public class Demo{internal const char HIGH_SURROGATE_START = '\ud800';internal const char HIGH_SURROGATE_END = '\udbff';internal const char LOW_SURROGATE_START = '\udc00';internal const char LOW_SURROGATE_END = '\udfff';internal const int UNICODE_PLANE00_END = 0x00ffff;internal const int UNICODE_PLANE01_START = 0x10000;internal const int UNICODE_PLANE16_END = 0x10ffff;public static bool IsHighSurrogate(char c){return ((c >= HIGH_SURROGATE_START) && (c <= HIGH_SURROGATE_END));}public static bool IsLowSurrogate(char c){return ((c >= LOW_SURROGATE_START) && (c <= LOW_SURROGATE_END));}public static char[] ConvertFromUtf32(int utf32){// U+00D800 ~ U+00DFFF 这个范围被 Unicode 定义为专用代理区，它们不能作为 Unicode 编码值。if ((utf32 < 0 || utf32 > UNICODE_PLANE16_END) || (utf32 >= HIGH_SURROGATE_START && utf32 <= LOW_SURROGATE_END)){throw new ArgumentOutOfRangeException("utf32");}if (utf32 < UNICODE_PLANE01_START){// 这是一个基本字符。return new char[] { (char)utf32 };}// 这是一个扩展字符，需要将其转换为 UTF-16 中的代理项对。utf32 -= UNICODE_PLANE01_START;return new char[]{(char)((utf32 / 0x400) + HIGH_SURROGATE_START),(char)((utf32 % 0x400) + LOW_SURROGATE_START)};}public static int ConvertToUtf32(char highSurrogate, char lowSurrogate){if (!IsHighSurrogate(highSurrogate)){throw new ArgumentOutOfRangeException("highSurrogate");}if (!IsLowSurrogate(lowSurrogate)){throw new ArgumentOutOfRangeException("lowSurrogate");}return (((highSurrogate - HIGH_SURROGATE_START) * 0x400) + (lowSurrogate - LOW_SURROGATE_START) + UNICODE_PLANE01_START);}}

前两年有个段子，说只有 2000 元以上的手机才能在输入法打出某个汉字。原因正是这个。

这里附上由我的开源项目：.Net 平台的高性能 Json 解析库：Swifter.Json：https://github.com/Dogwei/Swifter.Json。希望大家支持一下。

最后附上 Swifter.Json 内部使用的 Utf16 与 Utf8 互转的源代码：

    public static unsafe class EncodingHelper{public const char ASCIIMaxChar = (char)0x7f;public const int Utf8MaxBytesCount = 4;public static int GetUtf8Bytes(char* chars, int length, byte* bytes){var destination = bytes;for (int i = 0; i < length; i++){int c = chars[i];if (c <= 0x7f){*destination = (byte)c; ++destination;}else if (c <= 0x7ff){*destination = (byte)(0xc0 | (c >> 6)); ++destination;*destination = (byte)(0x80 | (c & 0x3f)); ++destination;}else if (c >= 0xd800 && c <= 0xdbff){c = ((c & 0x3ff) << 10) + 0x10000;++i;if (i < length){c |= chars[i] & 0x3ff;}*destination = (byte)(0xf0 | (c >> 18)); ++destination;*destination = (byte)(0x80 | ((c >> 12) & 0x3f)); ++destination;*destination = (byte)(0x80 | ((c >> 6) & 0x3f)); ++destination;*destination = (byte)(0x80 | (c & 0x3f)); ++destination;}else{*destination = (byte)(0xe0 | (c >> 12)); ++destination;*destination = (byte)(0x80 | ((c >> 6) & 0x3f)); ++destination;*destination = (byte)(0x80 | (c & 0x3f)); ++destination;}}return (int)(destination - bytes);}[MethodImpl(VersionDifferences.AggressiveInlining)]public static int GetUtf8Chars(byte* bytes, int length, char* chars){var destination = chars;var current = bytes;var end = bytes + length;for (; current < end; ++current){if ((*((byte*)destination) = *current) > 0x7f){return GetGetUtf8Chars(current, end, destination, chars);}++destination;}return (int)(destination - chars);}[MethodImpl(MethodImplOptions.NoInlining)]private static int GetGetUtf8Chars(byte* current, byte* end, char* destination, char* chars){if (current + Utf8MaxBytesCount < end){end -= Utf8MaxBytesCount;// Unchecked index.for (; current < end; ++current){var byt = *current;if (byt <= 0x7f){*destination = (char)byt;}else if (byt <= 0xdf){*destination = (char)(((byt & 0x1f) << 6) | (current[1] & 0x3f));++current;}else if (byt <= 0xef){*destination = (char)(((byt & 0xf) << 12) | ((current[1] & 0x3f) << 6) + (current[2] & 0x3f));current += 2;}else{var utf32 = (((byt & 0x7) << 18) | ((current[1] & 0x3f) << 12) | ((current[2] & 0x3f) << 6) + (current[3] & 0x3f)) - 0x10000;*destination = (char)(0xd800 | (utf32 >> 10)); ++destination;*destination = (char)(0xdc00 | (utf32 & 0x3ff));current += 3;}++destination;}end += Utf8MaxBytesCount;}// Checked index.for (; current < end; ++current){var byt = *current;if (byt <= 0x7f){*destination = (char)byt;}else if (byt <= 0xdf && current + 1 < end){*destination = (char)(((byt & 0x1f) << 6) | (current[1] & 0x3f));++current;}else if (byt <= 0xef && current + 2 < end){*destination = (char)(((byt & 0xf) << 12) | ((current[1] & 0x3f) << 6) + (current[2] & 0x3f));current += 2;}else if (current + 3 < end){var utf32 = (((byt & 0x7) << 18) | ((current[1] & 0x3f) << 12) | ((current[2] & 0x3f) << 6) + (current[3] & 0x3f)) - 0x10000;*destination = (char)(0xd800 | (utf32 >> 10)); ++destination;*destination = (char)(0xdc00 | (utf32 & 0x3ff));current += 3;}++destination;}return (int)(destination - chars);}public static int GetUtf8CharsLength(byte* bytes, int length){int count = 0;for (int i = 0; i < length; i += bytes[i] <= 0x7f ? 1 : bytes[i] <= 0xdf ? 2 : 3){++count;}return count;}public static int GetUtf8MaxBytesLength(int charsLength){return charsLength * Utf8MaxBytesCount;}[MethodImpl(VersionDifferences.AggressiveInlining)]public static int GetUtf8MaxCharsLength(int bytesLength){return bytesLength;}}