[RESOLVED] Prevent developer from placing a UserControl more than one time in Design Time

I am working on a UserControl and I am looking for a way to prevent developer to place this UserControl more than one time into a form, in Design Time. In other words, how can I detect if my UserControl is already placed into ParentForm or not, in Design Time and prevent the second placement if there is already one there?

I tried something like this example below... First I am not sure if this is the "correct" way and second, I can't find how to remove or stop the placement of UserControl in case there is already one.

Again, all this, in Design Time!!!

vb.net Code:

Private Sub MyUserControl_Load(sender As Object, e As EventArgs) Handles MyBase.Load 
    Dim _Count As Integer
    Dim _UserControl As MyUserControl
    For Each _UserControl In Me.ParentForm.Controls
        If _UserControl.Name.Contains("MyUserControl") Then
            _Count += 1
        End If
    Next
    If _Count > 1 Then
        MsgBox("Control have been placed.")
    Else
        MsgBox("Control haven't placed yet.")
    End If
End Sub

Try this:

VB.Net Code:

Private Sub Me_ParentChanged(sender As Object, e As EventArgs) Handles Me.ParentChanged
    Dim f = DirectCast(Me.FindForm, Control)
    If f IsNot Nothing Then
        'check to see if we are the only one on the form
        Dim c = f
        Do Until c Is Nothing
            If TypeOf c Is UserControl1 AndAlso c IsNot Me Then
                Throw New NotSupportedException("You cannot have more than one of these controls on your form at a time")
            End If
            c = f.GetNextControl(c, True)
        Loop
    End If
End Sub

Change UserControl1 to the name of your control...

The above will work ... but...
... You may want to show a message box before you throw the exception for an easier to understand message...
... I don't know of a way to cancel the control creation and only show a nice handled error though...
If you show your own message you can also choose to set Me.Parent = Nothing instead of throwing the exception to not insert the control ... but it also shows another error after stating: "'child' is not a child control of this parent"

Kris

Thank you very much my friend for your answer, it worked!!! Here is how I used your example with some changes...

vb.net Code:

Private Sub Me_ParentChanged(sender As Object, e As EventArgs) Handles Me.ParentChanged
        If Me.DesignMode AndAlso Me.ParentForm IsNot Nothing Then
            Dim _UserControl = DirectCast(Me.ParentForm, Control)
            If _UserControl IsNot Nothing Then
                Do Until _UserControl Is Nothing
                    If TypeOf _UserControl Is MyUserControl AndAlso _UserControl IsNot Me Then
                        Throw New ArgumentOutOfRangeException("", "You can place only one " & _UserControl.Name & " control per form.")
                    End If
                    _UserControl = _UserControl.GetNextControl(_UserControl, True)
                Loop
            End If
        End If
    End Sub

One more question...

vb.net Code:

Private Sub Me_ParentChanged(sender As Object, e As EventArgs) Handles Me.ParentChanged
    'Blah blah blah...
End Sub

and

vb.net Code:

Protected Overrides Sub OnParentChanged(e As EventArgs)[INDENT]MyBase.OnParentChanged(e)
    'Blah blah blah...
End Sub

is the same thing?

Quote:

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I don't know of a way to cancel the control creation

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Haven't got an ide in front of me to check but you could presumably just remove it:-

Code:

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f.Controls.Remove(Me)

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By the way, I think there's a typo in your code:-

Code:

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Dim c = f

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Should be:-

Code:

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Dim c = f.GetNextControl(c, True)

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Unless you actually want to check the control isn't the form for some reason


edit> crossed over

You may or may not want this:-

Quote:

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If Me.DesignMode

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While that's in they could add a second control to the form at run time.

Quote:

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is the same thing?

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Not quite. The "On" methods fire the events and are the intended method for firing the event from code, rather than calling the handling event directly. The correct way to wire up an event handler is the first one.

Quote:

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Originally Posted by FunkyDexter

You may or may not want this:-

Quote:

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If Me.DesignMode

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While that's in they could add a second control to the form at run time.

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Do you mean if developer try to add this control programmatically?

Quote:

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Originally Posted by FunkyDexter

You may or may not want this...

