cannot retrieve child count in canvas WPF

Question

2.00/5 (1 vote)

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Hello, In my project I have created a customized canvas named "MyCanvas" that implements drag and dropping.
Drag and dropping works well, but when i tried to count its child element using "VisualTreeHelper.GetChildrenCount Method ".
I got an error of "Value cannot be null. Parameter name: element".

Here is my code snippet
--------------------------

VB

Dim c As MyCanvas     
Protected Overrides Sub OnPreviewMouseDown(ByVal e As MouseButtonEventArgs)
            MyBase.OnPreviewMouseDown(e)
            Dim m As Integer
            m = VisualTreeHelper.GetChildrenCount(c) - 1
            MessageBox.Show(m)  'to display the number of 				                
				'child elements on the canvas
End Sub

[edit]code block added[/edit]

Posted 16-Nov-12 22:20pm

studiocode++

Updated 16-Nov-12 23:06pm

Nelek

v2

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Sergey Alexandrovich Kryukov · Accepted Answer · 2012-11-17T17:06:00

Solution 1

The problem is simple: c is null. I don't see any code where you initialize this member. Make sure it is properly initialized.

Also, you usually should use logical, not visual number of children. Simply use MyCanvas.Children.Count. But first, make sure you use proper instance if your class, not null.

—SA

Posted 17-Nov-12 17:06pm

Sergey Alexandrovich Kryukov

Comments

Shmuel Zang 18-Nov-12 1:33am

5'ed.

Sergey Alexandrovich Kryukov 18-Nov-12 11:05am

Thank you, Shmuel.
--SA