Función Primo

Public Class Form4
Dim a, i, c As Integer

Function mul(ByVal a As Integer) As String
c = 0
For i = 1 To 1000
If (a Mod i = 0) Then
c += 1
End If
Next
If c >= 3 Then
mul = "no es primo"
Else
mul = "es primo"
End If
End Function

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
a = TextBox1.Text
Label3.Text = mul(a)

End Sub
End Class