C# と VB.NET の質問掲示板

お初です。

自分の作ったコントロールの簡易版ですが、こんなんでどうでしょう？
ListBoxを変更すれば他のコントロールも乗せれますよ。

Public Class DropDownTextBox
    Inherits TextBox

    Private WithEvents ListBox1 As New ListBox
    Private ToolStripDropDown1 As New ToolStripDropDown

    Public Sub New()
        Dim ToolStripControlHost1 As New ToolStripControlHost(ListBox1)
        ToolStripControlHost1.AutoSize = False
        ToolStripControlHost1.Margin = New Padding(0)

        ToolStripDropDown1.DropShadowEnabled = False
        ToolStripDropDown1.AutoSize = True
        ToolStripDropDown1.Padding = New Padding(0)
        ToolStripDropDown1.Items.Add(ToolStripControlHost1)
    End Sub

    Public ReadOnly Property Items() As ListBox.ObjectCollection
        Get
            Return ListBox1.Items
        End Get
    End Property

    Protected Overrides Sub OnKeyDown(ByVal e As System.Windows.Forms.KeyEventArgs)
        Select Case e.KeyCode
            Case Keys.F1
                ListBox1.Height = ListBox1.Items.Count * (ListBox1.ItemHeight + 1)
                ToolStripDropDown1.Show(Me, -2, Me.Height - 3)
        End Select
        MyBase.OnKeyDown(e)
    End Sub

    Private Sub ListBox1_MouseClick(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles ListBox1.MouseClick
        Me.Text = ListBox1.SelectedItem.ToString
        ToolStripDropDown1.Hide()
    End Sub
End Class