Weird marshalling problem

[Dragon]

Hello,

This is the code:

~
Option Strict On
Imports System
Imports System.Windows.Forms
Imports Microsoft.VisualBasic
Imports System.Runtime.InteropServices
Class Test
Inherits Form

Friend Declare Auto Function GetWindowText Lib "user32.dll" ( _
ByVal hWnd as IntPtr, _
<MarshalAs(UnmanagedType.LPTStr), [In](), Out()> ByVal lpString As
String, _
ByVal nMaxCount as Integer _
) As Integer

Protected Overrides Sub OnLoad(e as EventArgs)
MyBase.OnLoad(e)
Text() = "All right!"
Dim tempStr As New String(chr(0),256)
Dim Result as Integer = GetWindowText(Me.Handle, tempStr,
tempStr.Length)
MessageBox.Show(Result.ToString & " " tempStr)
End Sub

<STAThread()> Shared Sub Main()
Application.Run(New Test)
End Sub
End Class
~

I expected it to msgbox "10 All right!" and it did it. In Windows XP.
But... testing it in Windows 98 gave strange result: "10 ". For some
curious reason string wasn't marshalled back, even though I included
OutAttribute.
However when I removed all attributes, it worked fine it both OS'es.
What's the difference? Is it documented or bug?

TIA
Roman

[Robin Tucker]

use ByRef. ie.:
Friend Declare Auto Function GetWindowText Lib "user32.dll" ( _
ByVal hWnd as IntPtr, _
<MarshalAs(UnmanagedType.LPTStr)> ByRef lpString As String, _
ByVal nMaxCount as Integer ) As Integer
"Dragon" <[e-mail]> wrote in message
news:um****************@TK2MSFTNGP12.phx.gbl...

Hello,

This is the code:

~
Option Strict On
Imports System
Imports System.Windows.Forms
Imports Microsoft.VisualBasic
Imports System.Runtime.InteropServices
Class Test
Inherits Form

Friend Declare Auto Function GetWindowText Lib "user32.dll" ( _
ByVal hWnd as IntPtr, _
<MarshalAs(UnmanagedType.LPTStr), [In](), Out()> ByVal lpString As
String, _
ByVal nMaxCount as Integer _
) As Integer

Protected Overrides Sub OnLoad(e as EventArgs)
MyBase.OnLoad(e)
Text() = "All right!"
Dim tempStr As New String(chr(0),256)
Dim Result as Integer = GetWindowText(Me.Handle, tempStr,
tempStr.Length)
MessageBox.Show(Result.ToString & " " tempStr)
End Sub

<STAThread()> Shared Sub Main()
Application.Run(New Test)
End Sub
End Class
~

I expected it to msgbox "10 All right!" and it did it. In Windows XP.
But... testing it in Windows 98 gave strange result: "10 ". For some
curious reason string wasn't marshalled back, even though I included
OutAttribute.
However when I removed all attributes, it worked fine it both OS'es.
What's the difference? Is it documented or bug?

TIA
Roman

[Dragon]

?

If I use ByRef, I get an ExecutionEngineException, which can't even be
handled!

[Robin Tucker]

Well, that sucks.

One moment. I'll sort a little test harness............

"Dragon" <[e-mail]> wrote in message
news:Oa**************@tk2msftngp13.phx.gbl...

?

If I use ByRef, I get an ExecutionEngineException, which can't even be
handled!

[Bart Mermuys]

Hi,

"Dragon" <[e-mail]> wrote in message
news:um****************@TK2MSFTNGP12.phx.gbl...

Hello,

This is the code:

~
Option Strict On
Imports System
Imports System.Windows.Forms
Imports Microsoft.VisualBasic
Imports System.Runtime.InteropServices
Class Test
Inherits Form

Friend Declare Auto Function GetWindowText Lib "user32.dll" ( _
ByVal hWnd as IntPtr, _
<MarshalAs(UnmanagedType.LPTStr), [In](), Out()> ByVal lpString As
String, _
ByVal nMaxCount as Integer _
) As Integer

Protected Overrides Sub OnLoad(e as EventArgs)
MyBase.OnLoad(e)
Text() = "All right!"
Dim tempStr As New String(chr(0),256)
Dim Result as Integer = GetWindowText(Me.Handle, tempStr,
tempStr.Length)
MessageBox.Show(Result.ToString & " " tempStr)
End Sub

<STAThread()> Shared Sub Main()
Application.Run(New Test)
End Sub
End Class

You should be using a StringBuilder if native code changes the string. See

http://msdn.microsoft.com/library/de...forstrings.asp

scroll to the bottom : "Fixed-Length String Buffers"
HTH,
Greetings

~

I expected it to msgbox "10 All right!" and it did it. In Windows XP.
But... testing it in Windows 98 gave strange result: "10 ". For some
curious reason string wasn't marshalled back, even though I included
OutAttribute.
However when I removed all attributes, it worked fine it both OS'es.
What's the difference? Is it documented or bug?

TIA
Roman

[Robin Tucker]

Ah yes. GetWindowText just plonks the text into an already existing buffer,
however, your marshalling attributes were probably causing it some confusion
as you were technically trying to marshal it as .... actually I have no
idea. It works. Stop whinging ;)
"Dragon" <[e-mail]> wrote in message
news:Oa**************@tk2msftngp13.phx.gbl...

?

If I use ByRef, I get an ExecutionEngineException, which can't even be
handled!

[Tom Shelton]

In article <um**************@TK2MSFTNGP12.phx.gbl>, Dragon wrote:

Hello,

This is the code:

~
Option Strict On
Imports System
Imports System.Windows.Forms
Imports Microsoft.VisualBasic
Imports System.Runtime.InteropServices
Class Test
Inherits Form

Friend Declare Auto Function GetWindowText Lib "user32.dll" ( _
ByVal hWnd as IntPtr, _
<MarshalAs(UnmanagedType.LPTStr), [In](), Out()> ByVal lpString As
String, _
ByVal nMaxCount as Integer _
) As Integer

Private Declare Auto Function GetWindowText Lib "user32.dll" _
(ByVal hWnd As IntPtr, _
ByVal lpString As System.Text.StringBuilder, _
ByVal nMaxCount As Integer) As Integer
Public Function GetWindowText (ByVal hWnd As IntPtr) As String
Dim buffer As New StringBuilder (256)

If GetWindowText (hWnd, buffer, buffer.Length) > 0 Then
Return buffer.ToString ()

' if you want an error check, maybe somthing like...
' Else
' Throw New Win32Exception (Marshal.GetLastWin32Error ())
End If

End Function

--
Tom Shelton [MVP]

[Dragon]

Okay, I'll move on to StringBuilder.

Thank you all for your replies.