VB.NET Accessing C DLL Problems

[webroten]

I've been working through trying to access a C DLL from VB.NET. I've read many online postings, but I'm still having problems. Now, my error is the "Attempted to read or write protected memory" error. Here's my Structure declaration:

<StructLayout(LayoutKind.Sequential, charset:=CharSet.Ansi)> _
Public Structure CONFIG_PARM
    <MarshalAs(UnmanagedType.LPStr)> Public address1 As String
    <MarshalAs(UnmanagedType.LPStr)> Public addrindex As String
    <MarshalAs(UnmanagedType.LPStr)> Public cdrom As String
    <MarshalAs(UnmanagedType.LPStr)> Public citystate As String
    <MarshalAs(UnmanagedType.LPStr)> Public crossref As String
    <MarshalAs(UnmanagedType.LPStr)> Public system As String
    <MarshalAs(UnmanagedType.LPStr)> Public elot As String
    <MarshalAs(UnmanagedType.LPStr)> Public elotindex As String
    <MarshalAs(UnmanagedType.LPStr)> Public llkpath As String
    <MarshalAs(UnmanagedType.LPStr)> Public ewspath As String
    <MarshalAs(UnmanagedType.LPStr)> Public dpvpath As String
    <MarshalAs(UnmanagedType.LPStr)> Public fnsnpath As String
    <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=124)> Public rsvd As String
End Structure
 
<StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Ansi)> _
Public Structure Z4OPEN_PARM
    <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=50)> Public rsvd1 As String
    <MarshalAs(UnmanagedType.I4)> Public status As Integer
    <MarshalAs(UnmanagedType.LPStr)> Public fname As String
    <MarshalAs(UnmanagedType.Struct)> Public config As CONFIG_PARM
    <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=1)> Public ewsflag As String
    <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=1)> Public elotflag As String
    <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=1)> Public llkflag As String
    <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=1)> Public dpvflag As String
    <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=1)> Public systemflag As String
    <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=511)> Public rsvd2 As String
End Structure

The Function declarations and the code used to call them are:

    <DllImport("zip4_w32.dll", CharSet:=CharSet.Ansi)> _
    Public Shared Function z4opencfgSTD(ByRef openparm As Z4OPEN_PARM) As Integer
    End Function
 
...
 
    Dim openparm As New Z4OPEN_PARM
    Dim configparm As New CONFIG_PARM
 
        Dim address1 As String = "e:\ams\ams_comm"
        Dim addrindex As String = "e:\ams\ams_comm"
        Dim citystate As String = "e:\ams\ams_comm"
        Dim crossref As String = "e:\ams\ams_comm"
        Dim fnsnpath As String = "e:\ams\ams_comm"
        Dim elot As String = "e:\ams\ams_elot"
        Dim elotindex As String = "e:\ams\ams_elot"
        Dim llkpath As String = "e:\ams\lacslink"
        Dim dpvpath As String = "e:\ams\ams_dpv"
        Dim systempath As String = "e:\projects\wrapz4\wrapz4\debug"
        Dim ewspath As String = ""
        Dim rsvd_string As New StringBuilder
        Dim rsvd As String = ""
        rsvd.PadLeft(124)
        Dim rsvd1 As String = ""
        rsvd1.PadLeft(50)
        Dim rsvd2 As String = ""
        rsvd2.PadLeft(511)
        Dim cdrom As String = ""
        Dim fname As String = ""
 
        configparm.address1 = address1
        configparm.addrindex = addrindex
        configparm.cdrom = cdrom
        configparm.citystate = citystate
        configparm.crossref = crossref
        configparm.system = systempath
        configparm.elot = elot
        configparm.elotindex = elotindex
        configparm.llkpath = llkpath
        configparm.ewspath = ewspath
        configparm.dpvpath = dpvpath
        configparm.fnsnpath = fnsnpath
        configparm.rsvd = rsvd
 
        openparm.status = 2
        openparm.rsvd1 = rsvd1
        openparm.rsvd2 = rsvd2
        openparm.elotflag = "Y"
        openparm.llkflag = "Y"
        openparm.ewsflag = "N"
        openparm.dpvflag = "Y"
        openparm.systemflag = "Y"
        openparm.fname = fname
 
        openparm.config = configparm
 
        Connection.Text = z4functions.z4opencfgSTD(openparm)

Why would I be getting the memory error, "Attempted to read or write protected memory. This is often an indication that other memory is corrupt."?

Thanks for your help.

[Frinavale]

I've been working through trying to access a C DLL from VB.NET. I've read many online postings, but I'm still having problems. Now, my error is the "Attempted to read or write protected memory" error. Here's my Structure declaration:

...

Why would I be getting the memory error, "Attempted to read or write protected memory. This is often an indication that other memory is corrupt."?

Thanks for your help.

I just encountered this type of error today.
What's happening is your C DLL is not properly using the memory allocated to it.

In my case I had to increase the size of the array that I passed to a method because this array was not large enough to hold the data that the function was manipulating. You would think that the developers of the DLL would have managed the memory correctly but this isn't the case.

I haven't really taken a close look at your code that you've posted...but I recommend going through and making sure that any variables declared in your .NET program are allocating enough memory for the C variables....

Especially Strings! C code is notorious for not handling memory allocation for Strings correctly.

(In your case...you may be providing Strings that are too long for the C DLL to handle)

[webroten]

That corrected the problem - thanks! I'm still having problems with the DLL reading my structure, but I don't think it's related to this issue. It could be something else with the marshalling or something.

[Frinavale]

That corrected the problem - thanks! I'm still having problems with the DLL reading my structure, but I don't think it's related to this issue. It could be something else with the marshalling or something.

I'm glad you solved the problem!
If you need help with your other issue start a new post.

Congratz!

-Frinny

[jm1576]

I am working on the same exact project. Were you able to solve your issue ? I have just received the CD from USPS. Could you post the rest of the code that call the other Z4 functions ?

[Frinavale]

I am working on the same exact project. Were you able to solve your issue ? I have just received the CD from USPS. Could you post the rest of the code that call the other Z4 functions ?

Hi Jm1576,

Webroten has only ever posted twice. This user is not very active.
Could you please post your exact error so that we might be able to help you?

-Frinny

[jm1576]

I am using the above code that was posted. This the error that I get

"Attempted to read or write protected memory. This is often an indication that other memory is corrupt."

[Frinavale]

I am using the above code that was posted. This the error that I get

"Attempted to read or write protected memory. This is often an indication that other memory is corrupt."

This is often a sign that the unmanaged code (the C code) is not using memory correctly. The previous person found that the problem was resolved when they allocated more memory for the C code to use...or else they shortened the strings that they were passing to the C code.

-Frinny

[jm1576]

Hi,
This is what I have changed.
<DllImport("zip4_w32.dll", CallingConvention:=CallingConvention.StdCall, EntryPoint:="z4opencfgSTD", ExactSpelling:=False), ReliabilityContract(Consistency.WillNotCorruptStat e, Cer.None)> _
Public Shared Function z4opencfgSTD(<[In](), Out()> <MarshalAs(UnmanagedType.LPStruct)> ByVal parm As Z4OPEN_PARM) As Integer
End Function

This is the error that I am getting now.
"Cannot marshal 'parameter #1': Invalid managed/unmanaged type combination (this value type must be paired with Struct)"