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.NET Forum / Languages / VB.NET / March 2008thrown exception in Marshal.PtrToStructure
Hello World, I am trying to read an intptr() returned from a call to EnumPrinters into a structure and keep getting an exception. The relevant code looks like: Public Structure pInfo <MarshalAs(UnmanagedType.LPStr)> Dim flags As String <MarshalAs(UnmanagedType.LPStr)> Dim pDescription As String <MarshalAs(UnmanagedType.LPStr)> Dim pName As String <MarshalAs(UnmanagedType.LPStr)> Dim pComment As String End Structure <DllImport("winspool.drv", EntryPoint:="EnumPrinters", _ SetLastError:=True, CharSet:=CharSet.Unicode)> Public Shared Function _ EnumPrinters(ByVal flags As Int32, ByVal pName As String, ByVal Level _ As Int32, ByVal pPrinterEnum As IntPtr, ByVal cbBuf As Int32, ByRef _ pcbNeeded As Int32, ByRef pcReturned As Int32) As Int32 End Function selectedItem = "\\" & CmB_Servers.SelectedItem server = selectedItem.ToString() EnumPrinters(flag, server, PRINTER_LEVEL_1, outC, 0, pcbNeeded, pcReturned) outC = Marshal.AllocHGlobal(pcbNeeded + 1) If EnumPrinters(flag, server, PRINTER_LEVEL_1, outC, pcbNeeded, pcbNeeded, pcReturned) > 0 Then Dim manyPr As pInfo Dim currentP As IntPtr = outC Dim i As Integer For i = 1 To pcReturned manyPr = Marshal.PtrToStructure(currentP, manyPr.GetType()) CmB_Printers.Items.Add(manyPr.pName) currentP = IntPtr.op_Explicit(currentP.ToInt32 + Marshal.SizeOf(manyPr.GetType)) Next Else MsgBox("unable to get printers") End If Marshal.FreeHGlobal(outC) ---- The exception thrown is: Object reference not set to an instance of an object I assume that is referring to the manyPr variable, but am not sure what I should be doing about it. Help! Thanks, Stephanie First, turn on Option Strict, then try this : <StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Unicode)> _ Public Structure pInfo <MarshalAs(UnmanagedType.LPTStr)> Dim flags As String <MarshalAs(UnmanagedType.LPTStr)> Dim pDescription As String <MarshalAs(UnmanagedType.LPTStr)> Dim pName As String <MarshalAs(UnmanagedType.LPTStr)> Dim pComment As String End Structure <DllImport("winspool.drv", EntryPoint:="EnumPrintersW", _ SetLastError:=True, CharSet:=CharSet.Unicode)> Public Function _ EnumPrinters(ByVal flags As Int32, ByVal pName As String, ByVal Level _ As Int32, ByVal pPrinterEnum As IntPtr, ByVal cbBuf As Int32, ByRef _ pcbNeeded As Int32, ByRef pcReturned As Int32) As Int32 End Function For i = 1 To pcReturned manyPr = CType(Marshal.PtrToStructure(currentP, GetType(pInfo)), pInfo) CmB_Printers.Items.Add(manyPr.pName) currentP = IntPtr.op_Explicit(currentP.ToInt32 + Marshal.SizeOf(GetType(pInfo))) Next > Hello World, > [quoted text clipped - 50 lines] > Thanks, > Stephanie Thank you for your suggestion,Bill. Unfortunately this throws an exception (Error: PInvoke item (field,method) must be Static.) in the first call to EnumPrinter. The definitions for the variables in that call are: Public Const PRINTER_ENUM_SHARED As Int32 = 20 Public Const PRINTER_ENUM_NAME As Int32 = 8 Public Const PRINTER_LEVEL_1 As Int32 = 1 Dim pcbNeeded As Int32 = 0 Dim pcReturned As Int32 = 0 Dim flag As Int32 = PRINTER_ENUM_NAME Or PRINTER_ENUM_SHARED What did I miss? Stephanie > First, turn on Option Strict, then try this : > [quoted text clipped - 76 lines] > > Thanks, > > Stephanie Mark the API declaration for EnumPrinters as Shared, or place it in a Module > Thank you for