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Important This document may not represent best practices for current development, links to downloads and other resources may no longer be valid. Current recommended version can be found here.

How to: Access All Open Forms of an Application

This example uses the My.Application.OpenForms property to display the titles of all the application's open forms.

The My.Application.OpenForms property returns all currently open forms, regardless of which thread opened them. This means that you should check the InvokeRequired property of each form before accessing it; otherwise it might throw an InvalidOperationException exception.

This example declares a function that obtains each form's title in a thread-safe manner. First, it checks the form's InvokeRequired property and, if necessary, uses the BeginInvoke method to execute the function on the form's thread. Then the function returns the form's title.

This example loops over the application's open forms and displays their titles in a ListBox control. For simpler code that displays only the forms directly accessible from the current thread, see My.Application.OpenForms Property.

Private Sub GetOpenFormTitles()
    Dim formTitles As New Collection

    Try 
        For Each f As Form In My.Application.OpenForms
            ' Use a thread-safe method to get all form titles.
            formTitles.Add(GetFormTitle(f))
        Next 
    Catch ex As Exception
        formTitles.Add("Error: " & ex.Message)
    End Try

    Form1.ListBox1.DataSource = formTitles
End Sub 

Private Delegate Function GetFormTitleDelegate(ByVal f As Form) As String 
Private Function GetFormTitle(ByVal f As Form) As String 
    ' Check if the form can be accessed from the current thread. 
    If Not f.InvokeRequired Then 
        ' Access the form directly. 
        Return f.Text
    Else 
        ' Marshal to the thread that owns the form.  
        Dim del As GetFormTitleDelegate = AddressOf GetFormTitle
        Dim param As Object() = {f}
        Dim result As System.IAsyncResult = f.BeginInvoke(del, param)
        ' Give the form's thread a chance process function.
        System.Threading.Thread.Sleep(10)
        ' Check the result. 
        If result.IsCompleted Then 
            ' Get the function's return value. 
            Return "Different thread: " & f.EndInvoke(result).ToString
        Else 
            Return "Unresponsive thread" 
        End If 
    End If 
End Function

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