Public Class Form1
    Dim r As New Random
    Dim i As Integer
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        i = r.Next(65, 90)
        Label1.Text = Chr(i)
    End Sub

    Private Sub TextBox1_KeyPress1(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress
        If e.KeyChar = i Then 'e.KeyChar 跟VB6里的KeyAscii意思一样
            TextBox1.Text = ""
            e.Handled = True '对应VB6的 KeyAscii=0 ,意思不一样...但是作用都是不让TEXTBOX收到输入
            i = r.Next(65, 90)
            Label1.Text = Chr(i)
        End If
    End Sub
End Class