Dim g As Graphics = Me.CreateGraphics

需要局部定义 每次实用需要定义--不能定义成全局

Public Class Form1
    Dim DrawState As Boolean
    Dim PreX As Single
    Dim PreY As Single
    Dim eP As New Pen(Color.Black,3)          ' 构造黑色画笔，并将它赋给对象变量eP
    'Dim g As Graphics = Me.CreateGraphics     ' 在窗体上构造一块画布，并将它赋给对象变量g

    Private Sub Form1_Load(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles MyBase.Load
        ' 将DrawState初始化为False，表示提笔
        DrawState = False
    End Sub

    Private Sub Form1_MouseDown(ByVal sender As Object,ByVal e As System.Windows.Forms.MouseEventArgs) Handles Me.MouseDown
        ' 当鼠标左键被按下时，把DrawState设为True，表示落笔开始画线
        Dim g As Graphics = Me.CreateGraphics
        If e.Button = Windows.Forms.MouseButtons.Left Then
            DrawState = True     ' 设置画图状态
            PreX = e.X          ' PreX和PreY保存了线条的起点。
            PreY = e.Y
        End If
        ' 当按住鼠标右键时，画一个直径为50的圆
        If e.Button = Windows.Forms.MouseButtons.Right Then
            g.DrawEllipse(eP,e.X - 25,e.Y - 25,100,100)
        End If
    End Sub

    Private Sub Form1_MouseMove(ByVal sender As Object,ByVal e As System.Windows.Forms.MouseEventArgs) Handles Me.MouseMove
        ' 当鼠标移动时，如果处于画线状态，则在(PreX，PreY)与(X，Y)之间画一条直线
        Dim g As Graphics = Me.CreateGraphics
        If DrawState = True Then
            g.DrawLine(eP,PreX,PreY,e.X,e.Y)
            PreX = e.X
            PreY = e.Y
        End If

    End Sub

    Private Sub Form1_MouseUp(ByVal sender As Object,ByVal e As System.Windows.Forms.MouseEventArgs) Handles Me.MouseUp
        ' 当释放鼠标左键时，解除画线状态
        If e.Button = Windows.Forms.MouseButtons.Left Then
            DrawState = False
        End If
    End Sub
End Class

改法1

Public Class Form1
    Dim DrawState As Boolean
    Dim PreX As Single
    Dim PreY As Single
    Dim eP As New Pen(Color.Black,3)          ' 构造黑色画笔，并将它赋给对象变量eP
    Dim g As Graphics     ' 在窗体上构造一块画布，并将它赋给对象变量g

    Private Sub Form1_Load(ByVal sender As System.Object,ByVal e As System.Windows.Forms.MouseEventArgs) Handles Me.MouseDown
        ' 当鼠标左键被按下时，把DrawState设为True，表示落笔开始画线
        'Dim g As Graphics = Me.CreateGraphics
        If e.Button = Windows.Forms.MouseButtons.Left Then
            DrawState = True     ' 设置画图状态
            PreX = e.X          ' PreX和PreY保存了线条的起点。
            PreY = e.Y
        End If
        ' 当按住鼠标右键时，画一个直径为50的圆
        If e.Button = Windows.Forms.MouseButtons.Right Then
            g.DrawEllipse(eP,ByVal e As System.Windows.Forms.MouseEventArgs) Handles Me.MouseMove
        ' 当鼠标移动时，如果处于画线状态，则在(PreX，PreY)与(X，Y)之间画一条直线
        'Dim g As Graphics = Me.CreateGraphics
        If DrawState = True Then
            g.DrawLine(eP,ByVal e As System.Windows.Forms.MouseEventArgs) Handles Me.MouseUp
        ' 当释放鼠标左键时，解除画线状态
        If e.Button = Windows.Forms.MouseButtons.Left Then
            DrawState = False
        End If
    End Sub


Private Sub Form1_Resize(ByVal sender As Object,ByVal e As System.EventArgs) Handles Me.Resize
        g = Me.CreateGraphics
    End Sub
End Class

改法2

将声明定义到模块中