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union MyType{
  int   i_Param;
  float   f_Param;
  DWORD  d_Param;
  //wchar_t wc_Param[50];
};

struct S_Header{
  unsigned short header;
  unsigned short type;
  MyType LParam,MParam,RParam,
    returnValue1,
    returnValue2;
};
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********************************************
        public struct S_Header
        {
            [FieldOffset(0)]
            public ushort header;
            [FieldOffset(2)]
            public ushort type;
            [FieldOffset(8)]
            public int i_LParam;
            [FieldOffset(8)]
            public float f_LParam;
            [FieldOffset(12)]
            public int i_MParam;
            [FieldOffset(12)]
            public float f_MParam;
            [FieldOffset(16)]
            public int i_RParam;
            [FieldOffset(16)]
            public float f_RParam;
            [FieldOffset(20)]
            public int i_returnValue1;
            [FieldOffset(20)]
            public float f_returnValue1;
            [FieldOffset(24)]
            public int i_returnValue2;
            [FieldOffset(24)]
            public float f_returnValue2;
        }
********************************************					

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[StructLayout(LayoutKind.Explicit)]
public struct S_Header
{
//...