C# helper function to validate whether or not the passed parameter string is a valid DNS name.

/// <summary>
/// Determines whether the DNS name is valid
/// </summary>
/// <remarks>
/// See RFC 2181, Section 11 https://tools.ietf.org/html/rfc2181
/// </remarks>
/// <param name="dnsName">The DNS name to validate.</param>
/// <returns>True if valid, false if not.</returns>
public static bool IsValidDnsName(string dnsName)
{
    if (string.IsNullOrEmpty(dnsName))
    {
        return false;
    }

    int len = dnsName.Length;
    if (len > 255)
    {
        return false;
    }

    int partLength = 0;
    for (int i = 0; i < len; i++)
    {
        char c = dnsName[i];
        if (c == '.')
        {
            if (i == 0 && len > 1)
            {
                return false; // Can't begin with a dot unless it's "."
            }
            if (i > 0 && partLength == 0)
            {
                return false; // No ".." allowed
            }
            partLength = 0;
            continue;
        }
        partLength++;
        if (partLength > 63)
        {
            return false;
        }
    }

    return true;
}