用CType转成你的窗口类型就可以引用了
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比如你的子窗口类是MyForm
单击父窗口Button2显示它包含字符串MyPath的内容的代码如下
Private Sub Button2_Click_1(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
MsgBox(CType(Me.ActiveMdiChild, MyForm).MyPath)
End Sub
将NewForm设置为全局变量就可以了，如果新建窗口比较多，就设置为数组，例如：
Public Class Form1
Dim NewForm(10) As Form
Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
NewForm(0) = New Form
NewForm(0).Show()
End Sub
End Class
根据补充信息，修改如下：
在窗体上加入控件command1,然后复制下面代码，运行，启动画图程序，单击command1，即可找到你要的画纸的句柄！
Option Explicit
Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName As String) As Long
Private Declare Function FindWindowEx Lib "user32" Alias "FindWindowExA" (ByVal hWnd1 As Long, ByVal hWnd2 As Long, ByVal lpsz1 As String, ByVal lpsz2 As String) As Long
Private Sub Command1_Click()
Dim huatu, huazi
Dim i As Integer
huatu = FindWindowEx(0, 0, vbNullString, "未命名 - 画图")
Do Until huatu = 0
huazi = FindWindowEx(huatu, 0, "AfxFrameOrView42u", vbNullString)
huazi = FindWindowEx(huazi, 0, "Afx:1000000:8", vbNullString)
i = i + 1
MsgBox "找到第" i "个画纸，它的句柄是：" huazi
huatu = FindWindowEx(0, huatu, vbNullString, "未命名 - 画图")
Loop
End Sub
