Converting string expression to boolean logic - C#

If you don't want to use some available libraries to parse that string you need to separate those characters and implement the logic based on comparison. So for example say we have "a || b", we can loop though each character and decide the appropriate operation based on char == '|'. For more complex situation I'd use a stack to keep track of each results, like this one that can handle && and || without parentheses:

public bool ConvertToBool(string op, bool a, bool b)
{
    var st = new Stack<bool>();
    var opArray = op.ToCharArray();
    var orFlag = false;
    var andFlag = false;

    for (var i = 0; i < opArray.Length; i++)
    {
        bool top;
        switch (opArray[i])
        {
            case '|':
                i++;
                orFlag = true;
                break;
            case '&':
                i++;
                andFlag = true;
                break;
            case 'a':
                if (orFlag)
                {
                    top = st.Pop();
                    st.Push(top || a);
                    orFlag = false;
                }
                else if (andFlag)
                {
                    top = st.Pop();
                    st.Push(top && a);
                    andFlag = false;
                    continue;
                }
                st.Push(a);
                break;
            case 'b':
                if (orFlag)
                {
                    top = st.Pop();
                    st.Push(top && b);
                    orFlag = false;
                }
                else if (andFlag)
                {
                    top = st.Pop();
                    st.Push(top && b);
                    andFlag = false;
                    continue;
                }
                st.Push(b);
                break;
        }
    }
    return st.Pop();
}