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So I ended up on this...

vb.net Code:

Private Sub Me_ParentChanged(sender As Object, e As EventArgs) Handles Me.ParentChanged
    If Me.ParentForm IsNot Nothing Then
        Dim _UserControl = DirectCast(Me.ParentForm, Control)
        If _UserControl IsNot Nothing Then
            Do Until _UserControl Is Nothing
                If TypeOf _UserControl Is CmosTitleBar AndAlso _UserControl IsNot Me Then
                    Throw New ArgumentOutOfRangeException("", "You can place only one " & _UserControl.Name & " control per form.")
                End If
                _UserControl = _UserControl.GetNextControl(_UserControl, True)
            Loop
        End If
    End If
End Sub

Quote:

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Originally Posted by Simonetos The Greek

So I ended up on this...

Code:

---

Private Sub Me_ParentChanged(sender As Object, e As EventArgs) Handles Me.ParentChanged
        If Me.ParentForm IsNot Nothing Then
            Dim _UserControl = DirectCast(Me.ParentForm, Control)
            If _UserControl IsNot Nothing Then
                Do Until _UserControl Is Nothing
                    If TypeOf _UserControl Is CmosTitleBar AndAlso _UserControl IsNot Me Then
                        Throw New ArgumentOutOfRangeException("", "You can place only one " & _UserControl.Name & " control per form.")
                    End If
                    _UserControl = _UserControl.GetNextControl(_UserControl, True)
                Loop
            End If
        End If
    End Sub

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This is incorrect : _UserControl = _UserControl.GetNextControl(_UserControl, True)
... it will only go into controls... look @ my first example...

Quote:

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Originally Posted by i00

This is incorrect : _UserControl = _UserControl.GetNextControl(_UserControl, True)
... it will only go into controls... look @ my first example...

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Yes but I can't understand this part

vb.net Code:

Dim c = f

What is f and what is c? If f is the ParentForm and c the UserControl, then why c=f?

Quote:

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What is f and what is c? If f is the ParentForm and c the UserControl, then why c=f?

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That confused me too but it's not necessarily wrong. It means that the first pass through the loop will check that the user control isn't the form and I can't think why you'd do that. But subsequent passes through the loop will check the controls because of this line within the loop: c = f.GetNextControl(c, True).

So at worst i00's code will do an extra pointless iteration but will still check the controls correctly. At best, he's thought of something I've missed and there's a good reason for checking against the form itself on that first pass.

Personally I think i00's solution is correct except that that one line should be Dim c = f.GetNextControl(c, True)

Quote:

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Originally Posted by FunkyDexter

That confused me too but it's not necessarily wrong. It means that the first pass through the loop will check that the user control isn't the form and I can't think why you'd do that. But subsequent passes through the loop will check the controls because of this line within the loop: c = f.GetNextControl(c, True).

So at worst i00's code will do an extra pointless iteration but will still check the controls correctly. At best, he's thought of something I've missed and there's a good reason for checking against the form itself on that first pass.

Personally I think i00's solution is correct except that that one line should be Dim c = f.GetNextControl(c, True)

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Let's see if he will give us a logical explanation like yours!!! :)

Ah, just had a thought. GetNextControl gets the next control in tab order after the control passed in, in this case c. On the first pass c won't be anything. By setting it to the form he gives it a start point.

Personally, I would just chance the Do Until loop to a For Each loop. Again, I'm afraid I don't have an ide in front of me but it should be something like:-

vb.net Code:

For Each c as Control in f.controls
   'check it's not your user control
Next

Quote:

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Originally Posted by FunkyDexter

Ah, just had a thought. GetNextControl gets the next control in tab order after the control passed in, in this case c. On the first pass c won't be anything. By setting it to the form he gives it a start point.

Personally, I would just chance the Do Until loop to a For Each loop. Again, I'm afraid I don't have an ide in front of me but it should be something like:-

vb.net Code:

For Each c as Control in f.controls
   'check it's not your user control
Next

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I think you are right my friend!!! I tried this and works 100%...

vb.net Code:

Dim _ParentForm = DirectCast(Me.FindForm, Control)
If _ParentForm IsNot Nothing Then
    For Each _Control As Control In _ParentForm.Controls
        If TypeOf _Control Is MyUserControl AndAlso _Control IsNot Me Then
            Throw New ArgumentOutOfRangeException("", "You can place only one " & _Control.Name & " control per form.")
        End If
        _Control = _ParentForm.GetNextControl(_Control, True)
    Next
End If