BackgroundWorker Blocking Question

[danadan]

Just wondering why this code blocks:

Public Class Form1
    Dim rand As New Random
    Private Sub BackgroundWorker1_DoWork(sender As Object, e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
        For i = 0 To 500000
            BackgroundWorker1.ReportProgress(1, rand.Next(0, 10000))
        Next
    End Sub
    Private Sub BackgroundWorker1_ProgressChanged(sender As System.Object, e As System.ComponentModel.ProgressChangedEventArgs) Handles BackgroundWorker1.ProgressChanged
        TextBox1.Text = e.UserState
    End Sub
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        BackgroundWorker1.RunWorkerAsync()
    End Sub
End Class

While this code does not block:

Public Class Form1
    Dim rand As New Random
    Private Sub BackgroundWorker1_DoWork(sender As Object, e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
        For i = 0 To 500000
            TextBox1.Invoke(Sub()
                                TextBox1.Text = rand.Next(0, 10000)
                            End Sub)
        Next
    End Sub
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        BackgroundWorker1.RunWorkerAsync()
    End Sub
End Class