I am using BinaryWriter to log data using a Structure in VB NET Compact Framework 3.5 and I do this by appending a block of the individual structure items as follows:
Code:
        Dim filestream As FileStream = New FileStream("mlog.bin",   FileMode.Append)
        Dim fwrite As BinaryWriter = New BinaryWriter(filestream)

        mdat.rtime = Now 'load the present time' 

        fwrite.Write(mdat.rtime)
...