c# 对DEVMODE结构的声明

DEVMODE结构包含两个联合，要使用精确的字节偏移量，才能将每个结构成员正确排序。
这里使用Ansi和unicode两个版本，对应以A或W结尾的EnumDisplaySettings函数，如果不对应各自版本的函数将出现不正确的结构，在Ansi，一个字符为1，在unicode，则为2。DeviceName在这里版本的字节偏移量是不同的，Ansi版本占了32*1，即32个偏移量；unicode版本占了32*2，即64个偏移量。另，这里使用了Pack=2，按2对齐，因此DeviceName还要填充2个字节，所以占了34或者66。
关于C#中使用联合，只要在结构中的几个字段成员具有相同的偏移量，那么它们就共享一个内存地址，该地址的偏移量取在这个共享的地址中，字节长度最大的那个成员的长度，另外，必须使用：LayoutKind.Explicit，用户自己定义每个字段成员的偏移量。

例：
[StructLayout(LayoutKind.Explicit, Pack = 2, CharSet = CharSet.Ansi)]
public struct Mystruct{
[FieldOffset(0)]
        public char c;
        [FieldOffset(0)]
        public POINTL Union1_POINTL;//POINTL 为一个结构包含两个int类型
        [FieldOffset(0)]
        public int Union1_DisplayOrientation;
        [FieldOffset(0)]
        public int Union1_DisplayFixedOutput;
};
如果Marshal.SizeOf(typeof(Mystruct));等于POINTL的长度，为8

好了，闲话不多说，下面声明DEVMODE结构的两个版本：
由于DEVMODE结构中包含一个匿名联合，这个匿名联合又包含一个匿名结构，c#不能使用匿名结构，因此在这里将该匿名结构的成员需要声明一个结构，将这些成员包起来。这里声明为Anonymity：
/// <summary>
    /// 此结构是API的DEVMODE结构在联合里的一个匿名结构
    /// </summary>
    [StructLayout(LayoutKind.Explicit, Pack = 2)]
    public struct Anonymity {
        [FieldOffset(0)]
       public short dmOrientation;
        [FieldOffset(2)]
       public short dmPaperSize;
        [FieldOffset(4)]
       public short dmPaperLength;
        [FieldOffset(6)]
      public   short dmPaperWidth;
        [FieldOffset(8)]
     public    short dmScale;
        [FieldOffset(10)]
      public short dmCopies;
        [FieldOffset(12)]
      public short dmDefaultSource;
        [FieldOffset(14)]
       public short dmPrintQuality;
    };//size==16
//---------------------------------------------------------------
/// <summary>
    /// POINTL结构
    /// </summary>
[StructLayout(LayoutKind.Explicit, Pack =4)]
    public struct POINTL {
    [FieldOffset(0)]
    public int X;
    [FieldOffset(4)]
    public int Y;
};

