[RESOLVED] Can't Loop Through Items in a Collection

[mksafi]

CAUTION: Noob using Win32 API ahead

I want to display a list of open windows on my computer in a rich text box. So, I'm trying to store the names of all windows in myString, and then set RichTextBox1.Text = myString.

Here's the code

Imports System
Imports System.Collections.Generic
Imports System.Collections.ObjectModel
Imports System.Text

Public Class Form1
    Private Function Display(ByVal cs As Collection(Of String))
        Dim builder As New StringBuilder
        For Each item As String In cs
            builder.Append(item & vbCrLf)
        Next item
        Return builder.ToString
    End Function

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        RichTextBox1.Text = Display(GetActiveWindows)

    End Sub

    ' CallBack delegate    
    Public Delegate Function CallBack(ByVal hWnd As IntPtr, ByVal lParam As Integer) As Boolean

    ' Imports
    Public Declare Function EnumWindows Lib "user32" (ByVal Adress As CallBack, ByVal y As Integer) As Integer
    Public Declare Function IsWindowVisible Lib "user32" (ByVal hwnd As IntPtr) As Boolean
    Private Declare Function GetWindowText Lib "user32" Alias "GetWindowTextA" (ByVal hwnd As Integer, ByVal lpWindowText As String, ByVal cch As Integer) As Integer
    ' /Imports

    Private ActiveWindows As New Collection(Of String)

    Public Function GetActiveWindows() As Collection(Of String)
        ActiveWindows.Clear()
        EnumWindows(AddressOf Enumerator, 0)
        Return ActiveWindows
    End Function

    Private Function Enumerator(ByVal hwnd As IntPtr, ByVal lParam As Integer) As Boolean
        Dim text As String = Space(Int16.MaxValue)
        GetWindowText(hwnd, text, Int16.MaxValue)
        If IsWindowVisible(hwnd) Then
            ActiveWindows.Add(text)
        End If
        Return True
    End Function
End Class

So, Display function above doesn't loop through the Collection of window titles. I don't know why. I just need to know why for educational purposes. I managed to display the list of window titles in the rich text box using another method...shown below.

Thanks,

Imports System
Imports System.Collections.Generic
Imports System.Collections.ObjectModel
Imports System.Text

Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        For Each item As String In GetActiveWindows()            
            RichTextBox1.AppendText(item)
            RichTextBox1.AppendText(vbCrLf)
        Next
        'RichTextBox1.Text

    End Sub

    ' CallBack delegate    
    Public Delegate Function CallBack(ByVal hWnd As IntPtr, ByVal lParam As Integer) As Boolean

    ' Imports
    Public Declare Function EnumWindows Lib "user32" (ByVal Adress As CallBack, ByVal y As Integer) As Integer
    Public Declare Function IsWindowVisible Lib "user32" (ByVal hwnd As IntPtr) As Boolean
    Private Declare Function GetWindowText Lib "user32" Alias "GetWindowTextA" (ByVal hwnd As Integer, ByVal lpWindowText As String, ByVal cch As Integer) As Integer
    ' /Imports

    Private ActiveWindows As New Collection(Of String)

    Public Function GetActiveWindows() As Collection(Of String)
        ActiveWindows.Clear()
        EnumWindows(AddressOf Enumerator, 0)
        Return ActiveWindows
    End Function

    Private Function Enumerator(ByVal hwnd As IntPtr, ByVal lParam As Integer) As Boolean
        Dim text As String = Space(Int16.MaxValue)
        GetWindowText(hwnd, text, Int16.MaxValue)
        If IsWindowVisible(hwnd) Then
            ActiveWindows.Add(text)
        End If
        Return True
    End Function
End Class

[Shaggy Hiker]

Since you claim to be new, are you familiar with breakpoints and stepping through code? Since you want to figure it out for educational purposes, a motivation that I thoroughly applaud, the best way to learn it is to figure it out yourself, and stepping through that loop would be the way to go.

