problem with passing c-struct to c#

Question

Hello,

after searching several topics in this forum, I couldn't found a solution for following problem:

I have structs in C:

typedef struct ObjectHeaderBase_t
{
    DWORD     mSignature;
    WORD       mHeaderSize;
    WORD       mHeaderVersion;
    DWORD     mObjectSize; 
    DWORD     mObjectType; 
} ObjectHeaderBase;

typedef struct ObjectHeader_t
{
    ObjectHeaderBase  mBase; 
    DWORD               mObjectFlags;
    DWORD               mReserved;
    ULONGLONG         mObjectTime; 
} ObjectHeader;

typedef struct Message_t
{
    ObjectHeader mHeader;
    WORD            mChannel; 
    BYTE            mFlags;
    BYTE            mDLC; 
    DWORD           mID;
    BYTE            mData[8];
} Message;<br/><br/><br/>

in C# I declared the structs as below:

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
    public struct ObjectHeaderBase
    {
      public uint      mSignature; 
      public ushort    mHeaderSize;
        public ushort    mHeaderVersion;
        public uint      mObjectSize; 
        public uint      mObjectType;         
    }

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
    public struct ObjectHeader
    {
      public ObjectHeaderBase mBase;
      public uint                mObjectFlags; 
      public uint                mReserved;
      public ulong               mObjectTimeStamp;
    }

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
    public struct Message
    {
      public ObjectHeader mHeader;
      public ushort          mChannel;
      public byte            mFlags; 
      public byte            mDLC;
      public uint            mID;
      [MarshalAs( UnmanagedType.LPArray, SizeConst=8) ]
      public byte[]          mData;
    }

with the following functions, I try to get the struct:

/// declaration in C
MYDLLAPI( BOOL) ReadObject( HANDLE hFile, ObjectHeaderBase* pBase);

///C#
[

DllImport( "myDll.dll", CharSet=CharSet.Unicode )]
extern static bool ReadObject(IntPtr hFile, [MarshalAs(UnmanagedType.Struct)] ref ObjectHeaderBase pBase);

...
Message msg;

msg.mHeader.mBase = HeaderBaseParam;
msg.mData = new byte[8];
bSuccess = ReadObject(mFileHandle, ref msg.mHeader.mBase);
...

when calling the function in C# the struct won't be passed. Normally the data in msg and msg.mHeader should be set by passing the mBase-data.
Because I thought the Dll is not working, I programmed code in c, which does the same as the c#-programme and it works fine.

Does anybody know where the misstake is? I tried to play with several Marshalling options but no success.

Thanks
jojo

Answer 1

Thanks to Captain Kernel ;-))

The fixed-command "fixed" it.
The problem was the byte-field of the message-struct.
When using:

public unsafe struct Message
{
      public ObjectHeader mHeader;
      public ushort          mChannel;
      public byte            mFlags;
      public byte            mDLC;
      public uint            mID;
      public fixed byte   mData[8];
}

it works fine.
Maybe there is another solution which is more elegant in C# without using unsafe. When somebody knows a way, please response.

thanks jojo