从大型文本文件中读取Unicode char

Question

如何读取Unicode char？喜欢"ä"

public static string Read(int length, string absolutePath)
{
    StringBuilder resultAsString = new StringBuilder();
    using (MemoryMappedFile memoryMappedFile = MemoryMappedFile.CreateFromFile(absolutePath))
    using (MemoryMappedViewStream memoryMappedViewStream = memoryMappedFile.CreateViewStream(0, length))
    {
        for (int i = 0; i < length; i++)
        {                  
            int result = memoryMappedViewStream.ReadByte();
            if (result == -1)
            {
                break;
            }
            char letter = (char)result;
            resultAsString.Append(letter);
        }
    }
    return resultAsString.ToString();
}

读取int（结果）为195，char铸件不是预期的结果。

Answer 1

不确定是否是您的要求，但是，您可以使用StreamReader

StreamReader sr = new StreamReader(stream, Encoding.Unicode);

Answer 2

如果您只想加载并将UTF-8文件读取到字符串变量中，代码可以像

一样简单

var text = File.ReadAllText(filePath, Encoding.UTF8);

如果您坚持处理UTF-8数据字节，则可以进行更复杂的解析。
这是一个粗略（但有效的）草图，可以使用您的原始代码：

StringBuilder resultAsString = new StringBuilder();
using (MemoryMappedFile memoryMappedFile = MemoryMappedFile.CreateFromFile(filePath))
using (MemoryMappedViewStream viewStream = memoryMappedFile.CreateViewStream(0, new FileInfo(filePath).Length))
{
    int b;
    while((b = viewStream.ReadByte()) != -1)
    {
        int acc = b;
        bool readUtfDataBytes(int bytesToRead)
        {
            while (bytesToRead-- > 0)
            {
                var nextB = viewStream.ReadByte();
                if (nextB == -1) return false; // EOS reached
                if ((nextB & 0xC0) != 0x80) return false; // invalid UTF-8 data byte
                acc <<= 6;
                acc |= nextB & 0x3F;
            }
            return true;
        }
        if (b >= 0xF0) // 1111 0000
        {
            acc &= 0x07;
            if (!readUtfDataBytes(3)) break; // break on malformed UTF-8
        }
        else if (b >= 0xE0) // 1110 0000
        {
            acc &= 0x0F;
            if (!readUtfDataBytes(2)) break; // break on malformed UTF-8
        }
        else if (b >= 0xC0) // 1100 0000
        {
            acc &= 0x1F;
            if (!readUtfDataBytes(1)) break; // break on malformed UTF-8
        }
        else if (b >= 0x80)
        {
            break; // break on malformed UTF-8
        }
        if (acc == 0xFEFF)
        {
            // ignore UTF-8 BOM
        }
        else
        {
            resultAsString.Append(Char.ConvertFromUtf32(acc));
        }
    }
}