To check string is palindrome or not in .NET (C#)

public bool IsPalindrome(string str, StringComparison comparisonType)
{
    bool valid = true;
    int halfway = str.Length / 2;
    int lastIndex = str.Length - 1;
    for (int i = 0; i < halfway; i++)
    {
        if (!str.Substring(i, 1).Equals(str.Substring(lastIndex - i, 1), comparisonType))
        {
            valid = false;
            break;
        }
    }
    return valid;
}

public bool IsPalindrome(string str)
{
    return IsPalindrome(str, StringComparison.OrdinalIgnoreCase);
}