实现功能：
a.ShowLine 参数：设置下划线是否可见
b.LineColor参数：设置下划线颜色
c.LineWidth参数：设置下划线粗细
d.LineOffset参数：设置下划线偏移量

Imports System.ComponentModel

<ToolboxBitmap(GetType(System.Windows.Forms.TextBox))>
Public Class TextBoxEx
    Inherits System.Windows.Forms.TextBox

    Protected _lineColor As Color = Color.Black
    Protected _lineWidth As Integer = 1
    Protected _lineOffset As Integer = 3
    Protected _showLine As Boolean = False
    Protected WithEvents Line As New Control()

    <DefaultValue(GetType(Color), "Black")>
    <Description("下划线颜色")>
    Public Property LineColor As Color
        Get
            Return _lineColor
        End Get
        Set(value As Color)
            If _lineColor <> value Then
                _lineColor = value
                RaiseEvent LineColorChanged(Me, Nothing)
            End If
        End Set
    End Property

    <DefaultValue(1)>
    <Description("下划线宽度")>
    Public Property LineWidth As Integer
        Get
            Return _lineWidth
        End Get
        Set(value As Integer)
            If _lineWidth <> value Then
                _lineWidth = value
                RaiseEvent LineWidthChanged(Me, Nothing)
            End If
        End Set
    End Property

    <DefaultValue(3)>
    <Description("下划线偏移量")>
    Public Property LineOffset As Integer
        Get
            Return _lineOffset
        End Get
        Set(value As Integer)
            If _lineOffset <> value Then
                _lineOffset = value
                RaiseEvent LineOffsetChanged(Me, Nothing)
            End If
        End Set
    End Property

    <DefaultValue(False)>
    <Description("指示控件是否显示下划线")>
    Public Property ShowLine As Boolean
        Get
            Return _showLine
        End Get
        Set(value As Boolean)
            _showLine = value
            RaiseEvent ShowLineChanged(Me, Nothing)
        End Set
    End Property

    ''' <summary>
    ''' 在 TextBoxEx.LineColor 属性值发生更改时发生
    ''' </summary>
    ''' <param name="sender"></param>
    ''' <param name="e"></param>
    ''' <remarks></remarks>
    Public Event LineColorChanged(ByVal sender As Object, e As EventArgs)

    ''' <summary>
    ''' 在 TextBoxEx.LineWidth 属性值发生更改时发生
    ''' </summary>
    ''' <param name="sender"></param>
    ''' <param name="e"></param>
    ''' <remarks></remarks>
    Public Event LineWidthChanged(ByVal sender As Object, e As EventArgs)

    ''' <summary>
    ''' 在 TextBoxEx.ShowLine 属性值发生更改时发生
    ''' </summary>
    ''' <param name="sender"></param>
    ''' <param name="e"></param>
    ''' <remarks></remarks>
    Public Event ShowLineChanged(ByVal sender As Object, e As EventArgs)
    ''' <summary>
    ''' 在 TextBoxEx.LineOffset 属性值发生更改时发生
    ''' </summary>
    ''' <param name="sender"></param>
    ''' <param name="e"></param>
    ''' <remarks></remarks>
    Public Event LineOffsetChanged(ByVal sender As Object, e As EventArgs)

    Public Sub onLineWidthChanged(ByVal sender As Object, e As EventArgs) Handles Me.LineWidthChanged, Me.LineOffsetChanged, Me.LineColorChanged
        If Me.ShowLine Then
            rePosLine()
        End If
    End Sub

    Public Sub onShowLineChanged() Handles Me.ShowLineChanged
        If Me.ShowLine Then
            BorderStyle = BorderStyle.None
            BackColor = Color.FromKnownColor(KnownColor.Control)
            rePosLine()
        Else
            BorderStyle = BorderStyle.Fixed3D
            BackColor = Color.FromKnownColor(KnownColor.Window)
            If Not IsNothing(Line) Then
                Line.Visible = False
            End If
        End If
    End Sub

    Protected Overrides Sub OnParentChanged(e As EventArgs)
        MyBase.OnParentChanged(e)
        If Me.ShowLine Then
            rePosLine()
        End If
    End Sub

    Protected Overrides Sub OnLocationChanged(e As EventArgs)
        MyBase.OnLocationChanged(e)
        If Me.ShowLine Then
            rePosLine()
        End If
    End Sub

    Protected Sub rePosLine()
        If Not IsNothing(Me.Parent) Then
            If Not Me.Parent.Controls.Contains(Line) Then
                Me.Parent.Controls.Add(Line)
            End If
            Line.Width = Me.Width
            Line.Height = Me.LineWidth
            Line.BackColor = Me.LineColor
            Line.Location = New Point(Me.Location.X, Me.Location.Y + Me.Height + Me.LineOffset)
            Line.Visible = True
            Line.BringToFront()
        End If
    End Sub

    Protected Overrides Sub Dispose(disposing As Boolean)
        MyBase.Dispose(disposing)
        If disposing Then
            If Not IsNothing(Line) Then
                Line.Dispose()
            End If
        End If
    End Sub

    Protected Overrides Sub OnLayout(levent As LayoutEventArgs)
        MyBase.OnLayout(levent)
        If Me.ShowLine Then
            rePosLine()
        End If
    End Sub
End Class