#region Ansi
/// <summary>
    /// 此结构是API的DEVMODE结构的Ascii版本,其中有个匿名结构由Anonymity结构代替
    /// </summary>
    [StructLayout(LayoutKind.Explicit, Pack = 2, CharSet = CharSet.Ansi)]
    public struct DEVMODEA {
        [FieldOffset(0)]
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 32)]
        public string DeviceName;//32+2
        [FieldOffset(34)]
        public ushort SpecVersion;//34
        [FieldOffset(36)]
        public ushort DriverVersion;//36
        [FieldOffset(38)]
        public ushort structSize;//38
        [FieldOffset(40)]
        public ushort DriverExtra;//40;
        [FieldOffset(42)]
        public int Fields;//44
        [FieldOffset(46)]
        public Anonymity Union1_Anonymity;
        [FieldOffset(46)]
        public POINTL Union1_POINTL;
        [FieldOffset(46)]
        public int Union1_DisplayOrientation;
        [FieldOffset(46)]
        public int Union1_DisplayFixedOutput;
        [FieldOffset(62)]
        public short Color;
        [FieldOffset(64)]
        public short Duplex;
        [FieldOffset(66)]
        public short YResolution;
        [FieldOffset(68)]
        public short TTOption;
        [FieldOffset(70)]
        public short Collate;//70 ??+2 补位
        [FieldOffset(72)]
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 32)]
        public string FormName;//104
        [FieldOffset(104)]
        public ushort LogPixels;//106
        [FieldOffset(106)]
        public int BitsPerPel;//110
        [FieldOffset(110)]
        public int PelsWidth;//114
        [FieldOffset(114)]
        public int PelsHeight;//118
        [FieldOffset(118)]
        public int Union2_DisplayFlags;//122
        [FieldOffset(122)]
        public int Union2_Nup;//122
        [FieldOffset(122)]
        public int DisplayFrequency;
        [FieldOffset(126)]//win95以上
        public int ICMMethod;
        [FieldOffset(130)]
        public int ICMIntent;
        [FieldOffset(134)]
        public int MediaType;
        [FieldOffset(138)]
        public int DitherType;
        [FieldOffset(142)]
        public int Reserved1;
        [FieldOffset(146)]
        public int Reserved2;
        [FieldOffset(152)]
        public int PanningWidth;
        [FieldOffset(156)]
        public int PanningHeight;//size==160 补两个定长的char数组各2位*/
    };

#endregion Ansi

#region UNICODE

/// <summary>
    /// 此结构是API的DEVMODE结构的Unicode版本，匿名结构由Anonymity结构代替
    /// </summary>
    [StructLayout(LayoutKind.Explicit, Pack = 2, CharSet = CharSet.Unicode)]//160+64
    public struct DEVMODEW {
        [FieldOffset(0)]
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 32)]
        public string DeviceName;//66
        [FieldOffset(66)]
        public ushort SpecVersion;//68
        [FieldOffset(68)]
        public ushort DriverVersion;//70
        [FieldOffset(70)]
        public ushort structSize;
        [FieldOffset(72)]
        public ushort DriverExtra;//74
        [FieldOffset(74)]
        public int Fields;//78
        [FieldOffset(78)]
        public Anonymity Union1_Anonymity;
        [FieldOffset(78)]
        public POINTL Union1_POINTL;
        [FieldOffset(78)]
        public int Union1_DisplayOrientation;
        [FieldOffset(78)]
        public int Union1_DisplayFixedOutput;//94
        [FieldOffset(94)]
        public short Color;
        [FieldOffset(96)]
        public short Duplex;//98
        [FieldOffset(98)]
        public short YResolution;
        [FieldOffset(100)]
        public short TTOption;//102
        [FieldOffset(102)]
        public short Collate;//104
        [FieldOffset(104)]
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 32)]
        public string FormName; //168,补2个位,对齐结构
        [FieldOffset(170)]
        public ushort LogPixels;
        [FieldOffset(172)]
        public int BitsPerPel;//174
        [FieldOffset(176)]
        public int PelsWidth;
        [FieldOffset(180)]
        public int PelsHeight;// 下面联合
        [FieldOffset(184)]
        public int Union2_DisplayFlags;
        [FieldOffset(184)]
        public int Union2_Nup;
        [FieldOffset(188)]
        public int DisplayFrequency;
        [FieldOffset(192)]
        public int ICMMethod;// win95以上版本
        [FieldOffset(196)]
        public int ICMIntent;
        [FieldOffset(200)]
        public int MediaType;
        [FieldOffset(204)]
        public int DitherType;
        [FieldOffset(208)]
        public int Reserved1;
        [FieldOffset(212)]
        public int Reserved2;
        [FieldOffset(216)]
        public int PanningWidth;
        [FieldOffset(220)]
        public int PanningHeight;//224

        //*/size=224==64+160
    };

    #endregion UNICODE
//调用函数： ansi
[DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Ansi)]
    public static extern bool EnumDisplaySettingsA(string DeviceName, uint ModeNum, ref DEVMODEA lpDevMode);

//unicode
[DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
    public static extern bool EnumDisplaySettingsW(string DeviceName, uint ModeNum, ref DEVMODEW lpDevMode);