转换词典<；字符串，字符串>；.KeyCollection到string[].Net 3.5

我正在尝试转换此AddressParser(http://usaddress.codeplex.com/)从.Net 4.0到.Net 3.5，我遇到了一个小问题。

我在下面的评论行中收到一个cannot convert from 'System.Collections.Generic.Dictionary<string,string>.KeyCollection' to 'string[]'错误。它特别强调rangelessSecondaryUnits.Keys

任何帮助转换这将是美妙的！

private static Dictionary<string, string> rangelessSecondaryUnits =
            new Dictionary<string, string>()
            {
                { "BA?SE?ME?N?T", "BSMT" },
                { "FRO?NT", "FRNT" },
                { "LO?BBY", "LBBY" },
                { "LOWE?R", "LOWR" },
                { "OFF?I?CE?", "OFC" },
                { "PE?N?T?HO?U?S?E?", "PH" },
                { "REAR", "REAR" },
                { "SIDE", "SIDE" },
                { "UPPE?R", "UPPR" }
            };
var rangelessSecondaryUnitPattern =
                @"(?<SECONDARYUNIT>" +
                string.Join(
                    "|",
                    string.Join("|", rangelessSecondaryUnits.Keys)) + //ERROR ON THIS LINE
                @")'b";

var rangelessSecondaryUnitPattern =
        @"(?<SECONDARYUNIT>" +
        string.Join("|", rangelessSecondaryUnits.Keys.ToArray()) +
        @")'b";