System.AccessViolationException on Converting Bytes To Structure

EDN Admin · Feb 16, 2011

Hi,
<p style="line-height:normal; margin:0in 0in 0pt <span style=" <span style="font-size:small <span style="font-family:Calibri I have been working on changing old VB6 code over to VB2010. VB6 program receives UDP byte data that is send from real time computer
system then moves the data into a VB6 TYPE (structure) for storage into database and display system. Now Iâm trying to do the same thing in 2010 but I keep getting an System.AccessViolationException error when I try to run a sample code from MSDN an
function called ConvertBytesToStructure. Below is the code I put together. It may be that I donât have the structure created correctly. Please help.
<p style="line-height:normal; margin:0in 0in 0pt <span style=" <span style="font-family:Calibri; font-size:small
<p style="line-height:normal; margin:0in 0in 0pt <span style=" 
Option Strict Off 
Option Explicit On
Imports SocketTools 
Imports SocketTools.SocketWrench 
Imports System.Runtime.InteropServices 
Imports System.Text
Public Class Form1
Inherits System.Windows.Forms.Form
Private WithEvents Socket As New SocketTools.SocketWrench
This the orginal VB6 TYPE Structure 
---------------------------------------------------------------------- 
Private Type tcsDataType Real data coming from TM TCS System 
MsgID As Long DINT 
SeqCount As Long {DINT} 
QDIntelSysCntr As Long {DWORD} 
StripID As String * 10 in TCS as StripID(0 To 9) As Byte 
ExitStripID As String * 10 in TCS as StripID(0 To 9) As Byte 
StripWidth As Single 
StripSetThick As Single 
ActStripLength As Single 
ActSpeed As Single 
ActFlatAct(0 To 53) As Single 
End Type
Private fromData As tcsDataType 
Private sendData As tcsDataType 
Private fromBuffer(524) As Byte Was 272 
Private sendBuffer(264) As Byte Was 272 
----------------------------------------------------------------------
 
Private Structure tcsDataStructure 
Dim MsgID As Int32 
Dim SeqCount As Int32 
Dim QDIntelSysCntr As Int32 
Dim StripID() As Byte 
Dim ExitStripID() As Byte 
Dim StripWidth As Single 
Dim StripSetThick As Single 
Dim ActStripLength As Single 
Dim ActSpeed As Single 
Dim ActFlatAct() As Single 
End Structure
Dim ByteClear() As Byte = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0} 
Dim FlatClear() As Single = {0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 
0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 
0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 
0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 
0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 
0.0, 0.0, 0.0, 0.0} 
Dim Tst As New tcsDataStructure With {.MsgID = 0, 
.SeqCount
= 0, 
.QDIntelSysCntr
= 0, 
.StripID =
ByteClear, 
.ExitStripID
= ByteClear, 
.StripWidth
= 0.0, 
.StripSetThick
= 0.0, 
.ActStripLength
= 0.0, 
.ActSpeed
= 0.0, 
.ActFlatAct
= FlatClear}
Function to Create structure must be called first 
Private Function BulidStr(ByVal Buff() As Byte, ByVal MyType As System.Type) As Object 
Dim MyGC As GCHandle = GCHandle.Alloc(Buff, GCHandleType.Pinned) 
Marshals data from an unmanaged block of memory to a newly 
allocated managed object of the specified type. 
Dim Obj As Object = Marshal.PtrToStructure(MyGC.AddrOfPinnedObject, MyType) 
Return Obj 
Free GChandle to avoid memory leaks 
MyGC.Free() 
End Function
Private Function ConvertBytesToStructure(ByVal data() As Byte, ByVal struct As System.Type) As tcsDataStructure 
Create an unmanaged handle to the data array
 
Dim gc As GCHandle 
Try 
gc = GCHandle.Alloc(data, GCHandleType.Pinned) 
Marshal the data at the pointer address into the structure specified
 
Dim obj As tcsDataStructure = DirectCast(Marshal.PtrToStructure(gc.AddrOfPinnedObject, struct), tcsDataStructure) 
Return the object created by the PtrToStructure call
 
