Cannot (always) use Objects as parameters

[Woinowski]

To show what I mean an example below.

Problem is: Sometimes you can use a parameter to assign it to the Value property of an object (e.g. of a TreeView Node ), sometimes you can't.

When it does not work you get an error message like "cannot convert [...].String to System.Windows.Forms.TreeView"

Funny thing is: In most cases it works directly at the beginning of a sub. It never (?) works further down in the body of a sub.

Following example sub ist not working (assume TempNode is a TreeView node which has been initialized):

[I have tried several variants]

' parameter ile is a file opened with FileOpen for writing
' node is a TreeView node object
Sub RecursiveWrite (file, node)
' First use of node sometimes works
TempNode.Value = node ' Remark: or node.value, try both
FileWrite(file, TempNode.text)

nodeCount = TempNode.Count - 1
for i = 0 to nodeCount
RecursiveWrite(file, TempNode.GetNode(i))
' or:
' TempNode.Value = TempNode.GetNode(i)
' RecursiveWrite(file, TempNode.Value)

' Reset TempNode, never works
TempNode.Value = node ' or node.value, does not matter :-(
node
End Sub

 

[Erel]

First, it should always be node.Value.
Are you sure that node is not an empty node?

 

[Woinowski]

Yes, you are right, it should also be .value on the right hand side of the "="

I am sure that it is not an empty node. Especially I had the case that within one call the first use worked and the second not, although I did not change 'node' within one call.

It might be the cause that recursion is the real problem. In general, if you do recursive calls I expect a stack of local parameters and variables. Is this expectation correct fpr B4PPC?

Unfortunaley I just overwrote my whole code with save as (desaster is one short click away :-( ), otherwise i could send you the complete thing. [I do not assume it is possible to decompile B4PPC-Programs]

 

[Erel]

When you use a recursive call a stack of local variables is created.
Note that objects (like a tree node) and controls are global and not local.

 

[Woinowski]

To prevent further theoretical discussions, I just coded some fully working example (see below, you can use copy&paste).

You need
Form1 : Form
Component added : TreeView.dll
Tree: TreeView
Node, TempNode: Node

---
Here's the error message:

An error occured on sub recursivedescent

Line numer: 70

TempNode.Value = nodeAsParameter
Error description:
Das Object mit dem Typ System.Int32 kann nicht in den Typ Systems.Windows.Forms.TreeNode konvertiert werden.
Continue?

[Buttons Yes/No]

Translation of error:

The object with type System.Int32 cannot be converted to type Systems.Windows.Forms.TreeNode.

---

Heres the sample code:

-- 8< -- snip -
Sub Globals
'Declare the global variables here.
Dim Result
End Sub

Sub App_Start
Form1.Show

' Initialize Tree
Tree.New1("Form1", 0, 0, 200,200)
Tree.AddNewNode("Root")
Node.New1
Node.Value = Tree.GetNode(0)
Node.AddNewNode("Level 1.1")
Node.AddNewNode("Level 1.2")
Node.AddNewNode("Level 1.3")
Node.Value = Node.GetNode(0)
Node.AddNewNode("Level 2.1")
Node.AddNewNode("Level 2.2")
Node.Value = Node.Parent
Node.Value = Node.GetNode(1)
Node.AddNewNode("Level 2.3")
Node.AddNewNode("Level 2.4")
Tree.ExpandAll

' Initialize TempNode
TempNode.New1

' Prepare Result of recursive call
Result = ""

' Initialize node for recursive call
Node.Value = Tree.GetNode(0)

' No recursion: Works fine!
UseNode(Node.Value)

' Recursice Call
RecursiveDescent(Node.Value)

' Use result
Msgbox(Result,"Result")
End Sub

Sub UseNode(nodeAsParameter)
' Assign Temp Node
TempNode.Value = nodeAsParameter

' Sample use
Msgbox(TempNode.Text, "It Works")
TempNode.Text = "New name"
Msgbox(TempNode.Text, "It Still Works")
End Sub

Sub RecursiveDescent(nodeAsParameter)
' Assign Temp Node (works!)
TempNode.Value = nodeAsParameter
Msgbox(TempNode.Text, "It Works in First Line")

