在 System.NullReferenceException 中第一次偶然出现的“WindowsApplication1.exe”类型的异常,这是什么意

Public Class Form1
Dim g As Graphics = PictureBox1.CreateGraphics
Dim p As New Pen(Color.Blue)
Dim sb As New SolidBrush(Color.Blue)
Dim a, b, x1, y1, w, h, t, s As Single
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
a = Val(TextBox1.Text)
b = Val(TextBox2.Text)
x1 = a
w = PictureBox1.Width
h = PictureBox1.Height
g.DrawLine(p, 0, h / 2, w, h / 2)
g.DrawLine(p, w / 2, 0, w / 2, h)
Timer1.Enabled = True
End Sub

Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
y1 = x1 ^ 2
g.DrawRectangle(p, w / 2 + 60 * x1, h / 2 - 60 * y1, t, 60 * y1)
g.FillRectangle(sb, w / 2 + 60 * x1, h / 2 - 60 * y1, t, 60 * y1)
s = s + 0.01 * y1
x1 = x1 + 0.01
End Sub
End Class
绘制y=x^2积分图的程序,s为积分值

我知道了，谢谢关注

如果以前没出现，那就是内存异常的问题。如内存有问题了。

最近这种异常很多啊，是vs问题，有时间正常有时间不正常

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