檢查N是不是質數

'檢查N是不是質數
Public Class Form1
    Dim i
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim n = 17
        Dim c = 0
        For i = 1 To n
            If n Mod i = 0 Then
                c = c + 1
            End If
        Next
        If c = 2 Then
            MsgBox(n & "是質數")
        Else
            MsgBox(n & "非質數")
        End If
    End Sub
End Class