' Do something for each node:
Result = Result & TempNode.Text & crlf

nodeCount = TempNode.Count - 1
For i = 0 To nodeCount
' This will overwrite TempNode
TempNode.Value = TempNode.GetNode(i)
RecursiveDescent(TempNode.Value)

' Reset TempNode after recursion (does not work!!!)
' Exactly the same as in first line of sub!
TempNode.Value = nodeAsParameter
Next
End Sub

-- 8< -- snip -

 

[Erel]

You can't pass an object value to a sub.
You should use the object's name instead:

Sub UseNode(nodeName)
   ' Assign Temp Node
   TempNode.Value = Control(nodeName).Value
...
End Sub
...
UseNode("Node")

 

[Woinowski]

Thanks for all your quick replies.

To sum it all up and "close" the bug

1. Why does the first line of my recursive function work then?

2. Using the name of the object won't help for the recursion - it is essential to be able to re-assign the original reference

3. As a workaround I think it should be possible to implement something similar to the iterators known form C# or Java. As soon as I have some code I'm gonna post it in the correct Main Category (Code Samples)

 

[Erel]

1. In a nutshell, Basic4ppc remembers the last object value returned and then it uses it the next time an object is required. Again you should not pass objects to subs as it will not work consistently.
2. Why won't it work? I think it should work properly.

 

[agraham]

Hi Erel

2. Why won't it work? I think it should work properly.

I see his problem (I think). There is only a single TempNode in the scope of the recursion, whereas he needs a different local node reference for each level of descent because he is recursing in a loop and needs the original reference to identify the next node to recurse into.

When you do "Node.AddNewNode("Level 1.1")" what is the name of the added node or is it anonymous? Is there any way of saving a node reference in a local variable -I've tried it and it doesn't error but also doesn't work!

 

[agraham]

How about this?

Sub Globals
   'Declare the global variables here.
   Dim Result 
End Sub

Sub App_Start
   form1.Show
Tree.New1("Form1", 0, 0, 200,200)
Tree.AddNewNode("Root")
   Node.New1
   Node.Value = Tree.GetNode(0)
   Node.AddNewNode("Level 1.1")
   Node.AddNewNode("Level 1.2")
   Node.AddNewNode("Level 1.3")
   Node.Value = Node.GetNode(0)
   Node.AddNewNode("Level 2.1")
   Node.AddNewNode("Level 2.2")
   Node.Value = Node.Parent
   Node.Value = Node.GetNode(1)
   Node.AddNewNode("Level 2.3")
   Node.AddNewNode("Level 2.4")
   Tree.ExpandAll
   Msgbox(GetControls("Tree"))
   ' Initialize TempNode
   TempNode.New1
   ' Prepare Result of recursive call
   Result = ""
   ' Initialize node for recursive call
   Node.Value = Tree.GetNode(0)
   ' Recursice Call
   RecursiveDescent("Node", 1)
   ' Use result
   Msgbox(Result,"Result")
End Sub


Sub RecursiveDescent(NodeName, Level)
   ' Do something for each node:
   Result = Result & Control(NodeName).Text & " "
   SubNode = "Node"&Level
   AddObject(SubNode, "Node")
   Control(SubNode).New1
   nodeCount = Control(NodeName).Count - 1
   For i = 0 To nodeCount
      Control(SubNode).Value = Control(nodeName).GetNode(i).Value
      RecursiveDescent (SubNode, Level+1)   
   Next
   Control(SubNode).Dispose
End Sub

 

[Cableguy]

Unfortunaley I just overwrote my whole code with save as (desaster is one short click away :-( ), otherwise i could send you the complete thing

Check out my backup utility in the samples forum....maybe it helps

 

[Woinowski]

I did not think of AddObject...

This is an extremely cool solution. Although I am now working without recursion I definitely will remember this one

So, thanks

 

[Woinowski]

Sample for Tree Iterator on TreeView

3. As a workaround I think it should be possible to implement something similar to the iterators known form C# or Java. As soon as I have some code I'm gonna post it in the correct Main Category (Code Samples)

You will find the workaround in the following open source project:

http://www.b4x.com/forum/showthread.php?t=1514