When does Uri.CheckHostName() return UriHostNameType.Basic instead of UriHostNameType.Dns or UriHostNameType.Unknown?

Question

The documentation at http://msdn.microsoft.com/en-us/library/system.urihostnametype.aspx and http://msdn.microsoft.com/en-us/library/system.uri.checkhostname.aspx is a bit unclear.

Uri.CheckHostName(string) returns UriHostNameType.Dns for a valid DNS hostname, and it returns UriHostNameType.Unknown when the string contains invalid characters and so on. Under what conditions does this method ever return UriHostNameType.Basic?

Answer 1

Now the .NET Core is open source we can know for sure.

Just like Mono, it never returns UriHostNameType.Basic.

Link to source code

Answer 2

It occurred to me that I could just check the Mono source code to answer my question. Here is the CheckHostName method from https://github.com/mono/mono/blob/master/mcs/class/System/System/Uri.cs:

    public static UriHostNameType CheckHostName (string name) 
    {
        if (name == null || name.Length == 0)
            return UriHostNameType.Unknown;

        if (IsIPv4Address (name)) 
            return UriHostNameType.IPv4;

        if (IsDomainAddress (name))
            return UriHostNameType.Dns;             

        IPv6Address addr;
        if (IPv6Address.TryParse (name, out addr))
            return UriHostNameType.IPv6;

        return UriHostNameType.Unknown;
    }

It appears that UriHostNameType.Basic isn't used at all. Maybe the Microsoft implementation can return this value?

Answer 3

UriHostNameType.Basic is used by the UriBuilder and Uri, when there is a DNS host name where at least 1 of the domain labels begins with a "-" (dash) or an "_" (underscore). For example, "_caldavs._tcp.google.com" will return UriHostNameType.Basic.

However, Uri.CheckHostName(string) appears to never return this value. For the same DNS host name above it returns UriHostNameType.Unknown.