用户控件--控制一个文本框只允许输入固定多个字符（按照字节计算）

Public Class maxtextbox
    Inherits System.Windows.Forms.TextBox

#Region " Windows 窗体设计器生成的代码 "

    Public Sub New()
        MyBase.New()

        '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

    End Sub

    'UserControl 重写 dispose 以清理组件列表。
    Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean)
        If disposing Then
            If Not (components Is Nothing) Then
                components.Dispose()
            End If
        End If
        MyBase.Dispose(disposing)
    End Sub

    'Windows 窗体设计器所必需的
    Private components As System.ComponentModel.IContainer

    '注意：以下过程是 Windows 窗体设计器所必需的
    '可以使用 Windows 窗体设计器修改此过程。
    '不要使用代码编辑器修改它。
    <System.Diagnostics.DebuggerStepThrough()> Private Sub InitializeComponent()
        components = New System.ComponentModel.Container()
    End Sub

#End Region
    Private h As Integer
    Private m_max As Integer = 10
    Public Property mlength() As Integer
        Get
            Return m_max
        End Get
        Set(ByVal Value As Integer)
            If Value >= 1 Then
                m_max = Value
            End If
        End Set
    End Property
    Public Function checkboolean() As Boolean
        Dim count As Integer = System.Text.Encoding.Default.GetByteCount(Me.Text)
        If count >= 10 Then
            h = Me.Text.Length
            Return True
        Else
            Return False
        End If
    End Function

    Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)
        Dim KeyAsc As Integer = m.WParam.ToInt32
        Select Case m.Msg
            Case &H102    '按键
                If checkboolean() Then
                    Me.MaxLength = h
                Else
                    Me.MaxLength = m_max
                End If
            Case &H302    '复制
                If checkboolean() Then
                    Return
                End If
        End Select
        MyBase.WndProc(m)
    End Sub
End Class