System.Text.Encoding.GetEncodingRare C# (CSharp) Method

        private static Encoding GetEncodingRare(int codepage)
        {
            BCLDebug.Assert(codepage != 0 && codepage != 1200 && codepage != 1201 && codepage != 65001,
                "[Encoding.GetEncodingRare]This code page (" + codepage + ") isn't supported by GetEncodingRare!");
            Encoding result;
            switch (codepage)
            {
                case CodePageUTF7:              // 65000
                    result = UTF7;
                    break;
                case CodePageUTF32:             // 12000
                    result = UTF32;
                    break;
                case CodePageUTF32BE:           // 12001
                    result = new UTF32Encoding(true, true);
                    break;
                case ISCIIAssemese:
                case ISCIIBengali:
                case ISCIIDevanagari:
                case ISCIIGujarathi:
                case ISCIIKannada:
                case ISCIIMalayalam:
                case ISCIIOriya:
                case ISCIIPanjabi:
                case ISCIITamil:
                case ISCIITelugu:
                    result = new ISCIIEncoding(codepage);
                    break;
                // GB2312-80 uses same code page for 20936 and mac 10008
                case CodePageMacGB2312:
          //     case CodePageGB2312:
          //        result = new DBCSCodePageEncoding(codepage, EUCCN);
                    result = new DBCSCodePageEncoding(CodePageMacGB2312, CodePageGB2312);
                    break;

                // Mac Korean 10003 and 20949 are the same
                case CodePageMacKorean:
                    result = new DBCSCodePageEncoding(CodePageMacKorean, CodePageDLLKorean);
                    break;
                // GB18030 Code Pages
                case GB18030:
                    result = new GB18030Encoding();
                    break;
                // ISO2022 Code Pages
                case ISOKorean:
            //    case ISOSimplifiedCN
                case ChineseHZ:
                case ISO2022JP:         // JIS JP, full-width Katakana mode (no half-width Katakana)
                case ISO2022JPESC:      // JIS JP, esc sequence to do Katakana.
                case ISO2022JPSISO:     // JIS JP with Shift In/ Shift Out Katakana support
                    result = new ISO2022Encoding(codepage);
                    break;
                case DuplicateEUCCN:
                case ISOSimplifiedCN:
                    result = new DBCSCodePageEncoding(codepage, EUCCN);    // Just maps to 936
                    break;
                case EUCJP:
                    result = new EUCJPEncoding();
                    break;
                case EUCKR:
                    result = new DBCSCodePageEncoding(codepage, CodePageDLLKorean);    // Maps to 20949
                    break;
                case ENC50229:
                    throw new NotSupportedException(Environment.GetResourceString("NotSupported_CodePage50229"));
                case ISO_8859_8I:
                    result = new SBCSCodePageEncoding(codepage, ISO_8859_8_Visual);        // Hebrew maps to a different code page
                    break;
                default:
                    // Not found, already tried codepage table code pages in GetEncoding()
                    throw new NotSupportedException(
                        Environment.GetResourceString("NotSupported_NoCodepageData", codepage));
            }
            return result;
        }