union marshalling issue in C#

Question

I have union inside structure and the structure looks like

struct tDeviceProperty {

    DWORD Tag;
    DWORD Size;     
    union _DP value;    
};

typedef union _DP
{
      short int           i;       
      LONG                l;      
      ULONG               ul;     
      float               flt;    
      double              dbl;    
      BOOL      b;        
      double              at;     
      FILETIME            ft;     
      LPSTR               lpszA;  
      LPWSTR              lpszW;  
      LARGE_INTEGER       li;     
      struct tBinary    bin;    
      BYTE                reserved[40]; 
} __UDP;


struct tBinary {
    ULONG size;     
    BYTE * bin;     
};

from the tBinary structure bin has to be converted to tImage (structure is given below)

struct tImage {
    DWORD x;
    DWORD y;
    DWORD z;
    DWORD Resolution;
    DWORD type;
    DWORD ID;
    diccid_t SourceID;
    const void *buffer;
    const char *Info;
    const char *UserImageID;
}; 

to use the same in c# I have done marshaling but not giving proper values when converting the pointer to structure. The C# code is follows,

tBinary tBin = new tBinary();
IntPtr tBinbuffer = Marshal.AllocCoTaskMem(Marshal.SizeOf(tBin));
Marshal.StructureToPtr(tBin.bin, tBinbuffer, false);


tDeviceProperty tDevice = new tDeviceProperty();
tDevice.bin = tBinbuffer;
IntPtr tDevicebuffer = Marshal.AllocCoTaskMem(Marshal.SizeOf(tDevice));
Marshal.StructureToPtr(tDevice.bin, tDevicebuffer, false);

Battary tbatt = new Battary();
tbatt.value = tDevicebuffer;
IntPtr tbattbuffer = Marshal.AllocCoTaskMem(Marshal.SizeOf(tbatt));
Marshal.StructureToPtr(tbatt.value, tbattbuffer, false);

result = GetDeviceProperty(ref tbattbuffer);

Battary v = (Battary)Marshal.PtrToStructure(tbattbuffer, typeof(Battary));

tDeviceProperty v2 = (tDeviceProperty)Marshal.PtrToStructure(tDevicebuffer, typeof(tDeviceProperty));

tBinary v3 = (tBinary)Marshal.PtrToStructure(tBinbuffer, typeof(tBinary));






[StructLayout(LayoutKind.Explicit)]
public struct tDeviceProperty
{
    [FieldOffset(0)]
    [MarshalAs(UnmanagedType.U2)]
    public ushort i;
    [FieldOffset(2)]
    [MarshalAs(UnmanagedType.I4)]
    public int l;
    [FieldOffset(6)]
    [MarshalAs(UnmanagedType.U4)]
    public uint ul;
    [FieldOffset(10)]
    [MarshalAs(UnmanagedType.R4)]
    public float flt;
    [FieldOffset(14)]
    [MarshalAs(UnmanagedType.R8)]
    public double dbl;
    [FieldOffset(22)]
    [MarshalAs(UnmanagedType.I4)]
    public int b;
    [FieldOffset(26)]
    [MarshalAs(UnmanagedType.R8)]
    public double at;
    [FieldOffset(34)]
    //[MarshalAs(UnmanagedType.Struct)]
    public IntPtr ft;
    [FieldOffset(42)]
    //[MarshalAs(UnmanagedType.Struct)]
    public IntPtr lpszA;
    [FieldOffset(43)]
    //[MarshalAs(UnmanagedType.Struct)]
    public IntPtr lpszW;
    [FieldOffset(45)]
    [MarshalAs(UnmanagedType.U8)]
    public ulong li;
    [FieldOffset(53)]
    [MarshalAs(UnmanagedType.Struct)]
    public IntPtr bin;
    [FieldOffset(61)]
    //[MarshalAs(UnmanagedType.Struct)]
    public IntPtr reserved;
}


[StructLayout(LayoutKind.Sequential)]
public struct tBinary
{
    public int size;
    public IntPtr bin;
}

[StructLayout(LayoutKind.Sequential)]
public struct Battary
{
    public uint Tag;
    public uint Size;
    public IntPtr value;
}

[StructLayout(LayoutKind.Sequential)]
public struct tDiccBatteryStatus
{
    public uint RefreshWear;
    public uint TotalWear;
    public ushort Voltage;
    public ushort Battery;
    public int BatteryOK;
    public int NeedRefresh;
    public int NeedChange;
    public ushort Temperature;
    public int Charge;
    public byte State;
    public byte ExternalPowered;
    public int CapacityLeft;
}