在textbox里放按钮并控制输入字符不能覆盖住按钮(包括设计阶段)

答案：
Imports System.Text 
Public Class mytextbox 
    Inherits System.Windows.Forms.TextBox 
Dim WithEvents btn As Button 
Dim WithEvents btn As Button 
    Public Sub New() 
        MyBase.New() 
 '该调用是 Windows 窗体设计器所必需的。 
 '该调用是 Windows 窗体设计器所必需的。 
        InitializeComponent() 
  '在 InitializeComponent() 调用之后添加任何初始化 
  '在 InitializeComponent() 调用之后添加任何初始化 
        btn = New Button() 
        btn.Size = New Size(23, 23) 
        Me.Controls.Add(btn) 
        btn.Dock = DockStyle.Right 
        btn.BackColor = SystemColors.Control 
        Me.Text = "" 
    End Sub 
 Public Event myclick(ByVal sender As Object, ByVal e As EventArgs) 
 Public Event myclick(ByVal sender As Object, ByVal e As EventArgs) 
    Private Sub btn_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btn.Click 
        RaiseEvent myclick(Me, e) 
    End Sub 
 Private Sub btn_MouseEnter(ByVal sender As Object, ByVal e As System.EventArgs) Handles btn.MouseEnter 
 Private Sub btn_MouseEnter(ByVal sender As Object, ByVal e As System.EventArgs) Handles btn.MouseEnter 
        Cursor = Cursors.Default 
    End Sub 
 Private Sub btn_MouseLeave(ByVal sender As Object, ByVal e As System.EventArgs) Handles btn.MouseLeave 
 Private Sub btn_MouseLeave(ByVal sender As Object, ByVal e As System.EventArgs) Handles btn.MouseLeave 
        Cursor = Cursors.IBeam 
    End Sub 
 Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message) 
 Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message) 
        Dim KeyAsc As Integer = m.WParam.ToInt32 
        Select Case m.Msg 
            Case &H102 
                If checklength() Then 
                    If KeyAsc <> 8 Then 
                        Return 
                    End If 
                Else 
                    Me.MaxLength = 0 
                End If 
            Case &H302 
                If checklength() Then 
                    Return 
                End If 
        End Select 
        MyBase.WndProc(m) 
    End Sub 
    '检查函数 
    Private Function checklength(Optional ByVal str As String = "") As Boolean 
        If str = "" Then 
            Dim leng As Integer = CInt(Me.CreateGraphics.MeasureString(Me.Text, Me.Font).Width) 
            Dim m As Integer = Me.Width 
            Dim p As Integer = btn.Width 
            If leng >= m - p - 5 Then 
                Return True 
            End If 
        Else 
 &nbs

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