WIN32 APIを利用したSystem.Text.Encodingの実装

Win32Encoding.cs

// WIN32 APIを利用したSystem.Text.Encodingの実装
// パクり元  http://jonskeet.uk/csharp/ebcdic/

public class Win32Encoding : System.Text.Encoding
{
    uint codePage;

    [System.Runtime.InteropServices.DllImport("kernel32.dll", SetLastError = true)]
    static extern int WideCharToMultiByte(uint CodePage, uint dwFlags, System.IntPtr lpWideCharStr, int cchWideChar, System.IntPtr lpMultiByteStr, int cbMultiByte, System.IntPtr lpDefaultChar, System.IntPtr lpUsedDefaultChar);

    [System.Runtime.InteropServices.DllImport("kernel32.dll", SetLastError = true)]
    static extern int MultiByteToWideChar(uint CodePage, uint dwFlags, System.IntPtr lpMultiByteStr, int cbMultiByte, System.IntPtr lpWideCharStr, int cchWideChar);

    public static Win32Encoding GetEncoding(uint CodePage)
    {
        return new Win32Encoding(CodePage);
    }

    Win32Encoding(uint CodePage)
    {
        this.codePage = CodePage;
    }

    public override int GetBytes(char[] chars, int charIndex, int charCount, byte[] bytes, int byteIndex)
    {
        int result;
        unsafe
        {
            fixed (char* pChars = chars)
            {
                fixed (byte* pBytes = bytes)
                {
                    int maxByteCount = GetMaxByteCount(charCount);
                    var lpChars = (System.IntPtr)(pChars + charIndex);
                    var lpBytes = (System.IntPtr)(pBytes + byteIndex);
                    result = WideCharToMultiByte(codePage, 0, lpChars, charCount, lpBytes, maxByteCount, System.IntPtr.Zero, System.IntPtr.Zero);
                }
            }
        }
        return result;
    }

    public override int GetByteCount(char[] chars, int index, int count)
    {
        int result;
        unsafe
        {
            fixed (char* pChars = chars)
            {
                var lpChars = (System.IntPtr)(pChars + index);
                result = WideCharToMultiByte(codePage, 0, lpChars, count, System.IntPtr.Zero, 0, System.IntPtr.Zero, System.IntPtr.Zero);
            }
        }
        return result;
    }

    public override int GetMaxByteCount(int charCount)
    {
        return charCount * 2; // !!!
    }

    public override int GetChars(byte[] bytes, int byteIndex, int byteCount, char[] chars, int charIndex)
    {
        int result;
        unsafe
        {
            fixed (byte* pBytes = bytes)
            {
                fixed (char* pChars = chars)
                {
                    int maxCharCount = GetMaxCharCount(byteCount);
                    var lpChars = (System.IntPtr)(pChars + charIndex);
                    var lpBytes = (System.IntPtr)(pBytes + byteIndex);
                    result = MultiByteToWideChar(codePage, 0, lpBytes, byteCount, lpChars, maxCharCount);
                }
            }
        }
        return result;
    }

    public override int GetCharCount(byte[] bytes, int index, int count)
    {
        int result;
        unsafe
        {
            fixed (byte* pBytes = bytes)
            {
                var lpBytes = (System.IntPtr)(pBytes + index);
                result = MultiByteToWideChar(codePage, 0, lpBytes, count, System.IntPtr.Zero, 0);
            }
        }
        return result;
    }

    public override int GetMaxCharCount(int byteCount)
    {
        return byteCount;
    }
}