Attempted to read or write protected memory error

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Hi! I'm trying to interface to an external dll but keep getting the error: "Attempted to read or write protected memory". googled a lot and tried a lot, but no progress. Does any one have suggestions please?
Joost.
here's the code:

C#

namespace iButtonReader
{
  class ButtonReader
  {
    [DllImport("IBFS32.DLL")]
    public static extern long TMExtendedStartSession(int portnum, int porttype, object reserved);

    [DllImport("IBFS32.DLL")]
    public static extern int TMEndSession(long sessionhandle);

    [DllImport("IBFS32.DLL")]
    public static extern int TMSetup(long sessionhandle);

    // documentation shows:
    // short far pascal TMFirst(long session_handle, void far *state_buffer);
    [DllImport("IBFS32.DLL", CallingConvention = CallingConvention.Winapi)]
    public static extern int TMFirst(long sessionhandle, ref byte[] statebuffer); 

    //[DllImport("IBFS32.DLL")]
    //public static extern int TMNext(long sessionhandle, ref byte[] statebuffer);

    //[DllImport("IBFS32.DLL")]
    //public static extern int TMRom(long sessionhandle, ref byte[] statebuffer, ref int[] ROM);

    private long _Handle = 0;
    private byte[] _Statebuffer = new byte[15360];

    public string Read()
    {
      try
      {
        string rom = string.Empty;
        int rslt = 0;
        _Handle = TMExtendedStartSession(1, 5, null);
        if (_Handle != 0)
        {
          rslt = TMSetup(_Handle);
        }
        rslt = TMFirst(_Handle, ref _Statebuffer); // <---------- Attempted to read or write protected memory.
        if (rslt == 1)
        {
          // tbd
        }
        rslt = TMEndSession(_Handle);
        return rom;
      }
      catch (Exception ex)
      {
        Debug.Print(ex.ToString());

        throw;
      }
    }
  }  
}

Posted 29-Aug-11 4:00am

joost.versteegen

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joost.versteegen · Answer 1 · 2011-08-29T20:05:00

Solution 2

As can be seen in "private byte[] _Statebuffer = new byte[15360]" the buffer size is 15360 bytes. The declaration can be seen in the comment of the function: "short far pascal TMFirst(long session_handle, void far *state_buffer)". thanks.

Posted 29-Aug-11 20:05pm

joost.versteegen

OriginalGriff · Answer 2 · 2011-08-29T06:19:00

Solution 1

How big should _Statebuffer be? If TMFirst is exceeding the buffer size, that would explain it.
Alternatively, what is the native declaration of TMFirst? Is the ref byte[] the correct parameter type?

Posted 29-Aug-11 6:19am

OriginalGriff

Maxim Kartavenkov · Answer 3 · 2012-10-23T20:22:00

Hello,

You make wrong way wrapper for your TMFirst function. You should not use byte[] as argument without specifing size of the buffer. As your functiona declaration does not have size argument you should use IntPtr as input type. It should looks:

C#

[DllImport("IBFS32.DLL", CallingConvention = CallingConvention.Winapi)]
public static extern int TMFirst(long sessionhandle, IntPtr statebuffer);

long _Handle;
byte[] _Statebuffer;
// Allocate memory
IntPtr _ptr = Marshal.AllocCoTaskMem(_Statebuffer.Length);
// Copy Data to that buffer
Marshal.Copy(_ptr,_Statebuffer,0,_Statebuffer.Length);
// Call your method
TMFirst(_Handle,_ptr);
// Copy data back to your array
Marshal.Copy(_Statebuffer,0,_ptr,_Statebuffer.Length);
// Free Memory
Marshal.FreeCoTaskMem(_ptr);

Regards,
Maxim.