Public Class Form1
    Private Sub TextBox1_KeyPress(ByVal sender As Object,ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress
        If Asc(e.KeyChar) = 13 Then
            Dim i,m As Integer,Tag As Boolean
            m = Val(TextBox1.Text)
            Tag = True

            i = 2
            Do
                If (m Mod i) = 0 Then
                    Tag = False
                    Label2.Text = m & "不是素数"
                    Exit Do
                End If
                i = i + 1
            Loop While (2 <= i And i <= m - 1)
            If i = m Then Label2.Text = m & "是素数"
        End If
    End Sub

    Private Sub TextBox1_TextChanged(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles TextBox1.TextChanged

    End Sub
End Class

随机数产生！

Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles Button1.Click
        Dim i,a,s,max,min As Integer
        max = 1 : min = 100
        Label1.Text = ""
        For i = 1 To 10
            a = Int(Rnd() * 100 + 1)
            Label1.Text &= a & "  "
            If a > max Then max = a
            If a < min Then min = a
            s = s + a
        Next
        Label3.Text = max & Space(15) & min & Space(15) & s / 10
    End Sub
End Class