Return obj 
Finally 
gc.Free() 
End Try
End Function

Private Sub UpdateListBox(ByVal message As String) 
ListBox1.Items.Add(message) 
ListBox1.SelectedIndex = ListBox1.Items.Count - 1 
ListBox1.ClearSelected() 
End Sub
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click 
Dim strLocalAddress As String 
Dim nLocalPort As Integer
strLocalAddress = TextBox1.Text.Trim() 
If strLocalAddress.Length = 0 Then 
strLocalAddress = "0.0.0.0" 
End If
nLocalPort = Int32.Parse(TextBox2.Text) 
If nLocalPort < 1 Or nLocalPort > 65535 Then 
MsgBox("Please specify a valid local port number", MsgBoxStyle.Exclamation, "Error") 
Exit Sub 
End If
If the Handle property is -1, this means that the UDP socket has not been 
created. Initialize the socket properties and create a datagram socket 
using the Bind method. By setting Blocking to false, the OnRead event will 
be raised when a remote host sends us a datagram. 
If Socket.Handle = -1 Then 
Socket.Protocol = SocketProtocol.socketDatagram 
Socket.Blocking = False
Bind to the specified IP address and port number 
If Not Socket.Bind(strLocalAddress, nLocalPort) Then 
MsgBox(Socket.LastErrorString, MsgBoxStyle.Exclamation, "Error") 
Exit Sub 
End If 
Button1.Text = "Stop" 
UpdateListBox(String.Format("Started listening for datagrams on {0}:{1}", strLocalAddress, nLocalPort)) 
Else 
Close the datagram socket using the Disconnect method 
Socket.Disconnect() 
Button1.Text = "Start" 
UpdateListBox("Stopped listening for datagrams") 
End If
End Sub
Private Sub Socket_OnRead(ByVal sender As Object, ByVal e As System.EventArgs) Handles Socket.OnRead 
Dim strPeerAddress As String = String.Empty
Dim nPeerPort As Integer = 0 
Dim msgBuffer(8192) As Byte 
Dim nBytesRead As Integer 
Dim nBytesWritten As Integer 
Dim LengthOfStru As Integer
 
The OnRead event will fire whenever a remote host sends a datagram to 
us on the port number that was specified. In this example, we limit 
the size of the datagram to 8K, but this could be any value up to 64K
Initialize the contents of the byte array
 
Array.Clear(msgBuffer, 0, 8192)
Read the datagram that was sent to us; if the datagram is larger than 
the maxiumum size of our buffer, the remaining data is discarded 
nBytesRead = Socket.ReadFrom(msgBuffer, 8192, strPeerAddress, nPeerPort) 
Debug.Print(String.Format("nBytesRead = {0}", nBytesRead)) 
If nBytesRead = -1 Then 
UpdateListBox(String.Format("Error {0}: {1}", Socket.LastError, Socket.LastErrorString)) 
Exit Sub 
End If
If nBytesRead > 0 Then 
Echo the datagram back to the remote host that sent it to us 
nBytesWritten = Socket.WriteTo(msgBuffer, nBytesRead, strPeerAddress, nPeerPort) 
UpdateListBox(String.Format("Read {0} bytes of data", nBytesRead)) 
For Index As Int16 = 0 To nBytesRead 
Debug.Print(String.Format("msgBuffer = {0}-{1}", Index, Conversion.Hex(msgBuffer(Index)))) 
Next 
Start here 
convert to bytes 
With Tst 
Debug.Print(.ActSpeed, .ActStripLength, .StripSetThick, .ExitStripID, .ActFlatAct)
End With 
LengthOfStru = Len(Tst) 
LengthOfStru = Marshal.SizeOf(Tst) 
Call BulidStr method to create structure
 
and copy byte array data to structure 
Tst = ConvertBytesToStructure(msgBuffer, Tst.GetType) 
Now use it and print it 
Debug.WriteLine(Tst.ActSpeed.ToString) 
Debug.WriteLine(Tst.SeqCount.ToString)
If nBytesWritten = -1 Then 
UpdateListBox(String.Format("Error {0}: {1}", Socket.LastError, Socket.LastErrorString)) 
Exit Sub 
End If 
End If
End Sub
End Class

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System.AccessViolationException on Converting Bytes To Structure

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