Public Class Form1

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

    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim n, i, s As Integer
        n = CInt(TextBox1.Text)
        'Is tere an integer number i between 1 and n such that n is divisible by i
        s = Math.Sqrt(n) + 1 'added 1 to eliminate possible rounding error
        For i = 2 To s
            If n Mod i = 0 Then 'i is such a divisor so n is not prime
                Label1.Text = CStr(n) & " is not prime"
                Return 'We accomplished the task
            End If
        Next
    'There are no such divisors so n is prime
        Label1.Text = CStr(n) & " is prime"
    End Sub

    Private Sub Label1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Label1.Click

    End Sub
End Class