Для 1251 примерно такой изврат выходит, дальше думать лениво :)

public static string GetString(byte[] bytes, int offset, int length) { if (bytes == null) throw new ArgumentNullException("bytes"); if (offset < 0 || offset >= bytes.Length) throw new ArgumentException("offset"); if (length < 0 || offset + length > bytes.Length) throw new ArgumentException("length"); var sb = new char[length]; length += offset; for (var i = offset; i < length; i++) { byte b = bytes[i]; if (b < 128) { sb[i] = (char)b; } else if (b >= 192) { sb[i] = (char)(848 + b); } else { switch (b) { case 128: sb[i] = '\u0402'; break; case 129: sb[i] = '\u0403'; break; case 130: sb[i] = '\u201A'; break; case 131: sb[i] = '\u0453'; break; case 132: sb[i] = '\u201E'; break; case 133: sb[i] = '\u2026'; break; case 134: sb[i] = '\u2020'; break; case 135: sb[i] = '\u2021'; break; case 136: sb[i] = '\u20AC'; break; case 137: sb[i] = '\u2030'; break; case 138: sb[i] = '\u0409'; break; case 139: sb[i] = '\u2039'; break; case 140: sb[i] = '\u040A'; break; case 141: sb[i] = '\u040C'; break; case 142: sb[i] = '\u040B'; break; case 143: sb[i] = '\u040F'; break; case 144: sb[i] = '\u0452'; break; case 145: sb[i] = '\u2018'; break; case 146: sb[i] = '\u2019'; break; case 147: sb[i] = '\u201C'; break; case 148: sb[i] = '\u201D'; break; case 149: sb[i] = '\u2022'; break; case 150: sb[i] = '\u2013'; break; case 151: sb[i] = '\u2014'; break; case 152: sb[i] = '\u0098'; break; case 153: sb[i] = '\u2122'; break; case 154: sb[i] = '\u0459'; break; case 155: sb[i] = '\u203A'; break; case 156: sb[i] = '\u045A'; break; case 157: sb[i] = '\u045C'; break; case 158: sb[i] = '\u045B'; break; case 159: sb[i] = '\u045F'; break; case 160: sb[i] = '\u00A0'; break; case 161: sb[i] = '\u040E'; break; case 162: sb[i] = '\u045E'; break; case 163: sb[i] = '\u0408'; break; case 164: sb[i] = '\u00A4'; break; case 165: sb[i] = '\u0490'; break; case 166: sb[i] = '\u00A6'; break; case 167: sb[i] = '\u00A7'; break; case 168: sb[i] = '\u0401'; break; case 169: sb[i] = '\u00A9'; break; case 170: sb[i] = '\u0404'; break; case 171: sb[i] = '\u00AB'; break; case 172: sb[i] = '\u00AC'; break; case 173: sb[i] = '\u00AD'; break; case 174: sb[i] = '\u00AE'; break; case 175: sb[i] = '\u0407'; break; case 176: sb[i] = '\u00B0'; break; case 177: sb[i] = '\u00B1'; break; case 178: sb[i] = '\u0406'; break; case 179: sb[i] = '\u0456'; break; case 180: sb[i] = '\u0491'; break; case 181: sb[i] = '\u00B5'; break; case 182: sb[i] = '\u00B6'; break; case 183: sb[i] = '\u00B7'; break; case 184: sb[i] = '\u0451'; break; case 185: sb[i] = '\u2116'; break; case 186: sb[i] = '\u0454'; break; case 187: sb[i] = '\u00BB'; break; case 188: sb[i] = '\u0458'; break; case 189: sb[i] = '\u0405'; break; case 190: sb[i] = '\u0455'; break; case 191: sb[i] = '\u0457'; break; } } } return new string(sb); } * This source code was highlighted with Source Code Highlighter.