Put a breakpoint on the For line of the loop. When execution pauses at the breakpoint, take a look at the collection (either hover over it, or use Shift+F9 on it) and see whether there are any items in it. Then step through the loop watching what happens in the stringbuilder. Frankly, I think you will find that the collection is empty, as the rest looks reasonable. The next step, then, would be to go figure out why the collection is empty, but this is a start.

[mksafi]

Actually, the Collection isn't empty. It contains 15 items in my case. And as I watch the code executes, I see that the code inside the loop is iterated 15 times, each time item has the value that it should and that value seems to be appended to builder, but for some reason the final value of the StringBuilder isn't output'd correctly on the rich text box.

[The Fire Snake]

Where are you using string builder???

[mksafi]

Where are you using string builder???

Inside Display function.

[techgnome]

Turn on Option Explicit and turn on Option Strict... Display doesn't return a string... it returns an object...
Private Function Display(ByVal cs As Collection(Of String))
SHOULD BE
Private Function Display(ByVal cs As Collection(Of String)) As String

-tg

[mksafi]

Turn on Option Explicit and turn on Option Strict... Display doesn't return a string... it returns an object...
Private Function Display(ByVal cs As Collection(Of String))
SHOULD BE
Private Function Display(ByVal cs As Collection(Of String)) As String

-tg

I changed the code as you suggested, but the problem persists.

Option Explicit On
Option Strict On
Imports System
Imports System.Collections.Generic
Imports System.Collections.ObjectModel
Imports System.Text

Public Class Form1
    Private Function Display(ByVal cs As Collection(Of String)) As String
        Dim builder As New StringBuilder
        For Each item As String In cs
            builder.Append(item & vbCrLf)
        Next item
        Return builder.ToString
    End Function

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        RichTextBox1.Text = Display(GetActiveWindows)

    End Sub

    ' CallBack delegate    
    Public Delegate Function CallBack(ByVal hWnd As IntPtr, ByVal lParam As Integer) As Boolean

    ' Imports
    Public Declare Function EnumWindows Lib "user32" (ByVal Adress As CallBack, ByVal y As Integer) As Integer
    Public Declare Function IsWindowVisible Lib "user32" (ByVal hwnd As IntPtr) As Boolean
    Private Declare Function GetWindowText Lib "user32" Alias "GetWindowTextA" (ByVal hwnd As System.IntPtr, ByVal lpWindowText As String, ByVal cch As Integer) As Integer
    ' /Imports

    Private ActiveWindows As New Collection(Of String)

    Public Function GetActiveWindows() As Collection(Of String)
        ActiveWindows.Clear()
        EnumWindows(AddressOf Enumerator, 0)
        Return ActiveWindows
    End Function

    Private Function Enumerator(ByVal hwnd As IntPtr, ByVal lParam As Integer) As Boolean
        Dim text As String = Space(Int16.MaxValue)
        GetWindowText(hwnd, text, Int16.MaxValue)
        If IsWindowVisible(hwnd) Then
            ActiveWindows.Add(text)
        End If
        Return True
    End Function
End Class

[techgnome]

I'm a little confused why you have it setup but, hey, what ever....

If you breakpoint on this line:
Return builder.ToString

What does Builder have?

Does it have what you expect it to?

-tg

[mksafi]

I'm a little confused why you have it setup but, hey, what ever....

If you breakpoint on this line:
Return builder.ToString

What does Builder have?

Does it have what you expect it to?

-tg

In QuickWatch, StringBuilder has four attributes(?)

Capatcity = 524304
Chars = In order to evaluate an indexed property, the property must be qualified and the arguments must be explicitly supplied by the user.
Length = 458766
MaxCapacity = 2147483647

I'm not sure if it should have something else?

