Question

这是我的代码：

Public Class Form1
  Public TheImage As Image = PictureBox1.BackgroundImage
  Public Function AppendBorder(ByVal original As Image, ByVal borderWidth As Integer) As Image
    Dim borderColor As Color = Color.Red
    Dim mypen As New Pen(borderColor, borderWidth * 2)
    Dim newSize As Size = New Size(original.Width + borderWidth * 2, original.Height + borderWidth * 2)
    Dim img As Bitmap = New Bitmap(newSize.Width, newSize.Height)
    Dim g As Graphics = Graphics.FromImage(img)

    ' g.Clear(borderColor)
    g.DrawImage(original, New Point(borderWidth, borderWidth))
    g.DrawRectangle(mypen, 0, 0, newSize.Width, newSize.Height)
    g.Dispose()
    Return img
  End Function

  Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    Dim OutputImage As Image = AppendBorder(TheImage, 2)
    PictureBox1.BackgroundImage = OutputImage
  End Sub
End Class

在PictureBox1内有一个实际的背景图像，我在Designer中添加了它。但是当我调试时，我收到错误消息：

InvalidOperationException未处理

我做错了什么？

Answer 1

 Public TheImage As Image = PictureBox1.BackgroundImage

那不行。执行此语句时，PictureBox1还没有值，直到InitializeComponent（）方法运行才会发生。你可能从未听说过，神奇的咒语是你输入“Public Sub New”。当您按Enter键时，您将看到：

Public Sub New()

    ' This call is required by the Windows Form Designer.
    InitializeComponent()

    ' Add any initialization after the InitializeComponent() call.

End Sub

这是构造函数，它是.NET类的一个非常重要的部分。请注意生成的“添加任何初始化”注释。这就是你初始化TheImage的地方。使它看起来像这样：

Public TheImage As Image

Public Sub New()
    InitializeComponent()
    TheImage = PictureBox1.BackgroundImage
End Sub

如果这一切仍然很神秘，那么请阅读书籍以了解更多信息。

Answer 2

这部分代码：

Public TheImage As Image = PictureBox1.BackgroundImage

在调用TheImage之前初始化InitializeComponent，因此此时尚未创建PictureBox1。当我把这件作品移到Form_Load时，一切都很完美：

Public TheImage As Image
'...
Private Sub Form1_Load(sender As Object, e As System.EventArgs) Handles Me.Load
  TheImage = PictureBox1.BackgroundImage
End Sub

如何为图像添加边框

2 个答案: