NullReferenceException?解决方法

NullReferenceException???
用VB在Partial Public Class Page1下定义变量出现未处理NullReferenceException提示框。。求解决办法。。。！

代码如下
Partial Public Class Page1

  Inherits PhoneApplicationPage

  Dim a1 = Convert.ToInt32(Button1.Content)

[解决办法]

VB code

Dim a1 as String'假設你的Button1存在a1=Convert.ToInt32(Button1.Content)