Public Class Form1

    Private Sub ComboBox1_KeyDown(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyEventArgs) Handles ComboBox1.KeyDown
        If e.KeyCode = 13 Then
            ComboBox1.Items.Add(ComboBox1.Text)
            TextBox1.Text = ComboBox1.Items.Count

        End If
    End Sub

    Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged

    End Sub

    Private Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
        If Timer1.Enabled = False Then
            Timer1.Enabled = True
            Button1.Text = "停止"
        Else
            Button1.Text = "开始"
            Timer1.Enabled = False
        End If
    End Sub

    Private Sub Timer1_Tick(ByVal sender As Object, ByVal e As System.EventArgs) Handles Timer1.Tick
        Dim a, n As Integer
        Randomize()
        n = ComboBox1.Items.Count
        a = Int(Rnd() * n)
        ComboBox1.SelectedIndex = a
        Label4.Text = ComboBox1.SelectedItem

    End Sub
End Class

textbox1.text还是不会显示
label4.text会显示

可能电脑有问题吧