无法在画布WPF中检索子计数 [英] cannot retrieve child count in canvas WPF

问题描述

您好，在我的项目中，我创建了一个名为"MyCanvas"的自定义画布，该画布实现了拖放操作.
拖放效果很好，但是当我尝试使用"VisualTreeHelper.GetChildrenCount方法"对它的子元素进行计数时.
我收到值不能为空.参数名称:元素"的错误.

这是我的代码段
--------------------------

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
--------------------------

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]添加了代码块[/edit]

[edit]code block added[/edit]

推荐答案

问题很简单:c为空.我看不到任何代码来初始化该成员.确保已正确初始化.

同样，您通常应使用逻辑数量，而不是视觉数量的子代.只需使用MyCanvas.Children.Count.但首先，请确保您使用正确的实例(如果您的类不是null).

—SA

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

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