your suggestion,Bill. Unfortunately this throws an exception > (Error: PInvoke item (field,method) must be Static.) in the first call to [quoted text clipped - 95 lines] >> > Thanks, >> > Stephanie Ok. Setting the EnumPrinters function as Shared fixed that exception. However, once past that, I am still getting the original exception (Object reference not set to an instance of an object) for the line: manyPr = Marshal.PtrToStructure(currentP, manyPr.GetType()) > Mark the API declaration for EnumPrinters as Shared, or place it in a Module > [quoted text clipped - 97 lines] > >> > Thanks, > >> > Stephanie go back and read the code in my first reply > Ok. Setting the EnumPrinters function as Shared fixed that exception. > However, once past that, I am still getting the original exception (Object [quoted text clipped - 115 lines] >> >> > Thanks, >> >> > Stephanie I believe I already did. Unless I missed something, my changes should have consisted of adding the line: StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Unicode)> _ changing the declaration for EnumPrinters to: <DllImport("winspool.drv", EntryPoint:="EnumPrintersW", _ and the code for the line that causes the exception to: manyPr = CType(Marshal.PtrToStructure(currentP, GetType(pInfo)), pInfo) plus the substitution a couple of lines later to: currentP = IntPtr.op_Explicit(currentP.ToInt32 + Marshal.SizeOf((GetType(pInfo)))) but I'm still getting the exception. > go back and read the code in my first reply > [quoted text clipped - 117 lines] > >> >> > Thanks, > >> >> > Stephanie and change the strings to UnmanagedType.LPTStr >I believe I already did. Unless I missed something, my changes should have > consisted of adding the line: [quoted text clipped - 147 lines] >> >> >> > Thanks, >> >> >> > Stephanie Did that too. > and change the strings to UnmanagedType.LPTStr > [quoted text clipped - 149 lines] > >> >> >> > Thanks, > >> >> >> > Stephanie post your complete code. > Did that too. > [quoted text clipped - 166 lines] >> >> >> >> > Thanks, >> >> >> >> > Stephanie Here goes: Option Strict On Imports System Imports System.IO Imports System.Text Imports System.Runtime.InteropServices Imports System.ComponentModel Imports System.Management Public Class Form1 Inherits System.Windows.Forms.Form Public Const PRINTER_ENUM_SHARED As Int32 = 20 Public Const PRINTER_ENUM_NAME As Int32 = 8 Public Const PRINTER_LEVEL_1 As Int32 = 1 <StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Unicode)> _ Public Structure pInfo <MarshalAs(UnmanagedType.LPStr)> Dim flags As String <MarshalAs(UnmanagedType.LPStr)> Dim pDescription As String <MarshalAs(UnmanagedType.LPStr)> Dim pName As String <DllImport("winspool.drv", EntryPoint:="EnumPrintersW", _ SetLastError:=True, CharSet:=CharSet.Unicode)> Public Shared Function _ EnumPrinters(ByVal flags As Int32, ByVal pName As String, ByVal Level _ As Int32, ByVal pPrinterEnum As IntPtr, ByVal cbBuf As Int32, ByRef _ pcbNeeded As Int32, ByRef pcReturned As Int32) As Int32 End Function Public Sub getServerPrinters() Dim pcbNeeded As Int32 = 0 Dim pcReturned As Int32 = 0 Dim outC As IntPtr = IntPtr.Zero Dim flag As Int32 = PRINTER_ENUM_NAME Or PRINTER_ENUM_SHARED Dim selectedItem As Object Dim server As String Dim status As Int32 selectedItem = "\\" + CmB_Servers.SelectedItem.ToString server = selectedItem.ToString() EnumPrinters(flag, server, PRINTER_LEVEL_1, outC, 0, pcbNeeded, pcReturned) outC = Marshal.AllocHGlobal(pcbNeeded + 1) If EnumPrinters(flag, server, PRINTER_LEVEL_1, outC, pcbNeeded, pcbNeeded, pcReturned) > 0 Then Dim manyPr As pInfo Dim currentP As IntPtr = outC Dim i As Integer For i = 1 To pcReturned manyPr = CType(Marshal.PtrToStructure(currentP, GetType(pInfo)), pInfo) CmB_Printers.Items.Add(manyPr.pName) currentP = IntPtr.op_Explicit(currentP.ToInt32 + Marshal.SizeOf((GetType(pInfo)))) Next Else MsgBox("unable to get printers") End If Marshal.FreeHGlobal(outC) End Sub ... End Class > post your complete code. > [quoted text clipped - 168 lines] > >> >> >> >> > Thanks, > >> >> >> >> > Stephanie You have the structure declared wrong. Note it should be LPTStr, not LPStr on each of the four fields. See my first reply again. > Here goes: > [quoted text clipped - 253 lines] >> >> >> >> >> > Thanks, >> >> >> >> >> > Stephanie I'm afraid that doesn't do it either. > You have the structure declared wrong. Note it should be LPTStr, not LPStr > on each of the four fields. See my first reply again. [quoted text clipped - 256 lines] > >> >> >> >> >> > Thanks, > >> >> >> >> >> > Stephanie try this : Option Strict On Imports System Imports System.IO Imports System.Text Imports System.Runtime.InteropServices Imports System.ComponentModel Imports System.Management Public Class Form1 Inherits System.Windows.Forms.Form Public Const PRINTER_ENUM_SHARED As Int32 = 20 Public Const PRINTER_ENUM_NAME As Int32 = 8 Public Const PRINTER_LEVEL_1 As Int32 = 1 <StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Unicode)> _ Public Structure pInfo Dim flags As Int32 <MarshalAs(UnmanagedType.LPTStr)> Dim pDescription As String <MarshalAs(UnmanagedType.LPTStr)> Dim pName As String <MarshalAs(UnmanagedType.LPTStr)> Dim pComment As String End Structure <DllImport("winspool.drv", EntryPoint:="EnumPrintersW", _ SetLastError:=True, CharSet:=CharSet.Unicode)> Public Shared Function _ EnumPrinters(ByVal flags As Int32, ByVal pName As String, ByVal Level _ As Int32, ByVal pPrinterEnum As IntPtr, ByVal cbBuf As Int32, ByRef _ pcbNeeded As Int32, ByRef pcReturned As Int32) As Int32 End Function Public Sub getServerPrinters() Dim pcbNeeded As Int32 = 0 Dim pcReturned As Int32 = 0 Dim outC As IntPtr = IntPtr.Zero Dim flag As Int32 = PRINTER_ENUM_NAME Or PRINTER_ENUM_SHARED Dim server As String server = "\\" & My.Computer.Name ' add code here for server name EnumPrinters(flag, server, PRINTER_LEVEL_1, outC, 0, pcbNeeded, pcReturned) outC = Marshal.AllocHGlobal(pcbNeeded) If EnumPrinters(flag, server, PRINTER_LEVEL_1, outC, pcbNeeded, pcbNeeded, pcReturned) > 0 Then Dim manyPr As pInfo Dim currentP As IntPtr = outC Dim i As Integer For i = 1 To pcReturned manyPr = CType(Marshal.PtrToStructure(currentP, GetType(pInfo)), pInfo) Debug.Print(manyPr.pName) currentP = IntPtr.op_Explicit(currentP.ToInt32 + Marshal.SizeOf((GetType(pInfo)))) Next Else MsgBox("unable to get printers") End If Marshal.FreeHGlobal(outC) End Sub That worked perfectly! I was then able to make the substitutions for my system and it's all good. Thank you so much, Bill. Stpehanie > try this : > [quoted text clipped - 62 lines] > > End Sub Glad I could help :) > That worked perfectly! I was then able to make the substitutions for my > system and it's all good. Thank you so much, Bill. [quoted text clipped - 68 lines] >> >> End Sub Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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