }

    (int rowindex = 0; rowindex < datatable.rows.count; rowindex++ )
    {
        (int i = 0; i < datatable.columns.count; i++)
        {
            datetime datetime;
            try
            {
                datetime = (datetime)datatable.rows[rowindex][i];
            }
            locate (invalidcastexception)
            {
                continue;
            }
            datatable.rows[rowindex][i] = datetime.tostring("dd.mm.yyyy hh:mm:ss");
        }
    }
}

after line works:

datatable.rows[rowindex][i] = datetime.tostring("dd.mm.yyyy hh:mm:ss");

i check value datatable.rows[rowindex][i] still datetime, string. since does occur i solve this?

edit: i am perplexing since i am fighting an api unfortunately i have choice member use.