Converting structure within structure to byte array for socket

Question

Hi,

I am trying to communicate with an external device and i am trying to send a byte array to the external device via sockets but i am always getting a response the message size is too small so i am not sure what i have done wrong. Between the data type there should be no alignment present and all numbers are represented in little endian format. The char array is not null terminated as mentioned in the protocol specifications.

I have to send data based on a struct that embeds 2 other struct. So here's my vb.net code for the struct used to convert to byte array and the sending part. Thanks in advance.

Public Structure MESSAGETYPE_OIP_Login
        Dim Header() As COMMANDHEADER
        Dim UserName() As PSTRING
        Dim Password() As PSTRING
    End Structure
   
    Public Structure COMMANDHEADER
        Dim MessageType As UInt32
        Dim Length As UInt32
        Dim Reserved1 As UInt32
        Dim Reserved2 As UInt32
    End Structure

    Public Structure PSTRING
        Dim Length As UInt32
        Dim CharArray() As Char
    End Structure


Dim client as System.Net.Sockets.TcpClient = New  System.Net.Sockets.TcpClient("192.168.20.93", 9401)

Dim networkStream As Net.Sockets.NetworkStream = client.GetStream()

        Dim Login As MESSAGETYPE_OIP_Login

        Login.Header.MessageType = 4485122
        Login.Header.Reserved1 = 0
        Login.Header.Reserved2 = 0
        Login.Header.Length = Marshal.SizeOf(Login.Header)

        Login.UserName.CharArray = "admin".ToCharArray
        Login.UserName.Length = Login.UserName.CharArray.Length

        Login.Password.CharArray = "admin".ToCharArray
        Login.Password.Length = Login.Password.CharArray.Length

        Dim StrtoBy() As Byte


        Dim ptr As IntPtr = Marshal.AllocCoTaskMem(Marshal.SizeOf(Login))
        
        ReDim StrtoBy(Marshal.SizeOf(Login) - 1)
        Marshal.Copy(ptr, StrtoBy, 0, Marshal.SizeOf(Login))
        Marshal.FreeCoTaskMem(ptr)

        networkStream.Write(StrtoBy, 0, StrtoBy.Length)

Answer 1

Is that really your code? It shouldn't even compile You're declaring for example Header as an array

Dim Header() As COMMANDHEADER

but accessing it like a single field

Login.Header.MessageType = 4485122

Marshal.SizeOf will not return the length you expect when used on structs with variable length arrays like PSTRING.CharArray. Therefore I don't think the Marshal class is the best solution here.

Have you considered writing member by member to the stream by wrapping a BinaryWriter around it instead?

---

Mattias, C# MVP

Answer 2

hi,

Sorry about that array i forgot to take it off after changing it to normal.

Yes, the thing worked when i write member by member to the stream.

When i use marshal.copy i realised that all the bytes are not in the order even after i specified structlayout = explicit and <fieldoffset> and marshal.structureToPtr works only when the structure members is only the basic .net datatype (not sure about this as i am pretty new to this) or i will get unable to access protected memory area.

I have 30 over different structures so writing stream by stream is very tedious that's why i am trying to look for ways where i can simplified this.