System.Uri.InFact C# (CSharp) Method

        private bool InFact(Flags flags)
        {
            return (_flags & flags) != 0;
        }

        //
        // Resolves into either baseUri or relativeUri according to conditions OR if not possible it uses newUriString
        // to  return combined URI strings from both Uris
        // otherwise if e != null on output the operation has failed
        //
        internal static Uri ResolveHelper(Uri baseUri, Uri relativeUri, ref string newUriString, ref bool userEscaped,
                                          out UriFormatException e)
        {
            Debug.Assert(!baseUri.IsNotAbsoluteUri && !baseUri.UserDrivenParsing, "Uri::ResolveHelper()|baseUri is not Absolute or is controlled by User Parser.");

            e = null;
            string relativeStr = string.Empty;

            if ((object)relativeUri != null)
            {
                if (relativeUri.IsAbsoluteUri)
                {
                    return(relativeUri);
                }

                relativeStr = relativeUri.OriginalString;
                userEscaped = relativeUri.UserEscaped;
            }
            else
            {
                relativeStr = string.Empty;
            }

            // Here we can assert that passed "relativeUri" is indeed a relative one

            if (relativeStr.Length > 0 && (IsLWS(relativeStr[0]) || IsLWS(relativeStr[relativeStr.Length - 1])))
            {
                relativeStr = relativeStr.Trim(s_WSchars);
            }

            if (relativeStr.Length == 0)
            {
                newUriString = baseUri.GetParts(UriComponents.AbsoluteUri,
                                                baseUri.UserEscaped ? UriFormat.UriEscaped : UriFormat.SafeUnescaped);
                return(null);
            }

            // Check for a simple fragment in relative part
            if (relativeStr[0] == '#' && !baseUri.IsImplicitFile && baseUri.Syntax.InFact(UriSyntaxFlags.MayHaveFragment))
            {
                newUriString = baseUri.GetParts(UriComponents.AbsoluteUri & ~UriComponents.Fragment,
                                                UriFormat.UriEscaped) + relativeStr;
                return(null);
            }

            // Check for a simple query in relative part
            if (relativeStr[0] == '?' && !baseUri.IsImplicitFile && baseUri.Syntax.InFact(UriSyntaxFlags.MayHaveQuery))
            {
                newUriString = baseUri.GetParts(UriComponents.AbsoluteUri & ~UriComponents.Query & ~UriComponents.Fragment,
                                                UriFormat.UriEscaped) + relativeStr;
                return(null);
            }

            // Check on the DOS path in the relative Uri (a special case)
            if (relativeStr.Length >= 3 &&
                (relativeStr[1] == ':' || relativeStr[1] == '|') &&
                IsAsciiLetter(relativeStr[0]) &&
                (relativeStr[2] == '\\' || relativeStr[2] == '/'))
            {
                if (baseUri.IsImplicitFile)
                {
                    // It could have file:/// prepended to the result but we want to keep it as *Implicit* File Uri
                    newUriString = relativeStr;
                    return(null);
                }
                else if (baseUri.Syntax.InFact(UriSyntaxFlags.AllowDOSPath))
                {
                    // The scheme is not changed just the path gets replaced
                    string prefix;
                    if (baseUri.InFact(Flags.AuthorityFound))
                    {
                        prefix = baseUri.Syntax.InFact(UriSyntaxFlags.PathIsRooted) ? ":///" : "://";
                    }
                    else
                    {
                        prefix = baseUri.Syntax.InFact(UriSyntaxFlags.PathIsRooted) ? ":/" : ":";
                    }

                    newUriString = baseUri.Scheme + prefix + relativeStr;
                    return(null);
                }
                // If we are here then input like "http://host/path/" + "C:\x" will produce the result  http://host/path/c:/x
            }


            ParsingError err = GetCombinedString(baseUri, relativeStr, userEscaped, ref newUriString);

            if (err != ParsingError.None)
            {
                e = GetException(err);
                return(null);
            }

            if ((object)newUriString == (object)baseUri._string)
            {
                return(baseUri);
            }

            return(null);
        }