how to learn if a column exists or not before calling GetOrdinal("columnname") method in C#?

Question

Is it possible to check if a column that is returned from database exist or not before getting ordinal of that column? Because if it doesn't exist then i dont want to call getordinal method ;

    //Here the place that i want to check if username column have returned from database or not.
    OrdUserName = reader.GetOrdinal("username");