[techgnome]

Consider this:

Option Explicit On
Option Strict On
Imports System
Imports System.Collections.Generic
Imports System.Collections.ObjectModel
Imports System.Text

Public Class Form1
    Private Function Display(ByVal cs As Collection(Of String)) As String
        Dim builder As New StringBuilder
        For Each item As String In cs
            builder.Append(item & vbCrLf)
        Next item
        Return builder.ToString
    End Function

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim windowText As String = ""

        GetActiveWindows 
        windowText = Display(ActiveWindows) 'Breakpoint here... does AcvtiveWinows have what you expect?
        RichTextBox1.Text = windowText 'Breakpoint here, does windowText have what you expect?

    End Sub

    ' CallBack delegate    
    Public Delegate Function CallBack(ByVal hWnd As IntPtr, ByVal lParam As Integer) As Boolean

    ' Imports
    Public Declare Function EnumWindows Lib "user32" (ByVal Adress As CallBack, ByVal y As Integer) As Integer
    Public Declare Function IsWindowVisible Lib "user32" (ByVal hwnd As IntPtr) As Boolean
    Private Declare Function GetWindowText Lib "user32" Alias "GetWindowTextA" (ByVal hwnd As System.IntPtr, ByVal lpWindowText As String, ByVal cch As Integer) As Integer
    ' /Imports

    Private ActiveWindows As New Collection(Of String)

    Public Sub GetActiveWindows() 
        ActiveWindows.Clear()
        EnumWindows(AddressOf Enumerator, 0)
    End Function

    Private Function Enumerator(ByVal hwnd As IntPtr, ByVal lParam As Integer) As Boolean
        Dim text As String = Space(Int16.MaxValue)
        GetWindowText(hwnd, text, Int16.MaxValue)
        If IsWindowVisible(hwnd) Then
            ActiveWindows.Add(text)
        End If
        Return True
    End Function
End Class

Re-arranged a few things...

-tg

[mksafi]

Consider this:

...

Re-arranged a few things...

-tg

Thanks for that, techgnome.

Yes, ActiveWindows does have what I expect. windowText, on the other hand, only contains the name of the first window in the Collection (Apologize for forgetting to mention this earlier. Printing only the name of the first window in the Collection has always been the case).

[techgnome]

Just for fun, change this:
builder.Append(item & vbCrLf)
to this:
builder.Append(item & " // ")
See if you're still only getting the first window... also check to see if you actually get the "//" to show...

-tg

[mksafi]

Just for fun, change this:
builder.Append(item & vbCrLf)
to this:
builder.Append(item & " // ")
See if you're still only getting the first window... also check to see if you actually get the "//" to show...

-tg

The first window still shows. The "//" don't show. The output doesn't look any different from what I had before.

[techgnome]

That tells me a lot then....

This is line is actually the source of the problem:
GetWindowText(hwnd, text, Int16.MaxValue)
When APIs return strings, they are NULL-terminated... so as you are stringing along your strings when adding them to string builder, when the NULL is hit on the end of the first string, that effectively ends the string.... so what gets appended after that "doesn't exist" ...

You can do one of two things.... 1) Strip the null character before adding it to your collection of string... or 2) strip the null character from the string before passing it to the string builder.

-tg

[mksafi]

That tells me a lot then....

This is line is actually the source of the problem:
GetWindowText(hwnd, text, Int16.MaxValue)
When APIs return strings, they are NULL-terminated... so as you are stringing along your strings when adding them to string builder, when the NULL is hit on the end of the first string, that effectively ends the string.... so what gets appended after that "doesn't exist" ...

You can do one of two things.... 1) Strip the null character before adding it to your collection of string... or 2) strip the null character from the string before passing it to the string builder.

-tg

Spot on!

So, I added item = Replace(item, Chr(0), "") before builder.Append(item) and now it works!

Much appreciated. Thanks!

[techgnome]

Excellent! Sorry for the run around initially.

Don't forget to mark the thread resolved (under Thread Tools at the top).

-tg