VB.Net 声明全局变量a=PictureBox1 之后为何不能使用？

Antoom 发布于 2016-10-28 23:28， 2933 次点击

程序关键部分简化如下：
Public Class Form1
    Dim a As PictureBox = PictureBox1

    Private Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
        a.Image = Image.FromFile(Application.StartupPath & "\1.jpg")
    End Sub
End Class
结果显示“未将对象引用设置到对象的实例”，图片不能导入。
为什么会出现这种情况？
如何解决？（保留全局变量的声明）

1 回复

梦幻倩影2016-10-29 10:18

Public Class Form1
Public a As New PictureBox  '定义公共变量
    Private Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
        a = Me.PictureBox1
        a.Image = Image.FromFile(Application.StartupPath & "\1.jpg")
    End Sub
End Class