PP in C#

public bool CheckPP(string post)
{
    List<int> start_parens;
    List<int> end_parens;
    int i;

    foreach (Char c in post)
    {
        if (c == '(')
        {
            // Start parentheses.
            start_parens.Add(i);
        }
        elseif (c == ')')
        {
            // End parentheses.
            end_parens.Add(i);
        }
        i++
    }

    i = 0;
    int length; // The length of text in parens.
    foreach (int l in start_parens)
    {
        length += (end_parens[i] - start_parens[i]);
        i++
    }
    if length >= (post.Length / 2) { return true; }
    return false;
}