How to pass a list of a class but also refer to a specific variable in that class

So here is a basic example and not the full code. I want to be able to refer to the specific variable in Symbol rather than have a bunch of if statements to know which values to add. I know i could make each instance of DataLine a list but that would not work for my overall goal.

public class DataLine
{
    public int open;
    public int close;
    public int high;
    public int low;
}

Public class Symbol
{
    public string Code;
    public string Name;

    public DataLine Day;
    public DataLine Week;
    public DataLine Month;
}

public int AddValues(string lineName, ref List<Symbol> symbol)
{
    int sum = 0;
    for(int i = 0; i < symbol.Count - 1; i++)
    {
        if (lineName == "day")
        {
            sum = sum + symbol[i].Day.close
        }
        else if (lineName == "week")
        {
            sum = sum + symbol[i].Week.close
        }
        else if (lineName == "month")
        {
            sum = sum + symbol[i].month.close
        }
    }
    return sum;
}

public void Main()
{
    List<Symbol> symbol = new List<Symbol>();
    //imagine here symbol has items added and variables in DataLine have values

    int daysSum = AddValues("day", ref List<symbol>);

}